testing the optical pick-up of mp3 vcd/dvd players
Contents
1. Warning Dis connect the motor from the main board The unavoidable wobble of any disk is essential in this case and sweeps the fo cus distance by more than enough to cover the entire focus range of interest It has been assumed that the spindle is driven by a conventional permanent magnet DC motor If it is a brushless DC motor some of the control electronics may be external to the motor and you will not be able to provide a DC voltage to get it rotate If this is the case you must use a stationary spindle but sweeping focus 2 Stationary spindle but sweeping focus This is a better method but requires a signal generator for the easiest use You can do this by hand using a variac or rheostat A better method is to use a 1 10Hz sinusoidal or triangular wave from a low frequency signal generator with a low impedance output or feeding an emit ter follower or audio amplifier to boost the current This signal is then fed into the coil along with the focus offset de rived from the power supply It is possible to dispense with these test set ups and just use the normal focus search of the CD player itself to provide the sweep However since you will be interfering with the proper feedback by removing selected sensors there is no assurance as to what the microcontroller does Therefore break ing the feedback loop as we are doing is preferred However if the CD player ap pears to make many attempt at focus this may be worth2. There is a threshold below which there is no output For a diode rated at a threshold of POWER ON OFF connecting the laser diode Slowly increase the current until you get a beam Use an IR detector for beam detection If you get the polarity back wards or are actually measuring across the internal photodiode the voltage across the diode goes above 3 volts or is less than 1V Then turn power off and re verse the leads Some laser diodes get de stroyed by a reverse voltage greater than 3V the reverse voltage rating is listed in the specification sheet Without a laser power meter however you have no way of knowing when the limit for safe beam power for the laser diode is reached For this test increase the current only until you get an indica tion on the IR detector or you see the red dot You are not trying to measure power just to see whether it works at all A typi cal threshold is around 30 mA Sometimes gt ae gt sl Sj SS MAINTENANCE be an increase in the laser output power beyond its normal range which may shorten its life substantially If the laser output power has de creased there is probably nothing that can be done as the feedback circuit is already doing its bit and the adjustment will have little effect Thus make sure the optics are as clean as possible before you touch the laser power The following will enable you to play a disk even if it has some problems with noise
3. Therefore a weak laser may not be salvageable as the feedback loop may have done all that is feasible 2 Photodiode sensors must be func tioning correctly for data recovery and fo cus and tracking feedback A 3 beam pickup has six segments the central segments A D are used for focus and data recovery and the outer segments E and F are used for tracking feedback In a single beam pickup segments E and F are absent proper focus has been established Thus if the CD rotates when in place but the bare spindle does not it is likely that fo cus is being established successfully potest ba connections to the optical pickup In order to perform many of the tests de scribed below you have to locate the drive and or signal connections to the optical pickup While there are many variations in the construction of optical pickups even from the same manufacturer they all need to perform the same functions So the inter nal components are usually quite similar Fig 41 shows the connections for a typical Sony pick up For laser diode as APRIL 2004 ELECTRONICS FOR YOU MAINTENANCE sembly and photodiode chip connections a single flex cable having 10 to 12 con ductors is used The actuator connections may also be included on it or a separate 4 conductor flex cable may be used The signals may be identified on the circuit board to which they attach with designa tions as shown in the figure The signals A C and B D ar
4. Touch the metal chas sis first to discharge yourself Basics of an optical pickup For information or music to be read off a CD several systems must work closely MAINTENANCE 3 Lens must be focused to within a fraction of a micrometre um of optimal to produce a diffraction limited spot This is less than 2 pm in diameter at the disk pits The lens is actually positioned sev eral millimetres from the disk surface and is maintained at the correct distance through optical feedback controlling the lens position using the focus coil 4 Lens must align to within a fraction of a micrometre of the centre of the track Tracks on a CD are spaced 1 6 ym apart Tracking is maintained via optical feed back controlling the radial lens position using the tracking coil or the radial posi tioning unit on some rotary positioners If the behaviour while the CD player is attempting to read the directory changes whether a disk is in place or not and there is no separate disk sensor some or all of these components are functioning correctly For example many CD players don t attempt to rotate the spindle until Sharp CD BA250 mini component system with 3 disk rotary changer system together 1 Laser must be emitting a coherent beam of sufficient power and stability The optical system must be clean and prop erly aligned Laser power is maintained constant via an optical feedback loop con trolling the laser diode current
5. is doing this position the IR detector above the lens If the laser is working you will get a posi tive indication of IR in about a 30 degree cone on either side of the lens While you have no way of knowing whether the power output is correct this is a reason able indication of laser operation Due to the wide angle of the beam the power decreases rapidly with distance So you need to be very close to the lens for a positive result If the lens moves smoothly in at least one direction up or down the focus ac tuator is functioning If the IR detector does not pick up a Fig 41 Connection diagram for a typical Sony pickup beam and you do not see the red dot either the laser diode is dead or power is not being applied by the control circuits Testing whether laser diode is being powered If there is no indication of an IR emission but the lens is moving up and down at tempting to focus the next step is to de termine whether the laser diode is being powered or is totally dead If you have a service manual follow the instructions given there for checking the laser diode usually by measuring a voltage drop across a sensing resistor or other test point If the reading is very low or 0 a fault is likely in the driver or the optical feedback circuitry If the reading is very high the laser diode is likely to be bad and the driver is unable to compen sate for low or no emission Look for a test point labeled l
6. stability and amplitude us ing the optical alignment adjustments and servo adjustments you would have achieved proper alignment To be continued
7. tower speakers tern this indicates proper functioning of all the major components of the optical pickup If however any of the following are observed testing of the laser diode focus and tracking actuators and or pho todiode array is suggested 1 The start up sequence does not com plete due to obvious failure of the pickup to perform some action For example there is no attempt to focus 2 Focus appears to be established but the directory is never displayed even though the disk spins at the cor rect speed or overspeeds or does not spin in the cor rect direction clock wise as viewed from the label side 3 The eye pat tern is weak dis torted or missing at the RF test point Try to eliminate alternative causes before undertaking these tests as there is a chance of dam age due to accidents or electrostatic dis charge There is a good chance that the tests will only con firm that the pickup is dead not many of the faults you will be able to locate are easy to fix The following descriptions assume that the pickup is still installed in the player but selected portions are disconnected when required This enables you to conveniently use the circuitry of the player to control certain functions for the live laser diode and pho todiode tests It is also possible to test the pick up in standalone condition but this requires an alternative power supply to drive the laser diode Since th
8. MAINTENANCE Part X TESTING THE OPTICAL PICK UP OF MP3 VCD DVD PLAYERS Identifying front end problems in CD players CDROM drives laser disk players and other optical drives is often thought to be a difficult task This part of the article describes techniques to check functioning of the laser diode focus voice coil actuator tracking voice coil actuator and photodiode array GP CAPT K C BHASIN RETD directory even erratically the tests de scribed here are unnecessary unless you suspect an intermittent behaviour of one of the sub systems as all major parts of the laser pickup assembly must be function ing properly in order to do this However there may be some marginally performing components such as a weak laser diode or shorted turns in the focus or tracking coil Sometimes a dirty lens results in symp toms that may be mistaken for much more serious problems For intermittent faults first carefully inspect the pickup assembly for bad sol der connections and hairline cracks in the flexible printed cables Interlock switches may be dirty or wornout Mechanical prob lems may result in intermittent behaviour as well the player is able to read the disk When and why to test the pickup If you have examined the RF test point with a scope and found a proper eye pat ELECTRONICS FOR YOU APRIL 2004 Fisher Electronics Slim 4000 vertical 4 CD stereo system with digital AM FM stereo tuner amp
9. a trial You may also need a new disk pref erably one you do not care much about as it may get scratched due to opening the drawer accidentally or doing some thing equally undesirable while the disk is still rotating Locate a 1 mega ohm resistor and se curely fasten it to a ground near the pho todiode connector Put the scope probe on the other end with its ground clipped to the same ground point as the resistor Bend the free lead of the resistor com pletely over so that it is able to hold the end of a wire like a mini clip lead Mark down exactly how the connector is wired so that as you remove individual wires you are able to get them back in the ELECTRONICS FOR YOU APRIL 2004 Fisher Electronics Slim 1400 digital AM FM CD R RW executive slim audio system proper spot Presumably you have already made a diagram of the photodiode connec tor wiring Component players often have connectors with individually removable socket pins A fine jeweller s screwdriver or paper clip may prove handy in removing these one at a time Turn on the power supply and adjust the focus to about mid range Start the spindle rotating or turn on the signal gen erator to provide a small sweep about 1 10 Vp p as measured on the coil Making the photodiode measurements Remove the wire corresponding to the pho todiode to be tested say A from the connector but leave the connector itself plugged into the main board Se
10. aser power Even if you don t know what the reading should be anything other than 0 would be a good indication that the driver is being enabled If you don t have the service manual carefully measure across the laser diode with a multimeter to determine whether there is any voltage when it is supposed to be active A normal reading is 2 to 3 V For Sony and other similar pickups there is a chip capacitor across the laser diode The trick is in being able to attach meter probes to these points without de stroying everything The flex cable may also have a pair of solder pads To attach fine wires to these for your multimeter use a fine point sol dering iron preferably grounded and tem perature controlled to prevent damage Warning For testing the laser diode use a digital multimeter with ESD protec tion to avoid damage to the laser diode Make all connections with power turned off as the momentary glitch from attach ing the probes and or an accidental short circuit can easily burn up the laser diode and other parts Testing the laser diode with an external power supply Consider the following only if there is no indication of laser output while the laser diode is connected to the player and you do not have schematics or a service manual to determine whether the laser power circuits are functioning Typically the current is in the 30 100mA range at 1 7 2 5V However the power curve is extremely non linear
11. ated at 5V may actually put out 8V or more when unloaded so check the current into a short circuit before the operating current is marked on the pickup Do not exceed this current If you detect a beam and there was none before the problem is most likely in the player s control or power circuits not in the pick up Laser power adjustment As mentioned earlier the laser diode may get destroyed when attempting to adjust its output power However if you suspect a weak laser as indicated by noisy play back or poor tracking performance and have eliminated all other possibilities such as servo adjustments attempt one of the procedures described here What adjusting the laser power prob ably does isn t compensating for a decrease in laser diode intensity but rather a buildup of dust and other junk on the optics possibly internal and inaccessible which reduces the beam intensity at the CD and the return beam intensity even more There is a subtle difference as the optical output of the laser diode itself is feedback controlled and shouldn t drift much and the result of an adjustment will find it to be only a few hundred mV there is likely to be a problem Caution A weak eye pattern can also be due to improper focus bias adjustment Check whether it is an electronic problem The laser power may be normal Turn the laser power adjustment with the player power switched off to avoid the possibility of electrical no
12. d on or only when the player is trying to focus or perform some other operation With an IR remote you should actually see the pulsed signal for each key code On a typical Sony CD player you could get about 0 1V signal at the RF test point using a VCR remote control as an IR source However even on a functional pick up due to the nature of the optics these responses may be very weak or undetect able Thus failure of either of the above tests is not a strong evidence of a bad For the tests using the internal laser diode you need to set up either an ad justable focus with continuously rotating spindle or a stationary spindle but sweep ing focus The latter is more straightfor ward but requires the optional signal gen erator for best results In each case the objective is to cause the lens disk distance to sweep through perfect focus without requiring the focus servo loop to be closed This results in a signal that includes the point of maximum signal amplitude on a periodic basis Alternative methods may be used to accomplish the same purpose Both techniques require the use of an ad APRIL 2004 ELECTRONICS FOR YOU MAINTENANCE justable power supply The test set ups are described below 1 Adjustable focus with continuously rotating spindle For the spindle motor you need a 1 5V battery or power supply with a suitable series resistor to cause the spindle to turn at approximately 1 2 Hz revolutions per second
13. e microcontroller will not be imposing its own will on those parts of the pickup still connected to the player this may be preferable Caution Whenever applying external power to any component totally disconnect it by un plugging or unsoldering label each wire if there is any ambiguity to prevent dam age to the circuitry on the logic board Tools documents and test equipment Only a minimum of tools and test equip ment are required for these testing tech niques to be effective An oscilloscope is desirable However a digital multimeter can work as a substitute as no high fre quency measurements are needed We as sume here that a scope is available It is also assumed that the sled drive or the drawer or spindle motors are functional as their correct operation may be required for some of the tests A schematic will help greatly Depend ing on the design of the unit you ll be able to infer enough about the front end elec tronics to get away without one The de sign of the components of the optical pickup is usually similar among CD players from different manufacturers which makes the tests relatively model independent What may differ are polarities of photodiodes laser diodes connector pinouts etc These can usually be determined quite easily Despite the incredible precision of the focus and tracking servos you can per form meaningful tests without sophisti cated or specialised test equipment The followi
14. e usually shorted to gether near the connector as these are al ways used in pairs The laser current test point if present is near the connections for the laser diode assembly It is usually possible to identify most of these connections with a strong light and magnifying glass by tracing back from the components on the optical block The locations of the laser diode assembly and the photodiode array chip are usually eas ily identified Some regulation and or pro tection components may also be present There is often a pair of solder pads on two adjacent traces Short these pads by applying a glob of solder using a grounded soldering iron This protects the laser di ode from electrostatic or other damage dur ing handling and testing and a multim eter can be safely used to identify compo nent connections and polarity Testing the laser diode while in the player Without a laser power meter it is difficult to fully verify laser functionality How ever determining that infrared is being emitted provides a reasonable assurance of laser operation For this test you need an IR detector A simple IR detector circuit is shown in Fig 42 This unit is also useful for testing of remote controls and other IR emitters However you need to gain access to the lens This may require the removal of the clamper assembly Once this is accomplished prepare to position the photodiode of the IR tester within 3 mm of the lens Plug the
15. ions or breaks in the flexible cables are not ruled out A defect found in the photodiode ar ray usually means that the laser pickup is not salvageable with reasonable effort Even if you could locate a replacement photodiode array aligning and soldering the surface mount package is quite a chal lenge without the factory jigs Assuming that these tests don t turn up anything the next step is to verify that the photodiodes are picking up an optical sig nal and evaluate the relative strengths of each segment using the laser diode optical system and disk combination For these tests to confirm proper operation the opti cal alignment must also be correct oar SEE covmvo paren ove nsroor Sony DVP NS700P DVD player A C B D E and F Since the A and C segments and B and D segments are usu ally shorted together on the circuit board this provides all the information needed to identify the photodiode connections It is not important to distinguish between A and C or B and D for the following tests though you will want to be able to sepa rate them With power off there is essentially no light on the photodiode array Unplug the photodiode connector from the main board Using your ohmmeter test each diode for open and short as you would test any signal diode There should be a the signal but only when the photodiode array is properly biased This signal may be present all the time the CD player is turne
16. ise causing cur rent spikes Mark the exact position of the laser power adjustment so you can get back to it if there is no effect or it makes things worse Turn the control the slightest bit clock wise Turn on the power and or note the eye pattern amplitude If the laser diode is not at the limit of its power you should see the amplitude change from what it was If it has decreased try the other di rection Note the playback quality Has it changed for the better If not laser power is probably not the problem If the ampli tude of the eye pattern is unchanged ei ther you are turning the wrong control or the laser is at its power limit and it may get damaged soon Try the same test in the counterclockwise direction if the am APRIL 2004 ELECTRONICS FOR YOU MAINTENANCE plitude decreases If there is improvement you can risk leaving the control at the new higher power setting realising that you may be shortening the life of the laser diode Do not push your luck by continuing to turn up the power unless you have tried all other alternatives If you do not have an oscilloscope you can still try the above procedure by carefully listening to the audio to deter mine whether there is any change It s a little bit riskier The laser power adjust ment may be very sensitive and you have no direct way of knowing how much you have increased the setting Testing the focus and tracking actuators If your CD pla
17. ng tools and test equipment are required 1 Basic hand tools including jeweler s precision screwdrivers 2 Digital multimeter 3 Oscilloscope for photodiode RF tests For most of the tests almost any scope will do as long as it has a DC coupled vertical amp 4 A 0 5V variable DC power supply 500 mA You can use a fixed 5V supply with a series adjustable resistor 100 ohms for focus and tracking actuator testing and 250 ohms for laser diode testing A highly regulated supply is not needed 5 Resistors 22 ohms 1W 5 ohms 1W 50 ohms and 1 mega ohm 6 Assorted test clip leads and a few centimetres of 25SWG solid hookup wire 7 IR detector circuit infrared IR de tector card or IR sensitive camcorder for laser diode tests 8 Low speed 1 10Hz sweep or func tion generator with low impedance out put or amplifier Precautions To minimise the chances of damage to the laser diode which is extremely sensi tive to static and excess current leave its connector plugged into the main board and do not attempt to test the laser diode with an analogue multimeter which on the low ohms scale may exceed the cur rent rating of the laser diode As with all modern solidstate equip ment preventing electrostatic discharges to sensitive components is critical An an tistatic wrist strap is desirable In any case work in an area where static charge build up is minimised not on a carpet prone to static build up
18. odiode segments are also pos sible In this case the outputs will not be independent Loading one segment s out put with a resistor may affect the output of other segments Such a discrepancy in photodiodes A to D prevents the establishment of a proper stable lens position at the optimal focal distance This prevents the formation of a proper eye pattern and subsequent data recovery A significant difference between E and F beyond the adjustment range of the tracking or E F balance control pre vents proper tracking However the signal amplitudes from photodiodes A through D and E and F may differ as photodiodes A through D operate off the main beam and E and F operate off the first order diffracted beams which are weaker As with the basic photodiode tests above a failure here usually requires replacement of the entire optical assembly As noted if the pick up s optical align ment is way off there could be significant differences in photodiode responses On component type units it is unlikely that the optical alignment shifts on its own Portables dropped down accidently or au tomotive units subject to constant bumps and vibration could have alignment prob lems however If this eventually results in improved uniformity of the photodiode response alignment could be the prob lem If you can more or less equalise the response reconnect the servos and attempt to get an eye pattern If you can optimise the eye pattern
19. or tracking Put the oscilloscope probe on the RF test point While the disk is playing you should see the eye pat tern Mark the exact amplitude of the peaks Also note the playback quality so you can recognise if it changes While the correct voltage for the eye pattern is not the same in all players typi cal values are in the 1 2V range If you Philips ultra slim DVD video karaoke player 30 mA the maximum operating current may be as low as 40 mA A sensing pho todiode is built into the same case as the laser diode to regulate beam power It is critical to the life of the laser diode that under no circumstances is the safe cur rent exceeded even for a microsecond Laser diodes are also extremely sensi tive to electrostatic discharge so take ap propriate precautions Also don t try to test them with a volt ohm meter which could exceed their safe current rating on the low ohms scale Even connecting the test leads can blow the laser diode from static Always make or break power or test connections with the player turned off Use a 0 5V DC linear supply with a 50 ohm resistor in series with the diode a switching supply s spikes may destroy the laser diode If you use a variable resistor make sure it is at its maximum resistance when you start so as to keep the current under 20 mA Keep in mind that a wall wart a small power supply brick with in tegral male plug designed to plug directly into a wall outlet r
20. pickups while six segments are used in 3 beam pickups These segments are usually designated A through F A B C and D form the main detector which is used for both focusing and data recovery Generally segments E and F are used in a 3 beam pickup for fine tracking feedback The same has been assumed in the following discussion All the six photodiodes are connected to a common point which during opera tion has a DC bias voltage of around SV If the photodiodes are connected in com mon anode configuration it will be nega tive If common cathode configuration is used it will be positive The reason is that the photodiodes need to be reverse biased for normal operation The outputs of the photodiodes feed several operational amplifiers which are set up to amplify the current from the photodiodes The nor mal connections may be at virtual ground potential or these may feed into large value resistors The connector to the photodiode ar ray is usually separate and typically has at least eight wires comprising connec tions to photodiodes A through F ground and bias voltage You need to identify the wires First locate the ground using the ohmmeter Then locate the bias it usually goes to a low value resistor and then to the supply Another way to identify the bias wire is to turn on the player and measure each of the possibilities The bias is the highest or the lowest with no noise or ripple It is powered all the
21. rry unduly as the pos sibility of shorted turns is really quite re mote An intermittent may only show up dur ing dynamic operation with certain CDs or other peculiar circumstances The in termittent could be at the solder connec tions or the fine printed ribbon cable that connects the moving lens assembly to the remainder of the pickup Testing the photodiode array The photodiode array in an optical pickup consists of an IC with typically four or six Panasonic SC AK410S Nitrix series mini system with 5 CD changer and 3 way speakers with a bit of compressed air not high pressure and then with Q tips and iso propyl alcohol Do not lubricate Repeat the tests after cleaning If both the tests are positive the focus and tracking actuators are functioning If either you were unable to locate both pairs of coils or one or both actuators did not move you have located a problem An open coil can be due to a cable problem or a break in the coil If the break is right at the solder connections which are usually visible once the plastic protective shroud is popped off you can repair it However this requires a great deal of manual dexter ity and patience as the wire is really fine Shorted turns in the fine coils or an intermittent are still possible Shorted turns reduce the frequency response of the servo reduce the reliability of focus or detector segments Four segments are used in the less common single beam
22. t the scope for 1V div vertical deflection in slow free running sweep mode Clip the A wire into the resistor Now turn on power to the CD player While the player attempts to focus slowly ad just the focus voltage while observing the scope As you approach proper focus you will see the signal increase greatly de pending on the polarity pass through a maximum and then decrease Depending on the design of the CD player you may need to turn it off and on several times before you locate the signal since the microcontroller may give up pretty quickly with no focus or tracking coil servos as you ve disconnected the actuators The service manual may guide you on how to force the laser to be powered all the time Leave the focus set near the middle of the region of high signal If you use the signal generator to per form the focus sweep you need to optimise the amplitude of the signal by adjusting the signal generator output and offset from your power supply You may not need to touch the settings for testing the remaining photodiode segments Repeat the testing procedure for each of the photodiodes A through F All should produce fairly similar signals say within 20 per cent of one another in amplitude If A B C and D or E and F differ from one another by more than say 20 per cent there is a serious optical align ment problem in the pick up Else the photodiode array may be bad Partially shorted phot
23. time Now identify the photodiode segments Very often the connections are marked on the circuit board for example there may be several labeled test points designated junction drop in the forward direction and very high resistance in the reverse direc tion If you use a digital multimeter in diode testing mode the junction drop is typically 0 7 V There may be a very slight difference between the readings for seg ments A to D and those for E and F An initial test of photodiode response can be made by using an external light source a flashlight or other incandescent bulb or IR remote control to illuminate the lens directly from above With the mul timeter connected to reverse bias each di ode segment illuminate the lens The re sistance reading should drop possibly dra matically Segments A to D should show reasonably similar sensitivities but these may differ for segments E and F which should be similar to each other Similarly with the photodiode connec tions restored to normal you can use an oscilloscope to monitor the RF test point A source of IR directed towards the lens from above may result in a detectable change in MAINTENANCE photodiode array Any unusual reading such as a signifi cantly lower resistance for one of the di odes a short or open diode a short be tween diodes or variations in sensitivities is an indication of a problem This is some what unlikely though bad solder connec t
24. unit in and turn it on On portables you need to defeat the door interlock by using a tooth pick or bit of cardboard Some CD play ers have a disk detection sensor separate from the laser assembly This needs to be defeated for this test to work without a CD in place If it is a simple optical sensor a piece of black tape or paper would suffice Once a CD is in place and the play button is pressed the laser must be pow ered You can detect this in a darkened room because there is usually a very faint ELECTRONICS FOR YOU APRIL 2004 R1 PD SENSE LD1 LD DRIVE ct J ccommon LASER DIODE ASSEMBLY VR HA a PD1 PD FOCUS B FOCUS DATA coi a PD F l a D PD oh q PD 5 TRACKING E TRACKING i 4 F TE K BIAS l PHOTODIODE CHIP FOCUS TRACKING ACTUATORS A F S PDI or TO SCOPE DIODE R2 27K Fig 42 IR detector circuit red light emission which you can see as a tiny red dot of light if you look at the lens from a safe distance of at least 15 cm 6 inches at an oblique angle Do not look into the lens directly from above or from very close as the invisible IR is much stronger than the faint red emission and potentially hazardous If you see the faint red light power is being applied to the laser diode With the laser lit the lens should go through a few focus search cycles typi cally two to eight While it
25. wer or in some cases actually remove the clamper In a por table or boombox the lens is readily ac cessible Unplug the CD player from the ELECTRONICS FOR YOU APRIL 2004 AC socket or remove the batteries as you don t need to use its internal power With power supply switched off or the variac turned all the way down connect the 24SWG leads to one of the located pairs of coils Now turn on the power and slowly adjust the variac or rheostat while watching the lens If you are con nected to the focus coil you ll see the lens moving up and down If you are con nected to tracking coil you ll see it mov ing from side to side If there is no motion turn off the power supply reverse the polarity and try again For a typical pick up the 4 5V power sup ply and a minimum of 22 ohm resistor should cause the lens to move through the entire range of motion up and down or side to side as appropriate Once you have exercised the first coil switch connections and repeat for the other If the motion is jerky the lens assembly is dirty Clean the lens assembly carefully first tracking and increase the needed servo driver power A CD player that is overly sensitive to slight disk defects even after all the proper adjustments have been per formed may conceivably be a result of this type of fault An additional symptom may be an unusually hot servo driver IC However many of these ICs run hot nor mally so don t wo
26. yer has a rotary positioner there may be no separate tracking coil as coarse and fine tracking may be combined Typical linkages between the lens coil assembly and the body of the pickup are a sliding shaft focus and rotation on the shaft tracking or a hinged hinge With a sliding shaft and rotation on the shaft the slide can get gummed up preventing reli able focus and tracking With a hinged hinge one or both hinges may break as these are often made of thin flexible plas tic Repair is not really possible To test whether the lens is focusing or tracking properly perform the following First identify the cable leading to the fo cus and tracking voice coil mechanism This is usually a 4 conductor cable sepa rate from the data and laser cable at least at the pickup end Disconnect it from the main board before testing Using a digital multimeter you can locate the pair of coils with very low resistance a few ohms One of these is focus coil and the other is tracking coil Construct one of the following test cir cuits 1 Take a 4 5V DC eliminator Con nect a variac at its output and a 22 ohm 1W resistor in series with a pair of 50cm 24SWG insulated wires 2 Take a 5V DC power supply con nected in series with a 100 ohm variable resistor and 22 ohm 1W fixed resistor with a pair of 50cm 24SWG insulated wires Gain access to the lens for visual in spection For this you need to eject the disk open the dra
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