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Troubleshooting Your Oscilloscope

1. d33MS 30Vu13H 531 9 11 13538 TANNAL 13539 8 Nid 3534 55 34OQ 10H 340 09910 AC V 8 1n03Wl L 33OQ 10H TROUBLESHOOTING THE HORIZONTAL SECTION NOTE Where applicable there is a KEY INDICATOR for each of the events in the sequence This KEY INDICATOR points out a predicted occurance that you should be able to observe on the display or on the front panel indicators EVENT QUIESCENT PERIOD ARMED DEFINITION CHARACTERISTICS Tnis is the reference starting point It is the interval between HOLD OFF and Trigger This state occurs when HOLD OFF has timed out and ended and the instrument is now waiting for a Trigger KEY INDICATOR The READY indicator will illurninate when the instrument is in this state and the instrument is in the SINGLE SWEEP MODE Verify this on the A SWEEP SEQUENCE FLOWCHART Set the TIME DIV control to 1 S div select SINGLE SWEEP Press the SINGLE SWEEP RESET button to arm it The READY light should turn on You can also observe this state by checking the state of the Tunnel Diodes if applicable Both diodes should be in the Low state less than 100 mV on the Anode and HOLD OFF should be inactive The Trigger circuit is Armed and is ready to generate a trigger If the TRIGGER SLOPE is set for a positive trigger the circuit will be ARMED qualified by the first negative trigger signal occurance in the QUIESCENT
2. 4 Joye nGay 12ouuoosiQ 52125 JO 9151 POUS 1527 5 2045 payesipul sj uous mun 900 510 5159 M07104 79945 C3 115 jeu 79942 Paresmes 20 payous i yous 51 addiy 510151599 T puy B addiy 2242 5592 3 2 001 ndino indino Ajddng 26 CHAPTER 4 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY 27 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY Some instruments use a high efficiency low voltage power supply in which the primary circuit compensates for changes in line voltage line frequency and load demand Compared to conventional regulated supplies high efficiency power supplies present more of a challenge to the service technician The High Efficiency Power Supply is a quick review of high efficiency supplies They are basically two stage AC to DC converters The line voltage is rectified then used to power an inverter that runs at approximately 25 kHz The inverter drives the primary of the power transformer which provides the necessary secondary voltages Regulation is usually accomplished by controlling the frequency at which the inverte
3. rsen 2 E NO AQV3H 19539 58 A1 ppp 72 TROUBLESHOOTING THE HORIZONTAL SECTION 475A A LOCKED KNOBS FAST 519 1SV3 SGONY Q33201 VSZt 13 16 9660 8 680 as md 18915 3300 10H 9 toro woud Sd33MS 135 SIXV Z NV 18 S31V9 11 1353 S3Q0 Q TANNAL 13539 340 8290 NO 2190 dWVH 504 3790 IH 11 NO P 010 IH 11 440 an NO 9890 D 009n 4443 5 8690 1 2010 22010 NYHL LNO 8690 440 8890 soon 433 5 MNVIENN LNOAWIL 440010H 44O 0649 NO 50 504 SdWvu NO 2860 330 2150 9 009 HOLIMS aLVS AV 13a 31V9 NIVIN LYVLS 5 NM pA olson NODIML Ll NO AQY3H 1 13538 58 73 475A A LOCKED KNOBS SLOW TROUBLESHOOTING THE HORIZONTAL SECTION 14 571 MOIS SAONA 133201 VSZP 8 O 3 5 9660 9 660 LYVLS 44 01 9 rtoiO 8 9650 1 Sd33MS 13 SIXV Z2 MNv 18 S31V9 11 13538 5 T3NNn 13538 440 8290 2 NO 2190 504 3 90 91 0090 01 nomio 12 IH LI 0834 440 8690 IH Zt Ni
4. If you said the Delay Line you are correct If you didn t go back and look at the block diagram Use a shorting strap or screwdriver to short tne ends of the Delay Line together If the display returns to about the center of the CRT the fault is before the Delay Line If the display is still off screen the problem is in the Vertical Output Amplifier the CRT or in the connections between thern In either case before or after the Delay Line use the general troubleshooting techniques presented in the of this section to further isolate the problem ISOLATING FAULTS IN THE VERTICALS OF LABORATORY OSCILLOSCOPES Tne Vertical Section of Laboratory Oscilloscopes is made easier because of the inodular nature of the plug ins You can divide and test the vertical section easily by simply swapping the plug in If the fault is removed when the plug in is swapped use the general troubleshooting tips in the back of this section to further isolate the problem If the fault is not affected by changing the plug in divide the remaining portion of the vertical section into half by using the CRT Readout Turn the READOUT INTENSITY up to about the 12 o clock position If the Readout is on the display in approximately the correct position but the sweep is deflected offscreen use the BEAM FINDER to check the fault is located in the circuitry before the Vertical Output Board This can be verified by shorting the output ends of the Delay Line toge
5. be lost in the compressed CRT Readout Rotate the INTENSITY control clockwise for the desired brightness of the display If the sweep appears and the display appears normal do a Performance Check to insure proper operation of the instrument If the display is only a dot that can be seen without pressing the Beam Finder the fault is probably in tne Horizontal Section This condition indicates that a Sweep Gate Main Gate has been generated but the Sweep Generator is locked up Refer to the section on Troubleshooting the Horizontal Section If you still do not have a display after the INTENSITY is turned up set the INTENSITY control to the 12 o clock position Set the READOUT INTENSITY to the 12 o clock position If you still do not have a sweep but the Readout is normal the fault is probably in the Horizontal Section Display Sequencer or in the Z Axis Section Rotate the TIME DIV control to the X Y position Center the CH and CH 2 POSITION controls If you get a bright spot on the CRT the fault is probably in the Horizontal Section 17 TROUBLESHOOTING YOUR OSCILLOSCOPE If you have no display on the CRT press the BEAM FINDER If the display is offset vertically troubleshoot the Vertical Section If the display is offset horizontally troubleshoot the Horizontal Section If you still have no display troubleshoot the Power Supply and Z Axis Sections Lab Instrument Oscilloscope Tips and Techniques NOTE The following tips a
6. Typical A Sweep Sequence 465 A Sweep Sequence 465B A Sweep Sequence 475A A Locked Knobs Fast 475A A Locked Knobs Slow 475A A Unlocked Knobs or A Intensified Typical Block Diagrams for Solid State Trigger Circuits Tunnel Diode Triggering Voltages Typical Curve Tracer Display for Tunnel Diodes Trigger Level and Slope Troubleshooting Flowchart Sweep Circuit Troubleshooting Flowchart Spot off Screen on Left Troubleshooting Flowchart Trace off Screen on Right Page 26 81 37 48 43 50 59 71 72 73 74 75 77 79 30 82 87 88 CHAPTER 1 TROUBLESHOOTING TECHNIQUES AND PROCEDURES TROUBLESHOOTING TECHNIQUES AND PROCEDURES To the technicians good troubleshooting techniques are lixe money in the pocket Thougn technicians may use different methods for troubleshooting most of the better ones have a defined repeatable pattern or technique that they use when troubleshooting The best troubleshooters relate the visual symptoms and the front panel controls to the systems and circuitry within the faulty instrument Armed with the understanding of these relationships they perform the major portion of their troubleshooting from the front panel using front and rear panel connectors for access to internal circuitry Furthermore they do most of their major troublehsooting without removing the cover trom the instrument and without using a lot of bulky test equipment They u
7. PERIOD You can set this condition when the scope is in the SINGLE SWEEP MODE Set the TRIGGER SLOPE to positive slope and rotate the LEVEL control fully clockwise Press the SINGLE SWEEP RESET then rotate the LEVEL control slowly counterclockwise to its stop The Trigger is now ARMED 60 EVENT ARMED Cont FIRED TROUBLESHOOTING THE HORIZONTAL SECTION DEFINITION CHARACTERISTICS You can test this state in those scopes with Tunnel Diodes in the Trigger Generator Connect the test probe to the anode of the ARM Tunnel Diode There should be about 400 500 mV at the anode ot the diode If not check the Tunnel Diode and tne Tunnel Diode Driver Circuitry In those instruments with an integrated Trigger Generator you cannot test this step Tnis is the Trigger froin the Trigger Generator Tne output from the circuit may be called Trigger Sweep Gate or Main Gate It may also have an Auto Disable Gate This occurance signifies that the Trigger Generator has processed a signal of the correct polarity and amplitude as set by the front panel controls In the previous step of the sequence you ARMED the Trigger Generator Now that you have it armed you can FIRE it by rotating the LEVEL control clockwise KEY INDICATOR If the instrument Triggers properly the TRIGGERED light will illuminate Note this on the A SWEEP SEQUENCE FLOWCHART The Trigger Pulse Sweep Gate will be sent to the Sweep Generator and to t
8. VOLTS DIV VARIABLE POSITION AC GND DC VERT MODE INVERT 20MHz BW LIMIT TRIGGER CONTROLS MODE SLOPE COUPLING SOURCE SOURCE SWEEP CONTROLS HORIZONTAL DISPLAY DELAY TIME POSITION A TIME DIV B TIME DIV VAR TIME DIV X10 MAG POSITION A TRIGGER HOLDOFF TRACE SEPARATION 0 5V Calibrated detent Midrange DC CH Off Off Full bandwidth AUTO if applicable E DC INT STARTS AFTER DELAY An 1 00 1 mS 1 mS 100 uS Calibrated detent Off iMidrange NORM As desired If you nave not yet read the section on Troubleshooting Techniques and Procedures do so before continuing in this book 12 TROUBLESHOOTING YOUR OSCILLOSCOPE The Power Supply As you can see from the block diagram the Power Supply provides the required operating potentials to all sections and plocks of the oscilloscope Therefore a fault in tne Power Supply Section will usually affect several major blocks of the block diagram You should use the same basic troubleshooting procedures for isolating faults to the Power Supply section that you would use for isolating faults to any other section of the instrument This starts with front panel troubleshooting using the controls indicators on the front panel along with the connectors and switches on the front and rear panels If there are no CRT or power on indications at power up check the POWER indicator If the POWER indicator is not lit
9. check the Line Fuse and the position of the Line Selector Switch if it is accessible from the outside of the instrument Also check the other front panel indicators such as the VOLT DIV and TIME DIV skirt coding lamps the TRIGGERED light GRATICULE ILLUMINATION and READY indicator If none of the indicators are on and if there is no display then the fault is probably in the L V Power Supply At this point if you suspect the Power Supply is faulty refer to the chapter of this book on TROUBLESHOOTING THE POWER SUPPLY Even if there is a display or if there are front panel indicators operating on tne instrument the Power Supply may still be faulty However if the indicators are operating norinally and if there is a display sweep or dot the fault is probably located in one of the other sections of the instrument If you have front panel indicators and a display for troubleshooting continue your fault isolation from the front panel 13 TROUBLESHOOTING YOUR OSCILLOSCOPE The Vertical Section When no spot or sweep can be seen on the display use the beam finder and the position control to see in which direction the spot or sweep is deflected First use the vertical position control to see whether the display can be centered If the sweep will not center on the display press the BEAMFINDER button and check for an on screen display If there is still no display on screen the problem is likely to be in either the Power Supply Secti
10. horizontal points on the CRT display Delaying sweep oscilloscopes are quite common and provide two separate complete sweep systems The first or delaying sweep provides a delayed sweep trigger just prior to the moment when the signal of interest occurs Generally a IO turn multiplier dial used witn the TIME DIV control provides a continuously variable sweep trigger and initiates the delayed sweep at the desired time Delaying sweep oscilloscopes provide both increased ineasurement resolution and accuracy Modern time base generators generally consist of five inain circuits a sweep gating multivibrator a Miller runup or rundown circuits sawtooth generator and disconnect diode hoidoff circuitry sweep lockout circuitry and automatic sweep generator circuitry In addition the sweep circuit provides the unblanking signal to the and often a sawtooth and or gate output on the instrument panel Sweep generators make use of operational amplifier techniques to obtain their required linearity As a result if circuit problems appear they are sometimes difficult to troubleshoot because of the feedback loops involved Usually the feedback loop must be broken in order to localize the circuit probiein 83 TROUBLESHOOTING THE HORIZONTAL SECTION When troubleshooting sweep circuits free run the sweep to be certain that the trigger circuitry is not inhibiting sweep operation Gate and sawtooth output connectors provide quick che
11. in Figure 1 If the trace deflects to about the center of the CRT the fault is located before the stage that is shorted out If the beain is still deflected off screen the fault is located after the connecting points FIGURE 1 Cominon Mode Operation of a Push Pull Amplifier 47 TROUBLESHOOTING THE VERTICAL SECTION This technique can aiso be used in those instruments that use Integrated Circuit Amplifiers Simply use a clip lead to short the signal input pins of the amplifier stage together as illustrated in Figure 2 FAR 84513 i TIS2O 450 FIGURE 2 Common Mode Operation of an IC Amplifier Notice that this is an ideal technique for some instruinents because of the configuration of the C s used in the Vertical Amplifier circuitry The example in Figure 2 is a Tek made Vertical Amplifier part number 155 0078 XX This IC is used in the Vertical Section of many Tek oscilloscopes such as the 465B 475 and 475A and also in Vertical Amplifiers such as the 7A26 These IC s can be shorted without using a shorting strap Carefully short pins and 6 together with the tip of a small tweeker Notice in Figure 3 that these pins are adjacent to each other on the IC and that they are marked by a dot t pin indicator on the body of the IC FIGURE 3 Vertical IC Amplifier 43 TROUBLESHOOTING THE VERTICAL SECTION Tracking Down a Noisy Circuit You can also use the successive approximatio
12. of any suspected areas Make waveform and voltage measurements at points that would divide the suspected section into equal halves Use other testing techniques such as common moding and plug in swapping to isolate the faulty section TROUBLESHOOTING TECHNIQUES AND PROCEDURES 6 ISOLATE THE FAULTY BLOCK Now that you have located the faulty section continue to break it into smaller possible faulty blocks through a system of successive approximation techniques Make your waveform and voltage measurements at the approximate inidpoints of the suspected faulty section If the measureinents are correct the fault is probably behind the ineasurement point If the measurement is irregular or faulty the fault is probably before the measure nent point Continue to test and measure until you have isolated the fault to a functional block Remember to continuously use the front panel display and indicator symptoms as you proceed In those instrurnents that have plug in modules troubleshoot plug in swapping In instruments that have circuit boards that are easily exchanged isolate the fault by board and assembly swapping Just remember with either of these methods always replace the boards and assemblies after you have located the fault ISOLATE THE FAULTY CIRCUIT Make waveform and voltage measurements at the approximate midpoints of the functional blocks to further isolate the fault Use special techniques such as short
13. start when shock excited i e rotating the TIME CM switch Cneck Start stop multivibrator circuit failure will show tnese characteristics Off tolerance precision 1 resistors in this circuit will sometimes cause this problem Sweep timing off at several of the slower sweep speeds below ms div Check Suspect precision timing resistors Many older oscilloscopes used brown A P resistors on the sweep timing switches These resistors changed value with age and should all be changed Sweep nonlinear or inaccurate at slow sweep speeds In extreme cases spot may stop part way through the sweep Check The disconnect diodes should be tested Check for proper operation by starting the sweep and then remove the disconnect diode to see if the problem clears itself The sweep will run for one sweep and stop Replace the diode and repeat the procedure if necessary to get a better look If this procedure clears up the problem the disconnect diode is faulty leaky Sweep nonlinear at some TIME CM settings normal operation at others Check Leaky Miller runup or rundown circuit Check for faulty transistor 85 TROUBLESHOOTING THE HORIZONTAL SECTION Sweep inoperative Check Check the sweep gate transistor and the sweep TD If these operate properly then check the fixed divider at the input of the sweep reset multiviorator for proper value Sweep inoperative check If normal troubleshooting doesn t produce a trace check
14. the BEAM FINDER the SWEEP is locked up Look for this fault in the Sweep Generator 63 TROUBLESHOOTING THE HORIZONTAL SECTION EVENT END OF SWEEP EOS DEFINITION CHARACTERISTICS If the SWEEP is locked up on the left side of the CRT you can test the Sweep Generator by removing the Disconnect Transistor in the Sweep Generator the SWEEP runs the Sweep Generator is working look for fault in the input gating For those instruments with components that are soldered in you can turn the Disconnect Transistor off by shorting the Emitter Base juction careful when trying tnis since a misconnection could cause catastrophic failure The ramp voltage is compared to a fixed voltage that correlates the beam deflection to the far right side of the CRT The comparison of these two voltages signifies the end of the sweep This event tells the Timebase to Blank the CRT and to initiate the retrace reset operation KEY INDICATOR If the SWEEP runs one tirne across the CRT and the CRT is blanked upon reaching the right side this section is operating normally KEY INDICATOR If a dot appears on the right side of the CRT without the use of the BEAM FINIDER this step has not been cornpleted though the SWEEP has ended You can verify the location of the fault by checking the ramp voltage with a scope or by causing the End of Sweep Comparator to disconnect Use the same technique described for disconnecting the
15. the Sweep 5tart Comparator This is true only if the sweep has not run If the sweep is in any other condition the problem is elsewhere 62 EVENT UNBLANK SWEEP TROUBLESHOOTING THE HORIZONTAL SECTION DEFINITION CHARACTERISTICS This event in the sequence is marked by the generation of the gate logics necessary to the CAT at the beginning of sweep The UNBLANK gate provides the unblanking logic to the Z Axis Amplifier to turn on the beam in the CRT KEY INDICATOR After the UNBLANK event has occurred you can see the display without having to press the BEAM FINDER The CRT will have a visible trace or spot If the display is a sweep that can only be seen by pressing BEAM FINDER the fault is probably in the Unblank Amplifier or in the CRT and Z Axis Section In many oscilloscopes the gate generator for the unblanking logic also generates the A GATE that is availaole to the rear panel connector of the instrument Cneck the schematics and measure the output of the A GATE OUTPUT if applicable The generation of a linear ramp to drive the Horizontal Section The sweep polarity and amplitude varies from scope to scope so the ramp may be either positive or negative in transition As the ramp runs through its voltage transition it drives the beain from left to right on the CRT KEY INDICATOR If a dot appears on the left side of the CRT that is visible witn normal intensity and without the use of
16. the most common type of failure in the Vertical Section Front end failures These failures usually occur when an operator connects a signal to the Vertical Input connection that exceeds the voltage rating of the instrument or when the instrument is grossly over ranged for example 100V dc is applied when the VOLTS DIV setting is at l0mV DIV Since these front end problems are more common you should first try to isolate them to or away frorn this area In oscilloscopes with plug in modules you can do your initial isolation to the plug in by swapping the suspect inodules with modules that you know are operating properly You can use the following procedures on those oscilloscopes and vertical plug ins with dual channel inputs 43 TROUBLESHOOTING THE VERTICAL SECTION ISOLATING FAULTY PREAMPLIFIERS Check Channel A Press and hold Beam Finder B Rotate the position control through its ranges and note any effect on the display C Check each position of the Volts Division Switch D Check each position of the Coupling Switch Any appreciable change in the display may indicate a fault in Channel operation Note these changes but do not remove the cover from the instrument or begin to troubleshoot the section yet Continue to the next step 2 Check Channel 2 A Repeat the steps as indicated for Channel B Invert the display and check the display to see if the trace position is inverted on the CRT If the trace
17. the sweep length circuit A diode failure or bad switch contact in the sweep length circuit may cause an inoperative sweep Sweep timing error at different sweep speeds Check Diodes used in the sweep disconnect circuit may be defective Replace if necessary Sweep jitter Cneck Diodes used in the sweep disconnect circuit may be defective Replace if necessary 86 TROUBLESHOOTING THE HORIZONTAL SECTION Does Sweep Check Remain Milter Reset Circuit Na Reptace Old Disc And Check Sweep Start Signat sweep is latched in reset state spot off screen on left side 87 TROUBLESHOOTING THE HORIZONTAL SECTION Short Firng Ground Dees seen Sweep Gate Gate Go High Clew Check Trigger Circuit If sweep is latched in rundown state trace off screen on right side 88
18. to normal operation 32 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY Here are some quick checks that may help you isolate faults in a failed supply Check for approxiinately 330V dc across the inverter switching transistors Check for start pulses at the anode of tne PUT SCR in the start up circuit Check for 32V across the inverter start up transistor Check to see if the over voltage transistor is turned on 533 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY Typical High Efficiency Supply Problems f the supply won t come up and it isn t evident what the problem is the following troubleshooting hints may help Scope inoperative No ourst operation Solution Check fuses If they are blown and continue to blow when replaced it indicates that an inverter is probably shorted Scope inoperative Burst operation hear ticking sound Solution Check resistance of scope power supplies if the values are given in the service manual Scope inoperative Burst operation Resistance normal Solution Check for leaky or shorted capacitors on the rectifier board Burst operation Semi regulated voltage normal Solution Possible CRT shorted Shorted component on high voltage board No inverter operation Solution Check components on inverter board use Curve Tracer Primary transistors and capacitors Check transistors and capacitors in the primary of the inverter Unstable inverter operation Solution Ch
19. 12 o clock position Press the BEAM FINDER and observe the display Ifa sweep appears on the display it indicates a fault in the Unolanking Logic of the instrument This particular problem may be located in either the Horizontal Section tne CRT and Z Axis Section or in the interface between the sections It could also be in the Vertical Section but you have already eliminated that section if you followed the procedures If you have only a DOT for a display when you press the 3EAM FINDER the fault is probably located in the Horizontal Section Refer to the section of this book on TROUBLESHOOTING THE HORIZONTAL SECTION If you still have no display when you press BEAM FINDER the fault is probably located in either the Power Supply or CRT Z Axis Section of the instrument You can easily verify this from the rear panel of the instrument by using the following techniques NOTE When using this technique do not leave the injected voltage on the rear panel EXT Z AXIS INPUT connector for more than a few seconds Remove the voltage immediately because the intense beam on the CRT can damage the CRT Before continuing turn the FOCUS control fully counter clockwise Inject 10 to 15 volts DC into the EXT Z INPUT connector on the rear panel If you still have no display refer to the sections of this book on Troubleshooting the Power Supply and on Troubleshooting the CRT and Z Axis Section If you get a very intensified dot on the CRT th
20. 3538 99 75 TROUBLESHOOTING THE HORIZONTAL SECTION Tips for Troubleshooting Trigger Circuits When troubleshooting a new trigger circuit take some time to familiarize yourself with the block diagram and schematics Spending a few minutes with the instrument manual can give valuable insight into the problem Tektronix trigger circuits are designed to respond to a wide variety of input signals Since many of these input signals are unsuitable as sweep initiating triggers signals are first applied to a trigger circuit where they are converted to a pulse of uniform amplitude and shape Thus regardless of the input signal it is possible to start the sweep with a pulse that has constant amplitude and risetime The trigger circuitry allows the operator to start the sweep on either slope of the waveform select any voltage level on the rising or falling slope of that waveform and filter out selected frequencies of the input signal for greater ease and stability in triggering The triggering of the general purpose oscilloscope may be broken down into five basic parts 1 vertical amplifier trigger pickoff circuitry 2 input coupling circuitry 3 input amplifier 4 trigger pulse generator and 5 autornatic triggering circuitry The trigger pickoff circuitry acts as a buffer to keep the trigger circuitry from changing the operation of the vertical amplifier yet pass the amplified vertical signal to the
21. AY timing capacitor reaches a pre deterinined level TIMEOUT is detected After tne delay is completed a gate is generated that will initiate an automatic sweep Once the capacitor is charged and AUTO DELAY TIMEOUT has occurred the capacitor will remain charged until a trigger is received by the Trigger Generator This means that there will be no waiting period at the end of the next sweep and holdoff interval KEY INDICATOR the following is true for inost Tek oscilloscopes however some of the newer scopes do not comply At the end of the AUTO DELAY TIMEOUT the TRIGGERED light will be turned off This function still operates even in the SINGLE SWEEP Mode of operation If the TRIGGERED light comes on and then goes off again when you initiate the sweep sequence it signifies that the entire sequence has been completed and that the Timebase is operating properly TROUBLESHOOTING THE HORIZONTAL SECTION EVENT DEFINITION CHARACTERISTICS AUTO DELAY TRIGGER Again you can easily verify the occurance of this event in Cont two ways on scopes that use the 155 0049 XX Sweep Control IC If it has occurred properly the TRIGGERED lignt should be off The AUTO DELAY input to the IC is level sensitive and should activate AUTO when the charge at pin 6 becomes greater than 4 2 If the charge on pin 6 is not greater than 4 2 Vdc with no trigger applied in the NORMAL Trigger Mode there is a problem in the AUTO DELAY R C Timi
22. FOR SERVICE TECHNICIANS CONCEPT TRAINING Troubleshooting Your Oscilloscope Getting Down to Basics mE Eu Tektro COMMIT EXCELLENCE d Troubleshooting Your Oscilloscope Getting Down to Basics TEKTRONIX INC Test amp Measurement Training Troubleshooting Your Oscilloscope Getting Down to Basics 068 0315 00 Copyright 1989 Tektronix Inc rights reserved Printed in the U S A Tektronix products are covered by U S and foreign patents issued and pending Information in this publication supersedes that in all previously published material Specification and price change privileges reserved TEKTRONIX TEK SCOPE MOBILE and are registered trademarks of Tektronix Inc TELEQUIPMENT is aregistered trademark of T Tektronix U K Limited TABLE OF CONTENTS Chapter TROUBLESHOOTING TECHNIQUES AND PROCEDURES Cnapter 2 TROUBLESHOOTING YOUR OSCILLOSCOPE Front Panel Diagnosis Isolating the Faulty Section The Basic Oscilloscope The Power Supply The Vertical Section he Horizontal Section Portable Ocilloscope Tips and Techniques Microprocessor Based Portable Oscilloscope Tips and Techniques Lab Instrument Oscilloscope Tips and Techniques Chapter 3 TROUBLESHOOTING THE POWER SUPPLY Checking Power Supply Regulation Resistance Measurements Transistor Supplies Cominon Power Supply Problems Chapter 4 TROUBLESHOOTIN
23. G THE HIGH EFFICIENCY SUPPLY The High Efficiency Power Supply Guidelines for Troubleshooting the High Efficiency Supply Typical High Efficiency Supply Problems Chapter 5 TROUBLESHOOTING THE CRT AND Z AXIS SECTION The High Voltage Supply Typical High Voltage Problerns CRT Conditions 2l 22 23 24 25 27 30 34 35 36 36 37 Chapter 6 TROUBLESHOOTING THE VERTICAL SECTION Isolating Faulty Preamplifiers Isolating Faults in the Verticals of Laboratory Oscilloscopes Troubleshooting Tips for the Vertical Section Common Operation Tracking Down a Noisy Circuit Chapter 7 TROUBLESHOOTING THE HORIZONTAL SECTION Isolating the Horizontal Amplifier Lab Instruments Oscilloscope Tips and Techniques Typical Horizontal Problems Isolating Faults in the Timebase Timebase Sequence Tips for Troubleshooting Trigger Circuits Tunnel Diode Triggering Trigger Operation Tips for Troubleshooting the Sweep Circuits Typical Sweep Problerns ii 42 44 46 47 47 49 51 23 54 56 57 28 76 79 51 83 84 LIST OF ILLUSTRATIONS Figure of Block Diagram Basic Logical Steps for Troubleshooting an Oscilloscope Regulation Indications of a Typical Transistor Power Supply Power Supply Troubleshooting Flowchart Troubleshooting Logic Not Unblanking Common Mode Operation of a Push Pull Amplifier Mode Operation of IC Amplifier Vertical IC Amplifier Fault Isolation Flowchart Vertical Section
24. IGGER VIEW you have another troubleshooting tool When TRIGGER VIEW is activated the Trigger Signal from tne Trigger Generator is nor nally injected into the Vertical Section s circuitry just before the Delay Line This point is ideal since it oreaks the Vertical Section up into approximate halves If you press TRIGGER VIEW and the instrument has a normal display the fault is probably located in the section before the Delay Line Driver Use the rear panel VERTICAL SIGNALS OUTPUT to help further isoate the fault You can also use the Trigger controls in the Horizontal Section to help isolate the fault further Set the TRIGGER MODE to NORM TRIGGER COUPLING to DC TRIGGER SOURCE to CH 1 INTERNAL and set the VERTICAL MODE to 1 Set the CH 1 VOLTS DIV to OmV DIV and inject 50mV signal into the CH 1 input Rotate the TRIGGER LEVEL control and observe the TRIGGERED indicator If the indicator lights it indicates that the circuitry before the Trigger Pickoff circuit is OK After you have finished troubleshooting this far and you have isolated the fault down to one of the major functional blocks remove the cover and verify your conclusions From this point on your troubleshooting should be done using the successive approximation method while using as few tools and test instruments as possible 45 TROUBLESHOOTING THE VERTICAL SECTION Where could you make a test that would divide the Vertical Section up into approximate halves
25. ILLOSCOPE AdODSOTUISO NV 5418 1 53315 TWOIDOT 5 83015 l 10 5318 WWHON 5 59385 d33A8 NOILO3S NOLLO3S ON SIXV Z ONY 31VO W LNOZINOH 1OOHS3 18nOHL 1048 1OOHS318nOHL NOILO3S SIXV 2 1H9 LOOHS3 18n0H1 S3lTddns H3MOd S3riddns 1 5418 1 3ONVWHO JH3d od TVWYON 5 20 CHAPTER 3 TROUBLESHOOTING THE POWER SUPPLY 21 TROUBLESHOOTING THE POWER SUPPLY The power supply is the most fundamental block of an oscilloscope The performance of the instrument is only as good as the condition of its power supply The following information will assist in checking and obtaining the optimum performance froin Tektronix power supplies Incorrect operation of all circuits usually indicates trouble in the power supply Check first for correct voltage of the individual supplies However a defective component elsewhere in the instrument can appear as a power supply trouble and inay also affect the operation of other circuits A short circuit in any regulated supply tnay cause the output level of all supplies in the instrument to drop to zero until the short is reinoved If the output level of all the supplies is incorrect check that the Line Voltage Selector Asse nbly is set for the correct
26. If the oscillator does not free run then the problem is probably due to loading of tne transformer by one of the secondary loads By lifting the anode of the rectifiers in the secondary supplies these stages may be eliminated 38 TROUBLESHOOTING THE CRT AND Z AXIS SECTION Only the inost positive anode leads need be disconnected in the high voltage anode supply Check the high voltage caps in a similar fashion by lifting component leads If the oscillator now oscillates reconnect the supplies one at a time to find which one is causing the loading For example if this procedure led to a problem in the grid supply then the next step would be to check for resistance measurernents from the intensity control to ground A good idea is to remove the CRT socket to see whether this has any effect on the circuit symptoms Also disconnect the CRT anode lead It is possible for a short in the CRT or extremely gassy tube to load one of the otner supplies preventing proper oscillator action If you suspect a leaky or shorted component in the grid circuit or cathode circuit you may try this alternative method for fault isolation Remove power from the instrument before you proceed Connect the nigh voltage output from a curve tracer or some other variable supply to the cathode grid circuit of the oscilloscope ensure that you connect it to the H V side of the components This will siinulate the negative H V of the cathode Supply Now check aro
27. Nid O17 Nid 03193135 4 ozen AZ t 9 AV 13d OLNV osn 9 Nid 330 580 oi NO 7980 430 2980 gt aLVD LYHYVLS d33AS 8 9980 OL 8 2980 OL GL 3813 aula osso zsevo IH 01 Nid O1 21 5 Z1 ppp 13538 55 71 TROUBLESHOOTING THE HORIZONTAL SECTION 4658 A SWEEP SEQUENCE ADNANOAS 5 8999 9 109v0 8 V 330 28570 330 281 330 18890 957109 S03 IH 91 LYVLS 4300 10H szen Q d33MS SIXV Z MNV 18 S31V9 TIY 1353 530010 13NNR 1 13539 dWVH 8 Nid O1 0 Nid IH 21 33OQ 0H 19715 7 02 8680 440 16990 9 d33MS 330 9191 seo IH Nid O1 Nid 03193135 OLAY 31 lt 9 LNOAWIL AV 1ad OLNV 340 09910 dWVH 9 Nid OLNV NO 992 0 szezn 340 98210 8NId 31 9 LNOAWIL LYVLS 5 AG IH Q1 3813 3813 IH 01 Nid O1 t Nid 143953110 7 E
28. OPE When troubleshooting a new instrument take some time to familiarize yourself with the block diagram Spending a few minutes with the instrument s Instruction Manual can give valuable insight into the operation of an instrument and to the particular problem that may occur in the instrument The following description of the Basic Block Diagram is not intended to teach oscilloscope fundaraentals It is merely intended to orient you to the sections of tne oscilloscope as represented and used in this book By relating your knowledge of the oscilloscope s operation with the functions of its internal sections and blocks you can become more proficient in isolating and repairing faults within most oscilloscopes This is true because the basic operating and functional principals are the same for most oscilloscopes The ability to isolate a fault to one of these blocks comprises a major portion the first 5 steps of the troubleshooting procedure as described earlier in this book Before troubleshooting establish a starting point for which the display can be both predictable and repeatable Set the front panel controls up as follows POWER CONTROLS Line Voltage Selector 115V POWER ON CRT CONTROLS INTENSITY Midrange or as desired FOCUS Best focused display SCALE ILLUMINATION As desired B INTENSITY if applicable Midrange or as desired 11 TROUBLESHOOTING YOUR OSCILLOSCOPE VERTICAL CONTROLS Both channels if applicable
29. Or That Component Check Effects Ot Adjustments In The Questionabie Circuitry To isolate Tont Smaiier Area Check And Calibrate To Specs Was Fauit A Cat Adjusiment gt 114 Was Faut isolated To One Component Using An Gsctoscope And Amplifier Troubie shotting Techniques tsotate Faut To Trigger circuit troubleshooting chart 32 TROUBLESHOOTING THE HORIZONTAL SECTION General Tips for Troubleshooting the Sweep Circuits Tektronix sweep circuits are designed to develop a linear sawtooth voitage over a wide range of sweep times Linear sawtooth voltages ensure that the waveform passes through a given number of volts during each unit of time The sawtooth rate of rise or fall is set by the nor nally calibrated TIME DIV control This sawtooth voltage is then processed in the horizontal amplifier and applied to the plates of the CRT resulting in the horizontal deflection of the electron beam As a result the cathode ray beain is swept horizontally to the right through a given number of graticule divisions during each unit of tine the rate being controlled TIME DIV control In this manner a baseline is produced that is proportional to discrete amounts of time determined by the TIME DIV control A time difference reading is inade by measuring the distance between two different
30. bstituting tne plug in components offers a quick means of cnecking a suspected stage however there are some basic rules that you must remember and follow in using this method of troubleshooting l Ensure that you use the proper components by part number when substituting 2 Ensure that the power is removed when making the substitution to prevent damage to the instrument and or to the new component 3 Be careful when removing and replacing components to avoid physical mechanical damage to the components 4 Keep the number of variables to a minimum substitute only one component at a time If a new component does not fix the fault restore the original component in the circuit 5 Though several components in a circuit have the same part number avoid inter circuit component swaps Failure to return components to their proper place within a circuit may require a coimplete calibration of the circuit If components are replaced properly the circuit may require only a performaace check and spot calibration after the repair is completed 6 Always return the components to their original places after the problem has been solved As stated before this will prevent the necessity of recalibrating the entire circuit after the repair TROUBLESHOOTING YOUR OSCILLOSCOPE After affecting repairs to a circuit always check the replacement components to insure that the proper replacements have been used this includes checki
31. ck of circuit operation and may provide a clue to the problem If no outputs are observed check to be certain that trigger inputs are gating the sweep gate circuits Holdoff and feedback operation may be checked by monitoring the emitter of the holdoft Circuit Check to see if the holdoff follower follows the action of the sweep length control These two blocks comprise most of the feedback path and if their follower action is inoperative the problem is quickly localized Typical Sweep Problems Sweep shortens at faster sweep speeds Check The sawtooth output emitter follower inay be loading the circuit Remove the sawtooth emitter follower and note whether the problem disappears lf the trouble is not in this stage then check the output stage of the horizontal amplifier Sweep nonlinear at the left side of the CRT Check Faulty holdoff circuit operation may be causing the problem Check holdoff emitter follower for improper circuit operation Sweep shortens on right side of the CRT when sweep is triggered Check An open diode in the positive trigger clipper circuit may inhibit positive clipping of the sweep gate input and cause premature rundown of the sweep Sweep tends to free run at different sweep speeds when triggered at other speeds Check Preset stability is misadjusted or lockout multivibrator circuit operation is weak 34 TROUBLESHOOTING THE HORIZONTAL SECTION Sweep will not run by itself put will
32. d 330 E010 PN oon AAO IOH 8690 NUHL 2010 2010 00 1Nauuno vus AE NO 8690 SOT qi 440 8890 eoon gt d3i3MS LNOSWIL 4A0Q10H 330 0649 50 504 SdWYH 2860 330 2150 9 0090 HOLIMS aLvS LYVLS 5 OLNV 440 2190 d 72 0091 NO 2 7 74 TROUBLESHOOTING THE HORIZONTAL SECTION 475A A UNLOCKED KNOBS OR A INTENSIFIED 8690 MC ONES o ca mm NO 8690 4 0090 _ 330 060 lt gt 439 890 n 8 0090 lt gt di3AS NV aNn ANOSWIL lt GAISISNSALNI HO SHONA VSZt S O 3 13 16 9650 9 9660 gt LYVLS Sd33MS 3 SIXV Z NV 18 S31V9 11 13539 5 TANNNE 13538 440 8290 vuile NO 2190 dWVu 504 3 90 IH 11 Nid NO 9960 us IH LU Nid 440 p 330 8680 433 5 7777 440010H 340 06 NO 150 804 SdWvu NO 2860 440 210 9 009 HOLIMS aLvS 31V95 LYVLS 5 OLNV 330 2190 K 7 K Wuv 143253100 sosyo gt essuo vy DILL NOAGV3H 1
33. d by heater to cathode leakage in the oscillator bad neons in the CRT grid circuit or leaky coupling capacitors in the unblanking circuitry These symptoms are often seen when high voltage tubular capacitors have been replaced with disc capacitors Tne problem is present at any speed and the frequency is usually about 10 kHz or less relative to the oscillator frequency Intensity modulation and contro problems can also be caused oy faults in the DC Restorer Grid circuits 40 Check CRT Bias Circuit Yes Normal No 15 Unblank ing Pluse Present No Check Z Axis Logic Circuit TROUBLESHOOTING THE CRT AND Z AXIS SECTION CRT Not Unblanking Sweep And Vertical Operating Intensity Set At 2 O clock Press Beam Find Release Does Replace Beam CRT HV Find Unblank Running Oscillator Operation Restored No Running Yes No Check Z Axis Amplifier Circuit Check Cath No And Anode Supplies Troubleshooting Logic 4l Component No Is Check Yes Chop Bik g NYSS Inhibiting Circuit Check Z Axis Done Cathode Regulator Circuit Does Circuitry Appear Normal Replace No Defective Yes Is HV OSC Running Yes Check CRT CHAPTER 6 TROUBLESHOOTING THE VERTICAL SECTION 42 TROUBLESHOOTING THE VERTICAL SECTION The vertical amp
34. d that area 50 CHAPTER 7 TROUBLESHOOTING THE HORIZONTAL SECTION 21 TROUBLESHOOTING THE HORIZONTAL SECTION we will assume at this point that you have followed the procedures in the section of this book on ISOLATING THE FAULTY SECTION We will also assume that you have isolated the fault to the Horizontal Section using the procedure under the sub title of HORIZONTAL If you have not read that section of the book and have not perforined the procedures for isolating to the Horizontal Section back and do so at this time Refer to the Table of Contents to locate this secton The Horizontal Section of the oscilloscope is responsible for the generation of the gate logic for unblanking the CRT and for the generation and processing of the sweep signal used to drive the horizontal section of the CRT The Horizontal Section consists of the Trigger Generator the Sweep Generator and the Horizontal Amplifier Once you have isolated a fault to the Horizontal Section you can begin to isolate it to the smaller sections and blocks The Front Panel is still the best tool you can use for troubleshooting milk the Front Panel controls for all symptoms and indications Don t forget that yo also have the signal outputs on the rear panel to help you troubleshoot the instrument For effective troubleshooting examine the simple possibilities before proceeding with extensive troubleshooting The first thing to do when isolating a
35. e inore inaccessible locations Because there are so few metal parts on the battery powered scopes there is less chance of the user coming in contact with dangerous potentials between the instruments Apply test equipment ground to the negative side of the Line Bridge Rectifier when troubleshooting the primary side of the inverter circuit Notice that this point is also the negative side of the storage capacitor potentials This point provides the circuit common for the rest of the circuitry Do not take any resistance measurernents or make any repairs in the supply while the neon in the Line Rectifier circuit is blinking This neon is part of a Relaxation Oscillator that operates when the voltage on the storage capacitors is above 80 Vdc When the neon is flashing it is a warning that dangerous potentials exist in the circuit Allow the charge on the storage capacitors to bleed down before proceeding 31 5 6 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY After the neon has stopped flashing make sure that the charge is completely bled off the capacitors before attempting repairs or resistance measurements Do not remove any of the protection circuitry such as the Inverter Controller and the Inverter Regulator Transistors and apply full power to the instrument The high efficiency supplies are capable of supplying enough current to burn the circuit boards interface cables and circuit components If you remove the
36. e problem is probably in the Horizontal Section Don t forget about the output signal connectors such as SWEEP GATE and SWEEP OUT on the rear panel that can help you to verify your findings 16 TROUBLESHOOTING YOUR OSCILLOSCOPE Microprocessor Based Portable Oscilloscopes Tips and Techniques NOTE This method can also be used on the other Portable Oscilloscopes however the results not de as conclusive as those obtained by using the BEAM FINDER Notice that the EXT Z AXIS input connector on the rear panel is now a TTL level input This input can be used to blank the display by the application of 5 volts however it can no longer intensify or unblank the display Therefore you must use alternative methods of troubleshooting in these portable scopes Since the CPU controls the Display Section and the Horizontal Section of the microprocessor based portables the BEAM FINDER does not automatically unblank tne when pressed It merely limits the horizontal and vertical deflection to help locate the sweep display so that it can be positioned on screen Therefore it is very inportant that you have the TRIGGER MODE switch in the AUTO LVL or AUTO position to insure that you have a bright baseline automatic sweep Also ensure that you have the sweep speed set to about mS DIV If the sweep speed is set too slow it appears as a dot when the BEAM FINDER is pressed and if it is offset to the top or bottom of tne CRT it may
37. eck semi regulated voltage on capacitor rectifier board Check for faulty components on inverter board 34 CHAPTER 5 TROUBLESHOOTING THE CRT AND Z AXIS SECTION 35 TROUBLESHOOTING THE CRT AND Z AXIS SECTION The High Voltage Supply Tne high voltage supply is fundamental to oscilloscope CRT performance Cathode ray tubes require operating voltages much higher than those provided by conventional power supplies To eliminate large vacuum tubes bulky and dangerous capacitors and heavily insulated transformer windings most Tektronix hign voltage power supplies use voltage inultipliers to generate high voltages with a considerable savings in cost and space By using a frequency of approximately 40 kHz to 60 kHz instead of 60 Hz the required filter capacitor values are reduced by a factor of 1000 Thus small and relatively inexpensive disc capacitors 0 02 0 03 rnicrofarad can be used instead of expensive 20 microfarad capacitors A class C oscillator usually develops the 40 60 kHz voltage that supplies the primary winding of the high voltage transformer Satisfactory regulation is achieved in most high voltage supplies by controlling the amplitude of the high frequency oscillator output It is important to remember that ORT circuits are very low current circuits as a result they are susceptible to leakage paths Typical High Voltage Problerns High voltage power supply problems are usually indica
38. ed secondary supply After a fraction of a second the start circuit tries to start the inverter again and the cycle repeats itself The chirp clicking noise you hear is the transformer core responding to those short bursts of 12 kHz energy The rate of the burst and the frequency of the audible chirp will vary according to the type of fault and the series of the instrument When the supply is operating norinally it runs at 25 kHz well above the audible range When troubleshooting high efficiency supplies there are sorne basic rules to observe for your safety and for that of the equipment Keep these in mind when working in and around the potentials present in the high efficiency supplies because a careless mistake in this circuit could be fatal At the front of the inverter supply tnere is 325 Vdc stored across the equivalent of about 1000 1500 uF of capacitance Now consider how inuch energy must be dissipated when this voltage is discharged it becomes something akin to a high voltage high current welding machine If discharged through a short the energy from these capacitors would produce considerable heat and light arc 29 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY Guidelines for Troubleshooting the High Efficiency Supply When working in the high efficiency power supply use the following guidelines to protect yourself and the equipment 1 Do not attempt to troubleshoot or take any voltage or wavefor n measureme
39. een reset Even though you have cornpleted the loop around the sweep sequence you are not finished Notice that there are two steps AUTO DELAY and AUTO DELAY TIMEOUT that go to the inside of the loop Even though these two steps are only used functionally when the instrument is in the AUTO TRIGGER MODE they can still provide you with indicators and test points that can indicate the health of the circuitry 67 TROUBLESHOOTING THE HORIZONTAL SECTION Even though the scope is operating in the SINGLE SWEEP MODE the two AUTO operations can provide you with a powerful troubleshooting tool Refer to the A SWEEP SEQUENCE FLOWCHART again and notice the KEY INDICATOR for the FIRE event When the Trigger occurred the front panel TRIGGERED indicator was lit Notice also that the indicator has not been turned off yet The TRIGGERED light is usually turned off as a result of the operaton of the AUTO events After HOLDOFF has ended the instrument enters the QUIESCENT state in which it is now waiting for a trigger Norinally if no trigger is received and processed the oscilloscope will remain in this state and no sweep will be generated However if a trigger is not received within about 100 mS when the scope is operated in the AUTO Trigger Mode the Timebase will initiate an AUTO Trigger This will cause an Auto Sweep to occur This gives the operator a bright baseline to check the operation of the scope and to see if any trigger signals are
40. em is typically caused by an open collector load to one of the stages An open decoupling resistor will also cause this Compression or expansion of the sweep as it is positioned from one side to tne other Solution This problem is typically caused by the diode network between the bases of the amplifier Check for leaky diodes Horizontal sweep control center position is shifted and control is nonlinear Solution Check for an open circuit in the center tapped load resistors of the output amplifier Nonlinear sweep Solution A faulty input emitter follower may also cause the problem Check for leaky capacitors and transistors in the input circuitry Position range off centered Solution Check the input compensated divider of the input pre ainplifier 56 TROUBLESHOOTING THE HORIZONTAL SECTION ISOLATING FAULTS IN THE TIMEBASE Troubleshooting faults within an amplifier section vertical or horizontal of an oscilloscope is pretty straight forward since a signal is usually processed straight through the circuitry This means that there are usually no loops or feedback paths within the amplifier sections to compound the troubleshooting process Therefore the successive approximation method of fault isolation works very efficiently This is not true for tne Timebase section because the circuitry within the Timebase does form a loop Trying to use the successive approximation raethod to troubleshoot a Timebase i
41. erly with the slope switch 2 Internal trigger DC level adjust allows the center of the LEVEL control to set exactly to zero volts in the DC mode 3 Trigger sensitivity controls the minimurn signal response minimurin sensitivity limited by noise When troubleshooting trigger problems a few simple steps can often deter nine which stage of the trigger is at fault Checking the operation of a trigger circuit using different sources modes slopes and coupling positions will often isolate a problera Observing the effect the level control gives additional information In checking trigger circuits always be sure that sufficient signal is being applied to obtain a large observable deflection about 1 cm For instance set the VOLTS DIV switch to 100 mV DIV to a selected vertical the apply a 500 mV signal to the corresponding vertical input 78 TROUBLESHOOTING THE HORIZONTAL SECTION ee 15 AL SEU TION Varying the trigger SOURCE switch between INTERNAL and EXTERNAL triggering checks the trigger pickoff circuitry Comparing operation in different trigger modes can usually localize a problem to a specific trigger stage e g noting a difference in operation of the trigger circuit in AUTO or NORM may suggest the faulty stage Once the problem has been traced to a specific block a close visual check may pinpoint the problem Substituting transistors offers a quick means of checking a suspected problem Always return the
42. et the front panel controls according to the Preliminary Front Panel Set up in the section of this book on ISOLATING THE FAULTY SECTION Use a shorting strap to common inode the paraphase amplifier at the input of the Horizontal Amplifier NOTE in some instruments the Horizontal Amplifier is located on a board that makes access difficult Be very careful when common moding the amplifier stages and making other tests on the board to prevent personai injury and further damage to the instrument while troubleshooting i the display is a horizontally positioned dot the fault is in the Timebase sub section and you should refer to the following section on ISOLATING FAULTS IN THE If the beam is still deflected horizontally to the edge of the display the fault is in the Horizontal Amplifier and you can isolate the fault by using the saine procedures and techniques described in TROUBLESHOOTING THE VERTICAL SECTION Lab Instruments Oscilloscope Tips and Procedures This step is easy in the Laboratory Oscilloscopes If you have a failure in the Horizontal Section of the instrument swap the Horizontal Timebase with a Timebase that is known to be good If the horizontal display is normal as a result of swapping the plug ins the problem is probably in the Horizontal Timebase that was removed If you suspect the fault is in the timebase refer to the section of this book on ISOLATING FAULTS IN THE If you stiil do
43. f the next page illustrates the various line conditions norinally encountered 22 TROUBLESHOOTING THE POWER SUPPLY Power Supply Noise ree WV V ZWN Regulation Indications of a Typical Transistor Power Supply Resistance Measurements Resistance measurements can be made on most supplies since the resistance values are relatively low Solid state power supplies because of their lower impedance qualities have supply resistances that are typically lower than 50 ohms NOTE When making resistance measurements reversing the meter leads may produce a different reading due to diodes or other active devices in the load circuits Silicon diodes can usually checked in the circuit and typically read about 2k ohms in one direction When a power supply diode fails it will usually be either a dead short or open If an in circuit check of a diode leaves doubt as to its condition lift one end of the diode to be sure of the reading Most silicon supply diodes reach about 2k ohms or about 2M ohms depending on direction of current flow NOTE With a DOM range setting of 2 ohms a silicon diode will typically read about 600 ohms in one direction and infinite in the other 23 TROUBLESHOOTING THE POWER SUPPLY Transistor Supplies Solid state supplies are used in modern electronic equipment Tne following points summarize the major characteristics of solid state supplies Lower o
44. fault in the Horizontal Section is to break the section down into two sub sections Timebase Consists of the Trigger Pickoff Circuit the Trigger Generator and the Sweep Generator This circuit is responsiole for processing an input trigger signal internal or external generating the gate logic for unblanking and sweep and generating a precise linear ramp for driving the Horizontal Amplifier Horizontal Amplifier Processes and amplifies the sweep voltage from the Timebase and provides the drive signal required for the horizontal deflection of the beam across the CRT 52 TROUBLESHOOTING THE HORIZONTAL SECTION ISOLATING THE HORIZONTAL AMPLIFIER When you were performing the procedures to Isolate the faulty section you had two major conditions that indicated a fault in the Horizontal Section One condition was the presence of a dot on the display instead of a sweep The other was a dot on the display that you could only see by pressing the BEAM FINDER Both of these conditions indicated faults in the Horizontal Section The following procedures and tips will nelp you isolate a fault within the secion In keeping with the successive approximation method of troubleshooting you must divide and test the Horizontal Section at about the midpoint of tne section This is quite simple in the oscilloscope because the two major sub sections of the Horizontal Section perform such different functions one is a generator and the other i
45. g this interval H O pin 17 should be high H O Notted pin 10 should be low and H O TIMING pin 8 should oe ramping up frorn 0 Vdc 66 EVENT HOLDOFF TIMEOUT TROUBLESHOOTING THE HORIZONTAL SECTION DEFINITION CHARACTERISTICS This event marks the end of HOLDOFF HOLDOFF timing is a function of an R C timing network that is controlled by the TIME DIV control The timeout is proportional to the sweep speed to insure that HOLDOFF is long enough to allow all of the sweep timing capacitors to fully discnarge before another sweep is initiated When the charge on the timing capacitor in the H O Timing reaches a predeterinined level HOLDOFF is reset ended and the instruinent enters the QUIESCENT state again To check this event on instruments that use the Tek nade Sweep Control IC 155 0049 XX measure the voltage at the H O TIMING input pin 8 of the IC If the voltage is greater than 4 2 Vdc HOLDOFF TIMEOUT is complete You can also check to see if the instrument entered the QUIESCENT state of the sequence as it should have Check H O pin 17 of the IC for a low H O Notted pin 10 for a high and H O TIMING pin 3 for a low Pin 8 of the IC will be low again because the R C timing capacitor is internally discharged by the IC Check other things from the TIMEBASE such as rear panel GATE OUTPUTS that will verify that HOLDOFF TIMEOUT has occurred Also check to see that the Tunnel Diodes if applicable have b
46. he Sweep and Z Axis Logic sections This state will also disable the AUTO capabilities of the sweep Logic Circuitry 61 TROUBLESHOOTING THE HORIZONTAL SECTION EVENT FIRED Cont SWEEP START GATE DEFINITION CHARACTERISTICS On those scopes with Tunnel Diodes TD you can check this state by connecting a test probe to the anode of the FIRE Tunnel Diode If the TD is fired the output voltage will be 400 500 mV If the voltage is not up to this level tne TD has not fired Check the Tunnel Diode and the Tunnel Diode Driver circuitry On the instruments wth integrated Trigger Generators measure the output of the Trigger Generator IC for the proper trigger level This trigger waveforin specifications usually provided in a Voltage and Waveform Chart located in the Maintenance Section of the Service Instruction Manual This is the output of the Sweep Start Comparator The Trigger from the Trigger Generator is processed through the Sweep Start Comparator to ensure a proper trigger output level This gate drives the Sweep Generator and the Unblanking Gate Generator KEY INDICATOR If the TRIGGERED light is illuminated but you still do not have a display press the BEAMFINDER If the display is a dot that is stuck to the left side of the display and it can only be seen by pressing the previous stages The TRIGGERED light indicates that the FIRE has been completed so the fault is probably in
47. ied to input pin 16 of the IC to initiate HOLDOFF This should be a very narrow pulse with a low of about 0 Vdc and a high of about 4 3 65 TROUBLESHOOTING THE HORIZONTAL SECTION EVENT H O START Cont HOLDOFF H O DEFINITION CHARACTERISTICS Check the HOLDOFF START Pulse with a test oscilloscope It should step high just as the sweep reaches the right side of the CRT If no pulse is present check the output of the End of Sweep Comparator the HOLDOFF START goes high active and remains high the circuit is locked up check the HOLDOFF and TRIGGER circuits This is the time interval and event in which the sweep and all gates are reset HOLDOFF is a timed event It is controlled by an R C Timing circuit and the time delay is usually dependent on the sweep TIME DIV control KEY INDICATOR Wnen HOLDOFF has been initiated the READY indicator on the front panel will go out Tnis is a very active interval in the sweep sequence During this time the Trigger Generator will be reset and disabled so that it cannot accept another trigger signal The circuitry in the Trigger Generator is being held off hence the name HOLDOFF During this interval all gates are reset the CRT is blanked and the sweep is retraced In instruments that utilize the Tek rnade Sweep Control IC 155 0049 XX you can test the operation of the circuitry during this interval by looking at the pin activity on the IC Durin
48. igger source adjust the controls and vertically center at least 1 cin of calibrator signal on the CRT display Set the LEVEL control to zero and piace the coupling control in the AC LF REJECT position This is typically the most difficult position in which to make trigger adjustments If the circuit functions properly in this position you can be assured that the circuitry is good Set the sweep speed for the appropriate speed to observe 5 10 cycles of the square wave signal Preset the trigger sensitivity if there is one to midrange The sensitivity adjustment determines the minimum circuit response in mV The trigger level centering assures proper slope and level operation in all coupling modes 81 TROUBLESHOOTING THE HORIZONTAL SECTION it is Trace Visible in Auta Made With No Frig Anpited is m Trace Visible When Trigger Signat is AppHed Check Sweep Gate Sweep Generator And Auto Move Sweep Runs Gut Not Triggered Unstable Fauity Auto Sweep Reset Circuitry Ang Check That Sweep 5 Rans Awo Get Sweep CXF isolate Smakest Made Running in Possitjle Block Using Front Pansi Controls Auto Mode Can Fault Be isolated To One Component Such As An IC XSTR Switch Ete from from Patel Yes Sweep Run in Auto Mode Replace
49. ing the input to the delay line to break the block up into its individual circuits Study circuit schematics and calibration procedures closely for any hints which verify the suspected faulty circuits Performance checks in the service manuals are often helpful at this point ISOLATE THE FAULTY COMPONENT After you have isolated the suspected circuit make a thorough visual inspection of the areas If you should spot a burned or opened component don t stop there Burned and open components are often only symptoms of another more real problem of the circuit Isolate the faulty component by making waveform voltage and resistance measurements in the circuit Current measurements are also helpful in locating faulty components especially in those cases where there are other burned components in the circuit Troubleshoot the circuit by component substitution in those circuits where it can be done non destructively Use special techniques such as piggy backing and swamping for troubleshooting those circuits in which it is impractical to swap components TROUBLESHOOTING TECHNIQUES AND PROCEDURES REPAIR THE INSTRUMENT AND VERIFY PROPER OPERATION Use accepted repair procedures to correct the fault After the fault has been corrected perform the instrument functions and operations which indicated the original fault to verify that the problem is corrected If the instrument seems to be functioning properly Performance Check
50. it as directed in the Service Instruction Manual Correct any additional faults or misadjustments that you find during the check CHAPTER 2 TROUBLESHOOTING YOUR OSCILLOSCOPE TROUBLESHOOTING YOUR OSCILLOSCOPE Front Panel Diagnosis The oscilloscope is an excellent tool for providing clues to faults within itself In addition to the CRT display the calibrator signals and the front panel indicators trace position indicators beam trace finder and pilot lights often provide sufficient information to isolate the problem For effective troubleshooting examine the simple possibilities before proceeding with extensive troubleshooting The following list provides a logical sequence to follow while troubleshooting Check control settings 2 Check associated equipment 3 Make a thorough visual check of the instrument Check instrument calibration 5 Isolate trouble to a block level 6 Check voltages and waveforms 7 Check individual components Observing the effects of different multi function switch positions can do inuch to identify a problem For example using the second channel of a dual trace unit can check vertical circuitry up to the point where switching occurs In the case of a delaying sweep or dual beam oscilloscope a portion of the circuitry may be used to display information on the oscilloscope itself Detecting a problem in all circuits may indicate a problem in the power supply Switching to the external hori
51. ld be thoroughly cleaned A problem similar to static charge is sometimes caused by the CRT gun support rods becoming charged This rod charge may sometimes be eliminated by deflecting the electron beam completely off screen horizontally turning the intensity fully CW and varying the position control rapidly from the upper extreme to the lower extreme After a few moments the rod charge should be dissipated Burrs or dark spots on the CRT screen can sometimes be minimized or eliminated by flooding the screen with a display and then turning the intensity up for several minutes This can be done by injecting high frequency signal i e 100 MHz from an SG 503 into the vertical input Set the output of the signal generator for a p p signal centered in the display then reduce the vertical deflection of the oscilloscope to lv div Set the oscilloscope TIME DIV to lins DIV Turn the intensity fully clockwise CW and leave it for about 3 to 5 minutes CAUTION THIS COULD CAUSE DEGRADATION IN THE CRT IF THE INTENSITY IS KEPT FULLY CW FOR A LONGER PERIOD THAN 5 MINUTES CAREFULLY WATCH THE OSCILLOSCOPE WHEN PERFORMING THIS PROCEDURE 37 TROUBLESHOOTING THE CRT AND Z AXIS SECTION Most modern general purpose oscilloscopes have a transistorized solid state high voltage supply Sorne of the more common troubleshooting symptoms are listed below 1 2 3 4 Inability to turn off the intensity is often ca
52. lifier develops a push pull version of the input signal from tne vertical preamplifier These simultaneous positive and negative going amplified signal voltages are then applied to the upper and lower vertical deflection plates deflecting tne CRT spot as it traverses the screen Thus an accurately amplified reproduction of the original signal is displayed on the CRT In addition many oscilloscopes provide a vertical signal output which allows the amplified signal to drive other devices Once you have isolated a fault to the Vertical Section of the oscilloscope you must use a more specialized technique to isolate the fault down to the olock circuit and coraponent levels You will now use more of the front panel vertical controls and less of the other controls and indicators through you will find that you can use the horizontal controls to help you troubleshoot tne Vertical Section As you troubleshoot the Vertical Section remember to milk the front panel for all the hints and clues availaole to help isolate the fault In most cases you can isolate tne faulty block level using only the front panel In many cases you can even isolate the problem right down to the circuit In any case don t overlook the obvious Norinally you would want to start troubleshooting in the middle of a faulty section to more quickly isolate the problem However you should examine the simple possibilities before proceeding with extensive troubleshooting What is
53. line that follows will identify the events in tne sequence by name definition and characteristics Refer to the A Sweep Sequence diagram on the following page to see the position of each event in the sequence Notice that the events are listed in the order of their occurance within the sequence NOTE You can observe some of these events better if you operate the oscilloscope in the SINGLE SWEEP MODE of operation Before continuing set the TIME DIV to S DIV SWEEP SINGLE SWEEP and INTERNAL TRIGGER 58 TROUBLESHOOTING THE HORIZONTAL SECTION 3ON3noO3S d33AS 9897 2 188590 Wou4 31V9 V 440 28970 330 18890 330 28140 d33AS ONIINV 18 40 15 4 85104 503 IH 91 LYVLS 44 01 31V5 V 28140 NYHL 2 86S O 1 wuld ere geo en 330 26770 2814 440 18170 eae 433 5 YNV TENN IH t Nid O1 03103735 ony 31 AC v 9 AV 13a OLNV 9 191410 01 992 0 440 98210 alvS LYVLS d33MS 8 952 0 lt gt 01 opel 11111107 ecco Q gt 9 x rsen 2 c 1 15 Pi 9 01 11 Nid 143953110 a OLNV IH OF Nid 111143 AQV3H M
54. line voltage and regulating range Most Tektronix manuals list the tolerances of the power supplies If a power supply voltage is within the listed tolerance the supply can be assumed to be working correctly f outside the tolerance the supply may be misadjusted or operating incorrectly When testing for shorts and overloads rernove the loads froin the output filter Check the resistance of each to isolate the load that s causing the short or overload Next look in the defective circuit for connections from the power supply directly to ground Diodes and capacitors are good place to start Remember ensure that you have rermoved instrument power before you make resistance checks Checking Power Supply Regulation Connect the oscilloscope under test to a variable auto transforrner Turn off the sweep and calibrator and monitor the individual supplies with a 1X probe AC coupled to the test oscilloscope Begin with the reference supply since other supplies are related to this reference Adjust the variable auto transformer to the point where the supply goes completely out of regulation noted by a large increase in ripple Next increase the line voltage to the point where the supply pulls into complete regulation and note this voltage this point is the low line regulation voltage Next increase the line voltage to the point where the supply starts to go out of regulation This point is the high line regulation voltage The figure at the top o
55. ming tnis procedure The second is that on soine instruments the CRT Readout may compressed to dots at the top and bottom of the CRT when you short the input to the board The second method of verifying this fault isolation is to use an oscilloscope Connect the probe of a test oscilloscope to one of the legs of the push pull input to the Horizontal Amplifier Board The signal should be a positive or negative going ramp of about 265 mV in amplitude This equates to the standard of 25 mV Div of deflection used in most Tek Lab Instruments After you have measured one side of the signal input place the probe on the other input and you should measure a 265 mV of opposite polarity to the first leg measured If either of the inputs are missing or appreciably lower than 265 mV in amplitude the probelm is in the input to the board If both of the input signals are good the problem is in the Horizontal Amplifier Board After you have isolated the fault of this far you are essentially down to the circuit level of troubleshooting Use the same techniques to troubleshoot this circuitry that was described earlier for TROUBLESHOOTING THE VERTICAL AMPLIFIER 55 TROUBLESHOOTING THE HORIZONTAL SECTION Problem Problern Problem Problern Problem Typical Horizontal Problems Sweep length decreases at fast sweep speeds Nonlinearity and sometimes sweep compression to the right Solution This probl
56. n When you press the BEAM FINDER if you have only a DOT for a display the fault is probably loated in the Horizontal Section Refer to the section of this book on TROUBLESHOOTING THE HORIZONTAL SECTION If you still have no display when you press BEAM FINDER the fault is probably located in either the Power Supply or CRT Z Axis Section of the instrument You can easily verify this from the rear panel of the instrument by using the following steps NOTE When using this technique do not leave the injected voltage on the rear panel EXT Z AXIS connector for more than a few seconds Remove the voltage immediately because the intense beam can dainage the CRT Before continuing turn the FOCUS control fully counter clockwise Inject 10 to 15 volts DC into the EXT Z INPUT connector on the rear panel If you still have no display refer to the sections of this book on Troubleshooting the Power Supply and on Troubleshooting the CRT and Z Axis Section If you get a very intensified dot on the CRT the is probably in tne Horizontal Section Don t forget about the rear panel output signal connectors such as SWEEP GATE and SWEEP OUT that can help you to verify your findings Once you have isolated the fault to the Horizontal Section you can use a plug in swapping technique to further isolate the problem Some hints on these techniques are covered in the section on Troubleshooting the Horizontal 19 TROUBLESHOOTING YOUR OSC
57. n method for tracking down noisy circuits and components When tracking down the source of noise you may use either the snorting Strap as described in the previous tips or you may elect to observe the output of the selected amplifier stages with an oscilloscope When using tne shorting strap short tne input to a stage and observe the display if the noise disappears the noise is being generated in one of the circuits before the shorted input the noise continues the fault is in the circuitry somewhere after the short Whether you use the shorting strap or an oscilloscope to isolate the noise simply divide the faulty section into smaller and sinaller pieces until you have isolated the fault down to one or two stages of circuitry After you have the source of the noise isolated to a couple of suspect circuits you will need an oscilloscope for further troubleshooting Set the VOLTS DIV on the test osciloscope to 5mV DIV or less and connect the test probe to the output of the suspect amplifier stage Lightly tap the suspected components in the circuit with a spudger a non metallic wand and observe the displays on the oscilloscopes you notice an appreciable increase in noise when a particular component is tapped check the component carefully or substitute and check those components that are connected to the suspect component A neat gun and a coolant spray may also help to isolate the fault to an individual component if the noi
58. ng Network If the charge at pin 6 is at the correct level the IC should generate an auto gate to initiate a sweep The AUTO GATE pin 4 should be high or the INVERTED AUTO GATE pin 3 should be low to initiate a sweep If the AUTO DELAY level is correct but no gate is generated check the IC The A SWEEP SEQUENCE FLOWCHART that we have been using for a reference has been intentionally generic to fit the operation of most Tek scopes The SWEEP SEQUENCE FLOWCHARTS on the following pages are for specific instruments Study them carefully and compare them to the circuit operation of the scopes for which they are indicated After you understand the sequence you may want to explore the operation of other oscilloscopes and generate more of the sequence charts for the other scopes 70 TROUBLESHOOTING THE HORIZONTAL SECTION 465 SWEEP SEQUENCE ADNANOAS d33MS st o1en gt 01 3571089 903 91 Nid LYVLS 4400 10H BLVD V SIC 2 71010 221000 gt 340 91010 10 00 430 90610 gt 80 d33MS 30vu13U SIXV Z TANNAL 13534 3119 V 440 Pei i Na p ot N 1N3HHfO Z 9 960 0 440 90610 330 2010 49 90610 gt 44300700H d33MS xNV318Nn 3 7010 oi IH p
59. ng the suffix last 2 numbers of the part number If the suffix numbers are not correct the circuit may still operate but it may not calibrate properly Even if it can be calibrated another parameter of the device may have been selected for operation within the circuit which nay cause failure or improper operation in later use of the instrument TROUBLESHOOTING YOUR OSCILLOSCOPE ISOLATING THE FAULTY SECTION The Basic Oscilloscope The simplified block diagram below shows the major components of an oscilloscope be an effective troubleshooter you must first understand the functional operation of the sections and blocks of the oscilloscope You must also be able to relate the front panel controls to the display and to the systems and circuitry within the instrument Throughout the instrurnent there are internal signal paths and internally generated signals that can be accessed from the connectors on the front and rear panels Your knowledge and use of these controls and ports can be powerful troubleshooting tools at your disposal Attenuator ertica Vertical Vertical Signal Q Preamplifier Output input And Delay Line Amplifier Driver Vertical Deflection Plates L V To Trigger CRT Circuits Power All Pick off Z Axis amp H V Supplies Blocks Amplifier Horizontal Defiection Plates Trigger Sweep Horizontal Circuits Generator Amplifier Figure of Block Diagram 10 TROUBLESHOOTING YOUR OSCILLOSC
60. not have a display after swapping the plug in the problem is probably in the mainframe and can now be further isolated to the Horizontal interface Board if applicable the Horizontal Amplifier the CRT or in the connections between them If you have no sweep but the CRT Readout is normal ensure that the READOUT MODE Switch is in the FREERUN position the fault is probably in the Interface Board If you have neither display nor Readout the problem is probably on the Horizontal Amplifier Board or in the CRT In either case the location of the fault can be verified by one or both of the two following methods 55 TROUBLESHOOTING THE HORIZONTAL SECTION The first method is to use a shorting straps to common inode the input of the Horizontal Board using the technique described in the Vertical Section of this book The signal is push pull coupled from tne Horizontal Interface Board or plug in to the Horizontal Amplifier Use a shorting strap to short the push pull signal together at the input of the amplifier The resultant display should be a dot near the center of the display If tne dot does return to the center of the display the fault is in the Interface or input connection to the Horizontal Amplifier If it does not return the problem is in tne Horizontal Amplifier NOTE There are two things to reinember if you use this technique First ensure that all vertical and horizontal position controls are centered before perfor
61. nts and around the supply without using an isolation transforiner and variac in series with the instrument under test It is necessary to isolate float the instrument under test because tne high efficiency power supply is not referenced to earth chassis ground The circuit common for this section of the instrurnent is taken from the negative side of the input storage capacitors 30 2 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY CAUTION Dangerous potentials may exist between equipment frames and connectors when an isolation transformer is used Remeinber that the device connected to the isolation transformer is floating and does not have a solid ground reference Your other test equipment will be operating at earth ground potentials on their chassis If you should touch both pieces of equipment at the same time you may be exposed to a dangerous potential difference Some of the inore experienced technicians prefer to use a battery powered oscilloscope intead of the isolation transformer when troubleshooting the high efficiency supply This technique has several advantages First there is less danger of shock hazard since the battery powered scope does not provide a current return path to any source to whicn the intrument under test is connected The battery powered scope also provides higher portability for allowing the user to easily view the scope display while making those tricky connections in of th
62. o find those hidden clues and symptoms that may be the key to understanding the fault Use the connectors on the front and rear panels to gain more information using signal injection and waveform testing It sometimes helps to record the results of your testing to allow repeatability of the tests and to avoid confusion as you troudleshoot further SYMPTOM VERIFICATION After you have worked out the details of the syrnptoims and indications of the fault verify them Repeat the operations and functions of the instrument that gave you faulty symptormns during the first two steps This assures the repeatability of fault symptoins and verifies the proper operation that you noted in the previous steps Performance of this step will decrease the possibility of sorne of those insidious operator errors that can beso costly to the technician while troubleshooting ISOLATE THE FAULTY SECTION ASSEMBLY If you have performed the first three steps correctly you now have the information you need for this step In essence tnese steps are performed concurrently eliminating the sections of the instrument that are operating properly you can derive the possible faulty sections Use tne front panel controls to further eliminate the good sections and to collect information that can help to isolate the fault Use the front and rear panel connectors for signal injection and waveform testing Remove the instrument cover and make a thorough visual inspection
63. on the Horiontal Section or the CRT Section If the trace is deflected to tne top or bottom of the display when you actuate the Beam Finder the fault is probably in the Vertical Section Most problems in the Vertical Section result from improper signal levels at the Vertical Input Connectors Therefore most catastrophic failures in tne Vertical Section occur in the circuitry in the front end of the Verticals By using the proper steps you can further isolate many of these problems to a sub assembly or circuit without removing the cover from the instrurnent Try some of these before removing the cover from the faulty instrument Tnese steps will vary sligntly according to the capabilities of the instrument Vertical systems containing plug ins are convenient since plug in swapping and substitution may speed the logical troubleshooting process Now that you have isolated the fault to the Vertical Section refer to the chapter on TROUBLESHOOTING THE VERTICAL SECTION 14 TROUBLESHOOTING YOUR OSCILLOSCOPE The Horizontal Section Up to this point you have essentially eliminated two sections the Power Supply and the Vertical Section as the source of the fault within the instrument Now you can further isolate the problem to one of the other sections of the oscilloscope or confirm that it is one of the previous sections checked such as the Power Supply Before continuing ensure that the front panel is set up properly for a dis
64. original components to their place after the problem has been cured In newer model oscilloscope most components are soldered in place therefore this practice becomes impractical Tunnel Diode Triggering Voltages generally across a tunnel diode in various states of operation OV 400 500 MV 600 MV 150 TO 350 SHORTED puma mme TD OFF OR TD ON TD OPEN TD SHORTED SHORTED OR LEAKY Tunnel Diode Triggering Voltages TROUBLESHOOTING THE HORIZONTAL SECTION A Curve Tracer is very useful to check tunnel diodes in the circuit in most cases If there is any doubt of device performance lift one end and connect test leads directly across the tunnel diode TD Set the vertical sensitivity on the Curve Tracer to cover tne sensitivity of the diode under test and set the horizontal to 1 V div Typical TOs have a horizontal switching voltage of about 5 volts The waveforin is not exactly the saine as an out of circuit check but in most cases it indicates whether or not the TD is working properly Be careful to not apply excessive mechanical strain excessive heat or excessive voltage to the TD Typical Curve Tracer Display for Tunnel Diodes 80 TROUBLESHOOTING THE HORIZONTAL SECTION Trigger Operation Trigger Level and Slope A simple method for checking trigger circuit operation is to apply a calibrator signal to the oscilloscope Using the INTERNAL tr
65. play Set the timebase to A or Main Sweep with the TIME DIV switch in the 1 mS DIV setting Set the trigger section up for FREERUN if available INTERNAL TRIGGER and AUTOMATIC if the instrument has AUTO Rotate the INTENSITY control clockwise for desired brightness of the display If the sweep appears and the display appears normal do a Performance Check to insure proper operation of the instrument A thorough Performance Check will sometimes reveal minor faults in an instrument that otherwise appears to operate normally If the display is only a dot that can be seen without pressing the Beam Finder the fault is probably in the Horizontal Section This condition indicates that a Sweep Gate Main Gate has been generated but the Sweep Generator is locked up Refer to the section on TROUBLESHOOTING THE HORIZONTAL SECTION Portable Oscilloscope Tips and Techniques NOTE These tips are applicable only to the Portable Oscilloscopes such as the 400 2200 and 2300 Series that are not microprocessor controlled and do not have CRT readout NOTE These techniques may not work on the new microprocessor based oscilloscopes such as the 2400 Series because the CPU controls tne Display Sequencer and the Horizontal Section The BEAM FINDER does not automatically unblank the CRT when pressed TROUBLESHOOTING YOUR OSCILLOSCOPE If you still do not have a display after the INTENSITY is turned up set the INTENSITY control to the
66. position inverts on the CRT when Invert is actuated it indicates a fault in the input section of tne Channel 2 Vertical Amplifier For most Tektronix oscilloscopes such a fault would typically occur within the first two active stages Source Follower and Amplifier of the Channel 2 Preamplifier If the display does not invert continue to the next step If the display is faulty in both Channel and Channel 2 it normally indicates one of two things l Both Channel and Channel 2 are faulty 2 Both Channel and Channel 2 are good and the fault is in one of the circuits after the Vertical Preamplifiers If one of the channels has a normal display and the other is faulty then you ll know to troubleshoot the faulty channel preamplifier If both are faulty continue troubleshooting to further isolate the problem 44 TROUBLESHOOTING THE VERTICAL SECTION If the fault cannot be isolated to one of the channel preamplifiers isolate the problem to one of the other blocks in the Vertical Section by successive approximation Make tests at the approximate midpoint of the Vertical Section If the section fails the test the fault is probably located behind the test point If the resulting display is good the fault is usually located before the test point By successively dividing the section into smaller sections and blocks you isolate the fault into smaller and smaller sections of tne instrument On those scopes equipped with TR
67. present When in the AUTO Mode tne Tiinebase will cause the sweep to freerun until a trigger is received to disable the AUTO function EVENT DEFINITION CHARACTERISTICS AUTO DELAY This is the waiting interval of the QUIESCENT period in which the instrument is waiting for a trigger This is a timed event whose interval is controlled by an R C Timing Circuit When a trigger is received and the FIRE event is completed the AUTO TRIGGER Mode is disabled by the Auto Disable Gate froin the Trigger Generator This discharges the AUTO DELAY Timing Network and holds it discharged until the end of the following HOLDOFF If a trigger is not received the AUTO DELAY circuitry will begin charging up at the end of HOLDOFF This charging time is the delay waiting time before a sweep is automatically generated 68 EVENT AUTO DELAY TIMEOUT TROUBLESHOOTING THE HORIZONTAL SECTION DEFINITION CHARACTERISTICS You can observe this event on instruments that use the 155 0049 XX Sweep Control IC but you iust have the oscilloscope triggered to see the norrnal operation of the circuit Connect the test probe to tne AUTO DELAY input pin 6 of the IC You should see a shallow ramp on pin 6 of the IC Vary the frequency of the input trigger signal and observe the change to the rarnp notice that the amplitude and duration change as the input trigger signal is varied This event marks the end of the waiting period When the charge on the AUTO DEL
68. protection circuits from the supply and apply full power you may cause a catastrophic failure of the supply and to several other circuits in the instrument Use a Variac variable transformer to control the line voltage applied to an instrument when troubleshooting a faulty Power Supply When you have reinoved any of the protection or regulating components from a high efficiency supply limit the input line voltage to the intrument to 70V ac Without this protection full power applied to line input can cause catastrophic failures Seventy volts is sufficient to allow the inverter to work and allows you to observe and raeasure the operating characteristics of the circuitry With the protection circuitry installed and operational you can gradually increase the line voltage to see at which point the protection and regulating circuitry begin to work Sometimes you can troubleshoot in a faulty supply by increasing the supply voltage just enough to cause it to enter the burst mode or by leaving it at a level just below burst Do not remove the regulated supply loads when troubleshooting because this will prevent the Primary Inverter Circuit from operating properly For isolation purposes you can remove a portion of the output load such as the High Voltage drive output from the secondary side of the inverter However you should load the power supply by installing at least 2 plug ins in the instrument to allow the circuit to come up
69. r runs thereby controlling the energy applied to the primary In soine high efficiency supplies further regulation takes place in the secondary circuitry Variations in line voltage are compensated oy changing the duration of the on time of the current driver while load variations are handled by changing the repetition rate of the on time Take a look at troubleshooting the supply There are two items you will find useful a power line isolation transformer and a current probe such as the TEKTRONIX P6021 The primary power supply circuit s common and guard box are elevated to line potential The isolation transformer reduces the shock hazard and allows grounding the power supply s common for troubleshooting The current probe is a convenient means of viewing the current waveforms associated with the current driver 28 TROUBLESHOOTING THE HIGH EFFICIENCY SUPPLY To troubleshoot a supply that won t start up your ears may provide the first clue If you listen closely you may hear a short burst a signal of about 12 kHz repeated several times per second This indicates a probable short in one of the secondary supplies What s happening is that when you turn the instrument on the start circuit will try to start the inverter The inverter will run at a frequency of approximately 12 kHz for one second at which time the start circuit turns off This happens when the inverter cannot produce adequate power to eep running because of the short
70. re valid for the Lab Oscilloscopes such as 7000 Series that use the plug in configuration and have CRT Readout Before continuing ensure that the front panel settings on both tne mainframe and the timebase are set up properly for a display Also be sure that any timebases installed are in the INDEPENDENT MODE Rotate the INTENSITY control clockwise for the desired display brightness If the sweep appears and the display looks normal do a Performance Check to insure proper operation of the instrument If the display is only a dot that can be seen without pressing the Beam Finder the fault is probably in the Horizontal Section This condition indicates that a Main Gate has been generated but the Sweep Generator in the timebase is locked up Refer to the section on TROUBLESHOOTING THE HORIZONTAL SECTION If you still do not nave a display after the INTENSITY is turned up set the INTENSITY control to the 12 o clock position Set the READOUT INTENSITY control to the 12 o clock position If Readout appears on the CRT but you still have no sweep the fault is located in either the Interface or in the Horizontal Section NOTE If the Readout does not appear on the CRT make sure that the READOUT MODE SWITCH usually located internally is set to the FREERUN position 18 TROUBLESHOOTING YOUR OSCILLOSCOPE If there is still no display press the BEAM FINDER button If tne display is offset on the CRT troubleshoot the appropriate sectio
71. s an amplifier This facilitates ease of further fault isolation On most oscilloscopes you can easily isolate the two sub sections by checking the signals at the rear panel A SWEEP OUTPUT and the SWEEP GATE OUTPUT If both of the output signals are present and appear to be normal the problem is probably located in the Horizontal Amplifier If one or both of the signals is missing or incorrect the problem is in the Timebase NOTE When making this check ensure that the front panel controls are set up according to the Preliminary Front Panel Set up presented in the front of this book Make sure that you have the Trigger Mode in FREERUN or A UTOMATIC You can also isolate the two sub sections another way on those oscilloscopes that have X Y MODE capabilities First make sure that all horizontal and vertical position controls are centered Rotate the TIME DIV switch to the X Y position You should have a dot near the center of the display If a dot does not appear the fault is probably in the Horizontal Amplifier You can verify this further by pressing the BEAM FINDER and observing the display If you get a centered dot the problern may be in the Timebase or in the CRT Section If you get a dot that is deflected to either side of the display the fault is probably located in the Horizontal Amplifier 53 TROUBLESHOOTING THE HORIZONTAL SECTION You can verify the fault location after removing the cover from tne instrument S
72. s something like trying to find the mid point around the edge of a circle However if you establish a starting point along the edge of the circle you can easily find a corresponding mid point Notice that the key here is to establish a reference point from which to start This is also the key to troubleshooting the Timebase Many technicians have trouole isolating faults within the Timebase of an instrument because they try to troubleshoot the circuitry without having a thorough understanding of the loop concept The best way to troubleshoot the Timebase is to troubleshoot the sequence of events needed to generate the sweep This sequence is predictable and repeatable and has many events that can be easily identified by the technician Learn this sequence and its identifiable events and you can easily apply the troubleshooting methods and procedures presented in the first section of this book Keep the following in mind as you progress through the remainder of this section WHEN ISOLATING FAULTS WITHIN THE TIMEBASE TROUBLESHOOT THE SEQUENCE NOT THE CIRCUITRY 57 TROUBLESHOOTING THE HORIZONTAL SECTION Timebase Sequence The first thing to do in troubleshooting the Timebase is to establish a reference starting point For this sequence consider the starting point to be the Quiescent period the interval starting right at the end of Hold Off this will be the reference point for all other events in the sequence The out
73. se appears to be consistent in ainplitude and frequency it is probably a result of osciliations in the circuitry on the board Because of the feedback nature of this kind of problem it is harder to isolate the source of the noise it may be the result of interaction between several circuits or components You inay be able to solve this problem by re orienting the lead dress of cables connectors and components wthin the circuitry When repositioning components be careful or you inay stress the leads this can cause fractures at either tne component end or board end of the component lead not adjust or reposition transistors that are soldered onto the board Since transistors are three lead components they form a triangular connection to the board Any stress against the orientation of the transistor may cause damage to the board or to the transistor 49 TROUBLESHOOTING THE VERTICAL SECTION you have trouble locating the source of the oscillations use a snooping loop to help Forn a inch loop in the middle of an easy clip connector Hold the loop so that it is parallel with the plane of the circuit board Slowly pass the loop closely over the top of the suspect circuitry and closely observe the oscillations on the display you notice an appreciable increase or decrease in the amplitude or frequency of the oscillations as you pass the loop over a particular circuit or area of the board troubleshoot the circuitry aroun
74. se the test equipment only after they have determined which functional block within the unit is faulty Good troubleshooting procedures can cut your troubleshooting time in half Through practice these procedures will become second nature then you can focus more on the symptoms of the faulty instrument and less on the actual methods or procedures of troubleshooting The following is an outline of the 3 Basic Steps for Troubleshooting Something to remember as you learn and use these procedures Do not remove the cover from the instrument until you have isolated the fault to a section and or block or until you have made all of the tests that you can from outside the instrument The cover will prevent distractions and will reduce bad habits of haphazard troubleshooting as you follow the procedures TROUBLESHOOTING TECHNIQUES AND PROCEDURES SYMPTOM RECOGNITION Note any irregularities in the display and in any other indicators on the instrument Milk the front panel notice how the front panel controls effect the display and indicators Notice any interaction between the front panel controls themselves and then between the controls and display This step is much like perforinance check Remember while performing this step it is just as important to recognize and note proper operation as it is for faulty symptoms SYMPTOM ELABORATION Again milk the front panel Develop the symptoms thoroughly Use the front panel controls t
75. sweep 65 EVENT FAST BLANKING HOLDOFF START TROUBLESHOOTING THE HORIZONTAL SECTION DEFINITION CHARACTERISTICS At the end of the sweep the CRT is blanked to prevent an intensification at the end of sweep and to prevent visible flyback sweep retrace This step provides immediate blanking of the CRT beam without having to wait for tne propogation delay in ending all other gates KEY INDICATOR This step has been completed if there is a dot visible at the right side of the CRT that is visible only upon pressing the BEAM FINDER Since this is a function of the unblanking logic you can measure the result of this at the rear panel of sorne instruments Check the A GATE OUTPUT at the rear panel signals out connectors If the gate is still high this step has not been completed If the gate has ended then FAST BLANKING should have been completed After the end of sweep has been detected a pulse is generated to initiate the HOLDOFF cycle This pulse is usually taken directly from the End of Sweep Detector though in some instruments it may be amplified inverted or buffered In most Tektronix oscilloscopes this pulse is a positive pulse that is fed back to the Sweep and Z Axis Logic Circuit According to the oscilloscope this HOLDOFF START Pulse may 5e called H O START A SWEEP RESET or GATE RESET In those instruments that use the Tek inade Sweep Control IC P N 155 0089 the HOLDOFF START Pulse is appl
76. t Excessive ripple divide by 10 for approximate solid state value A 50 mV to 1 5 V comparator speedup capacitor B 1 5 V to 8 V output filter i C 8 V or more input filter Off tolerance leaky speedup capacitor lift one end change both output voltage setting resistors Poor regulation at 117 V line weak comparator defective bridge bad filter component at 105 V line weak regulator Noisy output noisy coinparator or regulator noisy output voltage setting divider noisy transistor or poor connection noisy diode or noisy resistor 25 TROUBLESHOOTING THE POWER SUPPLY 600110111250 dois 01 xXoeqpeo 4 9pi oJg 1941 uy sde uy 213 sdey uedg 520151599 30 5 payous J04 5 42902 59 Ayiqeisuy ysey asion yeu2 oj Ayddns szamog 550 puy sJojsisue1 59 Jadoig 104 428142 puy uno iQ 5 ya uy 5 1045 uy sp 1045 o2ejdeu 10 42942 15592 3 104 42942 5 eur ul Sepoiq eur
77. ted by one of the following CRT symptoms intensity on CRT display 2 Full intensity on CRT display 3 No control over intensity and or focus of CRT display 4 Incorrect vertical and horizontal calibration The control grid supply is normally 40 50 V more negative than the cathode supply If these two supplies decrease their bias for some reason the high voltage supply can draw sufficient current to drive it out of regulation The intensity control varies the bias of the CRT 36 TROUBLESHOOTING THE CRT AND Z AXIS SECTION Most Tektronix cathode ray tubes will cut off when the grid is approximately 65 V more negative than the cathode f the CRT is weak you can even get down below the cutoff point of the tube CRT Conditions Gassy CRT s may be identified by their double peaking characteristic When the CRT is cold this fault norinally produces two very pronounced spots where the CRT turns on As the intensity control is advanced clockwise CW a dim trace appears and continues to decrease in intensity then it increases somewhat toward the CW extreme Once a tube begins to display this characteristic a self destructive process has begun and it is only a matter of time until the tube must be changed Gassy CRT s also often exhibit poor focus and brightness characteristics and a static charge phenomenon Static charge problems are typically caused by dirt so if this characteristic is noted the CRT face and cover shou
78. ther where it connects into the Vertical Output Board If the display is approximately centered on the CRT when the Delay line is shorted the fault is in the Vertical Interface Board the Delay Line or in the connections somewhere between the output of the plug in and the output of the Delay Line f the fault is not removed by shorting the Delay Line the fault is either in the Vertical Output Board the CRT or in the connections to the CRT Press the BEAM FINDER and observe the display If the display can be seen on the CRT the fault is probaoly on the Vertical Output Board 46 TROUBLESHOOTING THE VERTICAL SECTION After you have isolated the fault to the board or block use tne general troubleshooting tips that follow to further isolate the problem Troubleshooting Tips for the Vertical Section Remember as you continue to troubleshoot you should continue to milk the front and rear paneis for clues that inay lead you to the source of the problem You should also continue to use the successive approximation method of fault isolation Common Mode Operation A simple clip lead can oe invaluable for troubleshooting faults in which the beam is defiected off screen This technique is sometimes called common moding the stages In a transistorized circuit this is accomplished by connecting a clip lead between the bases of the transistor pair in a Push Pull Amplifier it also works on Amplifiers as illustrated
79. trigger circuit with minimum distortion Input coupling circuitry allows selection or rejection of various frequency components of the trigger signal The input amplifier provides gain to assure the trigger pulse generator of sufficient input for proper circuit operation The automatic triggering circuitry used in Tektronix instruments provides a baseline in the absence of a trigger signal and eliminates control of coupling and level controls 76 TROUBLESHOOTING THE HORIZONTAL SECTION Although trigger circuits vary in their complexity and sophistication the essentials are the same in all instruments Most trigger circuits incorporate a trigger sensitivity control to permit adjustment of the minimum signal size to which the circuit can respond Tne following figure illustrates simplified block diagrams for trigger circuits Individual trigger circuit designs vary but all circuits make use of some of the basic functions on the following page SOLID STATE TRIGGER LEVEL CENTERING COMPAR ATOR TRIGGER LEVEL SENSITIVITY PRE AMP TRIGGER GEN TRIGGER PICKOFF INT TO SWEEP INPUT COUPLING NJ OR PLUG IN Typical Block Diagrams for Solid State Trigger Circuits 77 TROUBLESHOOTING THE HORIZONTAL SECTION The basic internal adjustments of a modern oscilloscope are 1 Trigger level centering adjust controls trigger circuit symmetry to enable all coupling modes to work prop
80. und the grid cathode supplies for faulty components This is sometiines a useful way to check capacitor breakdown etc Typical resistance value in the grid circuit is 4 5 megohins to ground This holds true almost anywhere you measure in the circuit If the components check out properly tne problem is probably in the high voltage transformer possibly one of the windings has a leakage path to the core Problems in the high voltage anode supply sometimes show up as insufficient high voltage Check the output filter capacitors and the anode coupling capacitors Weak high voltage rectifiers will also indicate insufficient high voltage A poor connection at the CRT anode connector can show up as jitter in the sweep or as poor regulation NOTE All solder joints on high voltage chassis should have smooth surfaces Any protrusions may cause high voltage arcing particularly at high altitudes 39 TROUBLESHOOTING THE CRT AND Z AXIS SECTION Some of the more recent Tektronix oscilloscopes have a CRT bias control This adjustment is sometimes used as a maximum intensity control to allow the user to protect his CRT When the instrument is adjusted in this manner and the intensity is limited dimness probleins may occur at the faster sweep speeds If there is a brightness proolem with a cathode ray tube check to be sure that the CRT grid bias is properly set Intensity nodulation blank spots or uneven trace intensity is often cause
81. used by a weak D C Restorer diode in the control grid supply A control grid to cathode short in the CRT will exhibit similar symptoins To for this condition remove the socket from the CRT and note if tne CAT bias changes If the bias changes then the loading is caused by the CRT load The CRT filament supply should also be checked to insure that the problem is not caused oy leakage in the filament transformer Internal CRT grid shorts may not show up with onmmeter resistance checks No brightness with norinal intensity control settings but slight intensity as the control is moved further clockwise may indicate a weak CRT or cathode supply Similar symptoms will be present if a CRT is gassy or if unblanking is not received from the time base generator A gradual increase or decrease in intensity is a symptom of weak rectifier diodes in either the control grid or cathode supplies NOTE DC Restorer diodes should be replaced at the same tirne to prevent differential aging problems and to eliminate the possibility of a repeated failure due to stressed weakened diodes Don t forget calibration adjustments Check GRID BIAS Intensity Range adjustments and their associated circuitry Lack of high voltage is commonly caused by loading one or more of the secondary supplies is causing the oscillator to not run In most high voltage supplies this will cause the oscillator to free run at a frequency slightly higher than normal
82. utput impedance resulting in lower output ripple usually on the order of 2 mV to 5mV 2 Resistance of supplies is typically lower out checking is the same due to stacking of supplies 3 Less problems with regulators because of less heat dissipation 4 Supplies may be checked for shorts irnmediately after power is applied no time delay relays If the regulators are not operating properly or if the supply operates inter nittently occasionally fails or blows fuses use an auto transformer to vary the line voltage and make checks around the supplies and regulators To simulate the actual operating conditions of the circuitry you can also vary the temperature of the circuits in operation Use a hair dryer or heat gun to heat the Power Supply circuits to their operating temperatures Cool the circuitry with an aerosol circuit coolant such as Arctic Mist to drop the circuit temperature back down to a start up level By alternately heating and cooling the circuitry in operation you sirnulate the ambient conditions which the supplies normally encounter over a longer period of tirne 2 or 3 hours Often this check will reveal heat sensitive devices early and may eliminate a Call back or the need to recalibrate the instrument under test 24 2 TROUBLESHOOTING THE POWER SUPPLY Common Power Supply Problems Fuse blows when power is applied shorted diode in bridge shorted filter cap Fuse blows after delay overloaded outpu
83. zontal input X Y mode disconnects the sweep and is a means of determining whether a problem is associated with the horizontal amplifier or sweep generator At the same time it can indicate the condition of the unblanking circuitry Varying the trigger source switch between internal and external triggering checks the trigger pickoff circuitry If the sweep will free run by adjusting the stability and trigger level control additional circuitry may be checked Comparing operation in different trigger modes can often localize a problem to a specific trigger stage TROUBLESHOOTING YOUR OSCILLOSCOPE Vertical preamplifier plug in units are a quick way of checking performance to the vertical amplifier input Once a problem is isolated to a specific plug in unit plug in circuit boards if used may isolate the problem even further Once a problem has been traced to a specific block a close visual check may pinpoint the problem Often times the troubleshooting job can be shortened by spotting burned components or loose leads Another inethod of streamlining troubleshooting procedures is through the substitution method This method can be used at many levels of troubleshooting In troubleshooting instruments with plug in modules isolate the faulty sub systern by substituting known good plug ins for those that are questionable In those instruments that have plug in boards board substitution can help to quickly isolate a faulty board Su

Troubleshooting Your Oscilloscope

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