Lecson Technical Service Manual (AC1 and AP1)
Contents
1. 3 Distorted Output This will be due either to an internal fuse or power board component failure Check all joints on the board of the affected channel test all semiconductors in circuit using an ohm meter 4 High Hum level Check all earthing to board inputs Look for dry joints or broken tracks associated with CA and C5 on the affected channel 5 Crackling Inspect base plate and power board for dry joints check for leaking capacitors particularly C2 and C3 C6 and C7 Test for leaky transistors particularly in early stages 6 Massive failure Indicated by several charred components on a board and possibly high offset voltage at the output Replace all visibly damaged components and all semiconductors on the board or exchange board AP2 Differences A small number of AP2s serial numbers 5030 to 5100 approx were produced with different circuit and insulation details The testing of these units 1s identical to AP1 and quiescent current should be set to 30mA AGE ASL 4001 LH LLY 95 vSdn Sul AGE DIE LC TO A AA AAKE AMON A ES ON2. RAD AUXI or 2 are selected TR21 is switched on and the voltage on T8 is OV 200mV Selection of PUL or 2 allows DI or D2 to conduct switching off TR21 and the voltage on T8 rises to 12V 3V Normally 1t 1s not possible to select two or more inputs at once this condition would be the result of a failure on the pcb or bad magnet alignment Tape monitor TR22 and 23 Normally the amplifier 1s operated in the monitor off position SW6 closed In this mode TR22 and 23 produce control voltages for routing the signal from the input amplifier to the volume control For inputs RAD AUXI and AUX2 the voltage on T6 is OV 200mV and on T7 is 18V 3V For inputs PUI or PU2 the voltage on T6 is 18V 3V and on T7 is OV 200mV When monitor on 1s selected SW7 closed TR22 and 23 are both off and the voltage on T6 and T7 1s 18V 3V Remote In the normal manual mode TR24 is switched on allowing the input to be selected via SW1 5 Pushing the monitor switch to the remote position switches off TR24 and thus disables the input selector Inputs may be selected via the 5 pin socket at the rear D5 ensures that in the remote mode the tape monitor 1s off Input amplifier low noise This stage comprises TR28 30 32 34 36 and 66 Left TR29 31 33 35 37 and 67 Right The selected input signal which appears at T9 T10 Right is amplified by this stage and the output signal appears across R124 R125 Right The input ampli
3. current limit of 100mA to assist work on faulty boards 3 An output circuit consisting of zoebel network 5 6 Q and 0 1 5uF 2 5A fuse and switch able 8Q load 4 Input lead connected to variable output oscillator 5 Means for connecting an ammeter into either supply lead It is important that the board is not allowed to over heat on this jig and great care must be taken at all times When working on the heat sink changing driver or output transistors ensure that the insulation 1s not damaged If in doubt on re assembly please refer to Fig 3 Before switching on check that there is not continuity between any transistor and the heat sink tab Setting Quiescent currents It 1s not possible to adjust the quiescent current with the amplifier assembled 1t will therefore be necessary to set the unit out on the bench as described in section 2 3 l Connect only the board in the transformer half and insulate the flying leads which connect to the power board in the free half 2 Remove the internal 10A fuses fitted to the transformer half 3 Plug the unit into a mains supply and switch on 4 Check that 40V AP1 or 50V AP2 is present on the reservoir capacitors 5 Insert a multimeter set to 1A f s d in the fuse holder for the ve supply and check that the current 1s less than 10mA 6 Remove the multimeter switch off the amplifier and fit the ve 10A fuse 7 Insert the multimeter set to 1 A f s d in the ye fuse holder sw
4. current of each half must be adjusted as described in Section 4 2 Push the base plate on to its correct location on the transformer half taking care not to disturb any insulation and not to trap any wires Use one screw to hold this in place 3 Stand the amplifier on its base and offer up the free half extrusion and then slide the halves together about 1 4 Pull all the free wires to the top of the amplifier 5 Solder the red and black supply wires to the tags on the power transistors the red wire to the 2N 3442 the black to the BDX18 80 BDX20 6 Slide the free half down 3 4 and push on the middle connection purple wire 7 Push the free half down to the base plate taking great care that no wires are trapped between the extrusion and base plate 8 Fit the input push connections to the top of the power board and insulate any exposed part of this connection with P V C adhesive tape 9 Carefully insert the top plate ensuring that the speaker output connections on the power boards are not deflected by the capacitors 10 Replace all the screws in top and bottom plates 11 Fit new adhesive tape to the perspex top disc and press this into place with the LECSON logo adjacent to the left hand seam on the extrusions Circuit Description Power Supply The main circuit includes a thermal cut out switch and a 2 SA anti surge 20mm fuse The transformer has a dual 1 20V primary which is either connected in series or
5. is of the normal feedback type with the feedback network of VR2 and VR3 applied around the high gain bootstrapped amplifier stage formed by TR58 and TR60 The d c working point is controlled by the feedback network formed by R206 R208 and R250 Normally for 500mV rms output the shorting link across R204 is fitted removal of the link reduces the feedback and raises the output to 1 2V rms The output of this stage appearing across R210 1s fed to the cancel switch Cancel switch TR62 and 64 are series switches controlled by TR27 In the normal out mode TR27 is switched on thus TR62 is switched on and TR64 switched off and the signal coming out of the tone control stage is fed through to the balance control VR4 and hence to the output socket In the cancel mode TR27 turns off switching TR62 off and TR64 on In this mode the direct signal from the volume control stage 1s fed through to the balance control and output Power Supply Each channel of ACI has a regulated 30V supply of a constant current type This is used because of its good regulation and the high degree of protection offered to the circuits in a fault condition TR25 and 26 provide the constant currents each of 27mA 5mA the rail voltage is stabilised by ZD1 and ZD2 A 18V rail used for switching 1s derived from the raw d c across C33 via R74 ZD3 and C32 Dismantling 1 Ensure that all slider controls are fully retracted to the front edge of the unit 2 Pla
6. is possible to change the internal 1 OA 114 fuses 3 Invert the amplifier and remove the six self tapping pozidrive screws from the base plate The base plate may now be pulled to one side to operate on the base plate circuits Beware of mains voltage if the unit is switched on 4 With the base plate sitting in its normal position stand the amplifier on its base and pull the top plate up and away towards the transformer half 5 Unclip the top most connections input lead on the power board in the free half right channel 6 Slowly slide the free extrusion half upwards 2 with early serial numbers some persuasion with a soft faced hammer may be necessary to move the free half 7 Remove the middle connection to the power board in the free half 8 Slowly slide the free half upwards a further 3 4 9 Unsolder the red and black leads from the tags on the power transistors 2 3 2 4 3 0 3 1 10 The power board is now disconnected and the extrusion half can be completely detached Working on the unit The unit 1s best serviced with the amplifier opened up in this way 1 Lay the transformer extrusion half curved side downwards on a soft surface 2 Arrange the top and bottom plates so that the cable form is not strained and no cables or contacts short to each other or the casing 3 Lay the free half alongside the transformer half Re assembly I If the boards have been worked upon or exchanged the quiescent
7. top plate Common failure to switch would be either fracture of the switch casing or mis alignment of the magnets It is extremely important that the magnets on the top plate are placed in the correct orientation with respect to each other and in the correct position Observe this location carefully before removing any magnet If the magnets are not in the correct position and or orientation the switches may fail to operate or may interact from side to side 5 4 Constant Current Power Supply In order to achieve high regulation a constant current supply is used with zener diode stabilisation The service engineer must be aware of the fact that fault conditions tending to cause a stage of the amplifier to drain too much current may result in a collapsed or oscillating rail voltage and that this does not necessarily indicate a failure of the supply 5 5 Double sided Printed Circuit Board ACls up to serial number 1300 approx used a double sided p c b with griplet or pin connections between the two sides of the board The griplet and or pin system proved to be somewhat unreliable typical symptoms are 1 Crackling and or intermittent contact between sides of the board 11 Channel fading or low frequency disturbances 111 Microphony Amplifiers of serial number from 1300 approx onwards use a through hole plated board which is much more reliable No problems should be encountered but great care must be taken not to damage the tracks whe
8. 1 2 Input amplifier low noise TR28 37 3 Volume control TR38 45 4 Low filter TR46 49 5 High filter TRSO 57 6 Tone control TRSS 61 7 Cancel facility TR27 TR62 65 8 Power supply TR25 26 Input switching and control logic Five inputs are provided PUI and 2 are low level inputs for magnetic cartridge source and in these positions the input amplifier 2 2 1s arranged to have an RIAA playback response Three inputs RAD AUX1 and AUX2 are provided each with an input sensitivity of 125mV and a flat response 1s selected on the input amplifier All the signal routing in ACI is effected by series FET switches The series FET switch is ON when the instantaneous voltage between the gate source and drain is very small lt 100mV The switch is OFF with a gate source or gate drain pd of more than 4V In ACI any intermediate state is carefully avoided by the use of good quality coupling capacitors either tantalum bead or polycarbonate and so avoiding polarisation of the switches by leakage currents The consequence of a voltage remaining on any terminal of the FET between 160mV and 4V would be indeterminate switching and high distortion of 2nd order The PU inputs use single series FETs and the flat inputs employ dual series FETs This 1s arranged to give good input isolation and normally at 10kHz isolation between a desired and undesired input should be gt 55dB with desired input o c gt 100dB with desir
9. 3 B or i i s 108 UC734E B ie ALL PNP EXCEPT TR26229 N Bn C13 20 000 n volt BC214L C or 1 100 BC24K D or N assi MEO412 D ALL NPN EXCEPT TR 25426 i R29 cr r3 BC184L C or EU i BC184K D or i BC109 D t LEFT E OUT CAPACITORS RESISTORS RS i ALL CARBON FILM EXCEPT Were MARKED i 1 3 WATT 5 a an Eu ee pr TANTALUM BEAD TYPES i R ALL 470kn eg 2 50 2pF50V U AUX 2 i FERRITE 5 BEAD 1 Klo L E E E O EE e ae NN OI i Mes a pe IA ji mese aa gt es eo E A TT as O LE E AM 18V 1 SWITCHES PUI D1 de IN4148 22125 N BC301 870 _ spy RI 4 Hi i N4148 Pn o ge in 133 D9 en RD Qe 18V 400mW T36 30V aa 1 i E e i dia Y pe Cr R72 ca LED k 2225 _ c28 EN Vos 0135 10 R73 C33 30V 30V l aux an E 8 iita fi i 148 gt tic MB so IA JIN O a SEHE INPUT SWITCHES AND LOGIC O SEN ENEE SINI Yso cle ue E IAGRAM A ACI CONTROL UNIT CIRCUIT DIAG AP1 2 Power Amplifiers 1 0 2 0 2 1 22 General Instructions API and AP2 are high powered direct coupled audio power amplifiers enclosed in black anodised aluminium extruded cases the case also acts as the heat sink for the output transistors The power amplifiers have an input sensitivity of 500mV rms and are suitable for use with any signal source in the audio band Unconditional stability ensures that any load is suitable although reduced output is availa
10. Technical Service Manual lecson Lecson Audio Ltd Burrel Road St Ives Cambridgeshire England PE17 4LE Telephone St lves 0480 64508 Telex 32562 AB Lecson Stlves Contents 1 ACI Pre Amplifier 2 AP1 2 Power Amplifier SANA UN AR U ACI PreAmplifier 1 0 2 0 2 1 General Description The ACI control unit is a self powered pre amplifier of advanced design all input and output connections are standard DIN with the exception of the connection to the power amplifier which is via an 8 pin DIN socket This manual gives all circuit details and descriptions in sections 2 and 3 and it is recommended that these sections be read before commencing a repair Section 4 gives full testing and fault finding procedures and section 5 describes details and fault conditions which are unusual or particular to the electronics of the AC 1 Any components may be ordered direct from LECSON for servicing purposes Questions on service will be answered by the service department at St Ives Tel No 0480 64508 Circuit Description Circuit diagram for ACI The heavy line on the diagram illustrates the path taken by a signal fed into the RAD input left channel The circuit is unusually complex for a control unit because of the electronic switching which involves 36 field effects and 7 bipolar transistors Description of the circuit treats the control unit in blocks as follows 1 Input switching and control logic TRI 24 TR38 4
11. V 50mV rms 11 Increase the input to the ACI and reduce the volume control setting towards 0 keeping the output at 500mV mis Check that serious waveform distortion does not occur below 200mV rms input and that there are no clipping effects with an input of 800mV rms 12 Reduce input level to 2 6mV and set volume control for an output of 50mV rms about 4 Feed in an input of 2 6mV at 10kHz and check that the output is 13dB 1dB on 1kHz level Feed in an input of 2 6V at 100Hz and check that the output is 13dB 1dB Repeat this test for left and right channels 13 Reset input to 2 6mV 1kHz and set volume to maximum 8 Switch the tape switch to monitor and check that left and right channels have no output 14 Measure the tape output signal appearing on pins and 4 of the AUX1 or 2 sockets Check that this 1s 25 5mV rms 15 Reset the tape switch to off check that left and right channels reappear at the output 4 3 16 Set input to PU2 and note that the output of both channels disappears 17 Move the input lead from PUI socket to PU2 Note that the signals reappear at 500mV 50mV in the left and right channels 18 Set input switch to PUI and note that the output of both channels disappears 19 Set input switch to RAD Connect input lead to RAD with a signal of 100mV rms Check that an output of 500mV 50mV is present left then right 20 Move input switch to AUXI Check that the output signal has disap
12. ble for loads less than 402 or greater than Q Mechanical Details Description The API 2 is constructed from two semi cylindrical aluminium ribbed plates which slide together to form the cylindrical body of the power amplifier The whole is clamped together by the top and bottom circular aluminium plates which are each fixed by six self tapping pozidrive screws There is in addition a perspex top disc fastened to the top plate by double sided adhesive tape One half extrusion carries the mains transformer line fuses bridge rectifier and thermal cut out switch this is called hereafter the transformer half Both halves each carry a channel sub assembly of printed board and integral heat sink hereafter called power board The transformer half carries the left channel power board The top plate supports the two reservoir capacitors and the bottom plate carries the 7 pin DIN input socket the speaker outlets fuse holders and cut out relay The top plate transformer half and bottom plate are connected by a cable form Dismantling Switch Off 1 Remove the perspex top disc with a rubber sucker or alternatively by carefully inserting a knife blade between the edge of the disc and the body and slowly levering the disc away 2 Remove the six pozidrive self tapping screws The top plate now may be removed and set on one side taking care not to short out the reservoir capacitors in order to inspect the inside In this form it
13. ce the amplifier face down on a soft surface e g felt 3 Remove screws A 4 Carefully raise the amplifier off the top plate for a distance not exceeding 2 5 Placing hand between top plate and amplifier extract light emitting diode from its housing in top plate 6 The bottom plate is taken off by removing the five flange head fixing screws B Top Plate Assembly This 1s a laminated assembly which cannot be dismantled without special tooling therefore should any part of the above become damaged 1t will be necessary to replace in total Polished Cover Plate This item is held in position by double sided adhesive tape which can be softened by gently warming allowing the cover to be peeled off A replacement cover plate can be obtained from the factory complete with adhesive tape Fitting Remove protective paper backing from tape and offer new cover plate up to amplifier Ensure that cover plate is correctly positioned before pressing into place 4 0 4 1 Replacement of Sliders Should a slider become damaged the following procedure 1s to be followed After removing the top plate assembly as detailed above 1t will be necessary to remove the magnet carriers These are held on with an cyano acrylate adhesive AVDELBOND C and the only way of breaking the bond is by applying a sideways leverage After removing the magnet carriers the sliders can be withdrawn from the front Note the orientation of the magnet ca
14. e AP1 2 via a suitable attenuator The headphone outlet on ACI is not used all the time in order that switching anomalies if they exist on the low filter and cancel slider may be tested The tester can perform a useful check in the absence of any signal by listening to the background noise in the unit and by injecting hum into each input with a screwdriver blade but the full test requires a flat 100mV signal and a disc source of speech or music The primary functions of the listening test are To establish operation of both channels in cancel and out modes To establish operation of tone controls high filters low filters headphone mute To check operation of input selection circuits and equalisation switching To check tape monitor switching To check volume control 4 2 To establish a reasonable quality and quantity of background noise and hum Bench Testing of the Complete ACI The following equipment will be necessary to test the ACI Audio range oscillator with output variable between ImV and 1V rms sine and square Oscilloscope 1MHz with sensitivity of at least 200mV cm Electronic a c millivoltmeter 1 mV 1V rms To check noise and distortion in addition Low distortion oscillator t h d 0 01 Distortion factormeter capable of reading down to 0 01 t h d at 1kHz DIN shorting plug Inputs are connected to ACI using five DIN plugs and the output taken from an 8 pin DIN plug Some method should be avai
15. ed input fed from lt 1KQ For example in the PUI switch TRI R7 and R82 hold the d c potentials of the source and drain respectively to OV 50mV Cl ensures that the gate and source of the device follow 22 each other maintaining the switch on in the ON mode and off in the OFF mode The control voltage on the gate is fed from C3 via R1 When the voltage at ZI is OV the signal flows through the FET TRI when the voltage at ZI is 18V the signal at pin 3 cannot enter the amplifier TR19 and 20 shunt the input signal when the amplifier is turned off normally in use the gates are tied to the 18V switching supply and have no effect on the operation of ACI A tape monitoring facility is available when the monitor is switched on the signals on the AUX2 inputs are routed to the volume control stage via TR17 and 18 The control logic performs the following functions Input logic DI D2 and TR21 and SW1 5 This sets the input FET switch control voltages which appear on capacitors C3 T1 C6 T2 C11 T3 C16 T4 C21 T5 The desired input e g PU2 has OV 50mV on T2 and the potential of the other points T1 T3 T4 T5 is ISV 3V The selected signal is then able to enter the low noise stage see 11 and the output of the stage is always available at the tape output regardless of the monitor switch position DI D2 and TR21 produce the control voltage which adjusts the frequency response and gain of the input stage When
16. en seconds to change from PU mode equalization to flat r Switch to AUX1 Check by injecting hum that RAD left and right have switched off Check that AUX1 left and right have switched on s Switch to AUX2 Check that AUXI left and right have switched off and that AUX2 left and right have switched on Further Optional Tests using a signal source t Set controls as in test 2 Connect pick ups to PUI socket and advance volume control Check that noise 1s smooth and at a reasonable level Place pick up on record and adjust a comfortable listening level u Repeat tests 4 to 11 inclusive checking that the controls give the desired effects v Set tape switch to on note than music disappears after a small delay w Connect a flat 100mV rms signal to the AUX2 socket and check that this signal is correctly reproduced with the tape monitor on and that switching to monitor off restores the original disc signal after a delay x Switch to PU2 observe that the disc signal disappears y Connect disc signal to PU2 and check that both channels are correct with the mode switch still set to stereo z Using the flat 100mV signal check that RAD AUX 1 and AUX2 switch on and off correctly Testing of PU inputs 9 Connect an input lead to the PUI socket of the ACI Set all controls as for step 1 and then switch the input to PU 1 10 Feed in an input of 1kHz sinewave 2 6mV rms and check that for left and right channels the output is 500m
17. fier has a gain of 1 2X or 2dB when RAD AUX and 2 are selected and an RIAA response with a gain of 10X or 20dB 1kHz when PUI or 2 are selected Four transistors form the basic amplifier TR28 and 32 are common emitter gain stages with high frequency correction applied by C47 C51 C53 The output of TR32 across R110 is buffered by the emitter follower TR34 which has a constant tail current supplied by TR36 2 3 2 4 2 5 2 6 The d c working point is maintained by d c feedback applied via R120 R118 and R108 to the emitter of TR28 In the PU mode TR30 is switched on and TR66 off Here a c negative feedback is supplied by the network of R120 R118 C57 CSS and R96 In the flat mode TR30 is switched off and TR66 on the feedback network being in this case for a c TR66 and R106 The input stage 1s capable of producing an output of 28V p p with PU and monitor off selected Volume control The volume control stage is of the shunt feedback type with variable feedback applied around the gain stage TR42 and 44 by the volume control VRI TR42 operates as a high gain bootstrapped common emitter amplifier and its output is buffered by TR44 which acts as an emitter follower The d c working point is controlled by the potential divider R142 and R144 and by the d c feedback resistor R146 The signal fed to the base of TR42 the virtual earth 1s fed either through TR38 R128 R134 for PUI or 2 or th
18. icates all the control settings of 4 2 1 D C Tests Quiescent d c levels are shown on the circuit diagram These should be checked using a 20 0000 V multimeter at the appropriate test points Switching voltages can be checked against the following test table Input Switching PUI selected T1 OV 50mV T2 T5 12V 5mV T6 tape off 15V 3V tape monitor 15V 3V T7 tape off OV 50mV tape monitor 15V 3V TS 15V 3V PU2 selected T1 15V 3V T2 OV 50mV T3 T5 12V 5V T6 tape off 15V 3V tape monitor 15V 3V T7 tape off OV 50mV tape monitor 15V 3V TS 15V 3V RAD selected 11 2 12V 5V T3 OV 50mV T4 5 12V 5V T6 tape off OV 50rnV tape monitor 15V 3V T7 tape off 15V 3V tape monitor 15V 3V TS OV 50mV AUXI selected T1 3 12V 5V T4 OV 50mV T5 12V 5V T6 7 8 as RAD AUXz2 selected T1 4 12V 5V T5 OV 50mV T6 7 8 as RAD Tape Switching T6 T7 see Input switching Voltage on C24 tape off 12V 5V tape monitor OV 50mV Mode Check voltage across C117 OV 500mV High Filter Voltage measured at two points out fl f2 f3 Junction of R172 and 173 12V 5V OV 12V 5V 0 5V 0 3V Junction R176 and 177 12V 5V 12V 5V OV 0 5V 0 3V Low Filter Set to Out no magnet applied T29 OV 50mV Junction R75 and R76 10V 5V Junction R154 and RI55 12V 5V Set to cancel T29 15V 3V Junction R75 and R76 OV 50mV Junction R154 and RI55 OV 5V Set fl T29 OV 50mV Junction R75 and R76 10V 5V Junction R154 a
19. ions on uncasing of the ACI It is often advantageous to test the AC uncased during fault finding In addition to the a c tests carried out in Test T2 it is of course possible to measure the quiescent d c voltage throughout the circuit N B It 1s of course not possible to measure noise and hum with the unit uncased With ACI uncased the general procedure is as below Place the board the right way up on a foam block or other protective surface taking care that nothing can short the underside of the p c b At all times take extreme care because mains voltages are exposed at the rear right hand corner around the outlet socket The mains supply is switched on by holding the microswitch button depressed This 1s best done with adhesive tape The slider controls can be easily moved The switching which is normally activated by the magnets in the top of the ACI has to be effected using free magnets and four magnets are supplied with this manual Remember to remove these magnets before recasing The procedure for activating any function 1s to rest a magnet on top of the appropriate reed switch note that several switch positions have no reed Thus to set up the ACI as for test 1 in section 4 2 the controls would be as follows Place a magnet on the RAD input reed Middle of left row Place a magnet on the tape off reed Set volume slider to maximum Set the other 3 slider controls half way Switch the microswitch on This now dupl
20. itch on and check that the current is less than f s d 8 Adjust VRI on the power board for a current of 135mA API or 30mA AP2 after 30 seconds If any difficulty 1s experienced disconnect the blue output lead and readjust 9 Switch off and connect the following leads to the board in the free half Input screened lead red supply lead to 2N 3442 Do not connect output lead 10 Insert multimeter set to 1 A f s d between the ve fuse holder and the BDX18 BDX20 solder tag 11 Switch on and set current as described in step 8 12 Switch off and discharge the reservoir capacitors with a 10A 2W resistor 13 The amplifier can now be reassembled as described in 2 4 Likely Faults 1 No output at all Check all fuses mains supply and control unit connecting lead Check that loudspeaker switch is set to the correct position Ensure that the muting relay is not 4 5 operating switch off and listen for 1t dropping out Check that there 1s zero offset voltage at the output terminals Check internal fuses 2 No output one channel Check as in 1 ensure that there is not a large d c offset this indicates major failure of power board Remove top plate and ensure that the input signal tags are not shorted together or shorting to the top plate itself common fault Remove bottom plate and check for continuity from the board outputs top of speaker fuses to the output terminals Inspect board for dry joints or burnt out components
21. lable for simply changing the signal from left to right input and switching the oscilloscope and meter from left to right output Procedure 1 Set AC1 switches as follows Input RAD Tape Off Volume 8 Treble 0 Bass 0 Balance 0 Mode Stereo High F Out Low F Out Phones On Power On Connect an input lead from the oscillator to the RAD input connect the output of the ACI to the oscilloscope voltmeter and DFM 2 Feed in a sinewave input of 100mV into the left channel check that the output voltage 1s left 500mV rms Check that the waveform on the oscilloscope is a clear sinewave Check that this disappears when balance control is moved to right 4 Return balance control to 0 Switch output to right check that the crosstalk output is less than 5mV rms To ensure that no intermittent connections are present To ensure that the switching of ACI does not interact magnetic or electrical faults A flow chart testing procedure 1s given below Follow through all the tests before attempting a repair so that all faults may be isolated 1f possible 3 Feed the same signal into the right channel Check that the output signal on the right channel is 500mV 50mV rms Check that this disappears when the balance control is moved to left 4 Return balance control to 0 Check that output of left channel is less than 5mV rms So far 1t has been established that the amplifier is working with the right gain and that all the stages seem to functi
22. n removing a component 5 6 Modifications Mains click and radio interference Units up to serial number 1200 were not fitted with components R217 218 221 222 226 227 C117 118 or ferrite beads on TR28 29 42 43 These components can be added to units to reduce the susceptibility to outside interference and are available in sub assembly form from LECSON 3 3 gt D 3 2 SIGNAL INPUT R R104 100k LECSON AUDIO LTD BURREL ROAD ST IVES HUNTINGDON Tite IC CORRE SPOND W PONENTS eg TR4BLEFT TR49RIGH USES E ING ODD NUMBERED COM T CHANNEL IS IDENTICAL NOT SHOWN HAND CHANNEL AMPLIFIER RIGH DRG No 30V z U U I U i i U i I I I I U i I OV ora I C85 Y i 100n Re 1 100k i i i c87 i 100n 3 i 1 ov c89 i R162 110725 Fre erro Er N E E 8k2 i i R154 i i i i I I I N i EN 1 SN API Dr u E i i 18V i R155 i f t n i N i HIGH I OPTION It i 13 i A OO SEE A AN i eee RS akat SD CE alee eirer ran TAPE OUT LEFT i TE OU Te ee N 11 O e A RMAL OUTPUT OUTPUT CIRCUIT SHOWING ER woo 2 Fund osaa I CEL OUTPUT ds TT TRANSISTORS i 2k2 1no0 IR22 CANCEL OUTPUT D komma ren i i BF244B A or R2 DV LECSON 6 A or i All voltages shown MEF 10
23. nd R155 50V 50mV A C Tests Using the general procedure outlined at the beginning of section T3 all a c tests may be carried out as in section T2 In addition for the controls set as described at the beginning of section T3 refer T2 1 the following a c voltages are considered normal for an output of 500mV rms T9 10 100mV 25mV rms 2 6mV 5mV rms T11 12 120mV 27mV rms 28mV 5mV rms T13 14 98mV 25mV rms 2 4mV 0 5mV rms T15 16 OV rms 27mV V rms T17 18 ImV rms approx T19 20 550mV 50mV rms T21 22 550mV 50mV rms T25 26 520mV 50mV rms T2 510mV 20mV rms With PUI or 2 input arranged for 500mV rms output the voltages shown in brackets would apply 5 0 Special Details Using the test procedures described in section 4 it will be possible to locate the area of any fault condition and to solve 1t by normal techniques The ACI does however have some unusual features which on first acquaintance may be confusing Jol Field Effect Transistors The FET used in the AC1 is an n channel junction FET Common failure modes are Short circuit drain source gate This results in switches being on even when switched off Typical symptoms would be An input not switching off and breaking through on all input positions Lack of RIAA equalization or incorrect gain in any mode TR66 38 40 Low gain on all inputs TR19 20 Filter failure TR50 53 Cancel failure e g dead or heavily distorted in the cancel or ou
24. off low filter switch to out treble to 0 h Move treble to 4 and observe equal cut in brilliance on both channels Reset treble to 0 check that bass control gives boost and cut I Reset bass control to 0 Check that mode switch gives mono in the mono mode This will be observed by a central image of the noise j Reset mode to stereo Check that high filter gives progressively more out in brilliance in both channels as it is advanced through flto f3 Reset to out k Check low filter gives deep bass cut set to fl Reset to out Set tape switch to on observe that the noise levels drop considerably Set tape switch to rem observe small rise in hiss level m Set tape switch to on check that AUX2 inputs are functional by injecting hum with a screwdriver blade or similar into pin 3 left then pin 5 right n Set tape switch of off check that both AUX2 inputs are disconnected by injecting hum as in test 13 Correct result should give no hum o Check by injecting in the same way that the left and right inputs of PUI pin 3 and 5 are connected and that PU2 left and right are not connected p Switch to PU2 note low frequency switching noises are normal at full volume Check that PU left and right are connected and that PUI left and right are not connected q Switch to RAD Check by injecting hum that both channels left and right pin 3 and 5 are connected N B The circuits take up to t
25. on 4 Increase the input signal and ensure that both channels will deliver 2V rms at least 5 Feed a square wave 1kHz into the left RAD input Adjust the input to give 0 5V pKc pk output Check that the output waveform is clear with no overshoot and with flat tops 6 Repeat test 5 for the right channel 7 To test quickly the functioning of the tone and filter controls make the following tests and in each case refer to the diagram which shows the desired output waveform Repeat for left and right channels a Set treble control to 4 refer to Fig 2 b Switch low f to cancel Check that original square wave 1s restored at about 500mV pk pk c Reset treble to O low f to out d Set treble to 4 Refer Fig 3 e Reset treble to 0 f Set bass control to 4 refer Fig 4 g Set bass control to 4 refer Fig 5 h Set bass control to 0 Switch mode to mono Check that output is halved and that this appears in both channels I Reset mode to stereo Set high filter to f 1 refer Fig 6 I Set high filter to f2 refer Fig 7 k Set high filter to f3 refer Fig 8 1 Set high filter to out m Set low filter to f 1 refer Fig 9 For more accurate testing of these functions the following performance is expected of these controls Sinewave input of 10mV rms 10kHz Treble control should give 9dB at 4 9dB at 4 Set treble control to 0 High f should give the following responses Out OdB fl 3dB at 10kHz f2 5dB a
26. parallel depending on the local supply See instruction manual Some export models are fitted with voltage selector switches The secondary circuit consists of a bridge rectifier feeding two 10 000pF 6 800pF AP2 reservoir capacitors The capacitors are connected to the boards via two 10A 1 4 fuses fitted internally The supply is 40V API 50V AP2 2 4 0 4 1 4 2 Power Board The amplifier 1s a directly coupled class B circuit using complementary output triples The output circuit TR acts as a voltage follower with high power gain and low distortion Each half of the input signal is handled by the two halves of the input and driver TR for the ve half and TR for the ye half Quiescent voltages are shown in the circuit diagram Negative feedback 1s applied through R R and C RI and C1 form an input filter The quiescent current is adjusted by a VR to 136mA for APi and 30mA for AP2 Tantalum capacitors are used throughout to achieve high d c stability Protection against short circuit loads or adverse load conditions is provided by the delayed V I limiters TR and fly back is protected through DI and D2 Output Circuits The base plate carries the output circuits the zobel networks R and C headphone alternators R67 70 muting relay RLI and output fuses and the A AB B output switch Testing Procedures Complete Amplifier 1 Asacomplete unit the amplifier is tested in the normal way Input signals are fed into
27. peared left then right isolation should be gt 60dB 21 Move input plug to AUX socket Check that an output of 300mV 50mV is present left then right 22 Move input switch to AUX2 Check that the output signal has disappeared left then right 23 Move input plug to AUX2 socket Check that an output of 500mV 50mV rms is present left then right 24 Move input switch to AUXI Check that output signal has disappeared left then right 25 Move tape switch to monitor Check that an output signal appears of 450mV 50mV left then right 26 Move tape switch to remote Check that the output disappears left then right This completes the functional testing of ACI Measurement of noise hum and distortion 27 Set the controls of ACI as in 1 Connect a low distortion signal of 100mV rms to the radio input 1kHz Make a fine adjustment of the input to get an output of 500mV Measure the total harmonic distortion and check that this 1s less than 0 05 28 Remove input plug and fit a shorting plug to the RAD input Measure the noise and hum content of the output of the ACI Check that the combined noise and hum is less than 70dB for both channels 29 Move the shorting plug to the PUI input socket and switch input to PUI Check after 20 seconds that the noise is less than 70dB and that noise and hum are less than 60dB Bench Testing of the cased printed circuit board of ACI Refer to section 3 for instruct
28. rough TR40 and R132 for RAD AUXI or 2 When the monitor on is selected TR38 and 40 are off and the AUX left signal 1s fed in via TR17 and R132 R124 and R136 maintain the switches TR38 and 40 with OV 200mV d c on drain and source The output from the volume control stage at T21 is fed to the low filter stage and to the cancel switch Low filter The low filter is a 2nd order active filter formed by C85 C87 C89 TR48 and R166 this filter is disabled by switching on TR46 which shorts the signal across C85 and 87 With the filter off the gate of TR46 is tied to OV by R156 switching the filter on SW9 applies 12V 3V to the gate of TR46 switching this transistor off allowing the filter to operate High filter This stage TR50 52 54 and 56 is a 3rd order active feedback filter TR54 and 56 form a super emitter follower and the filter frequencies are selected by SW13 15 With this high filter off TRSO and 52 have gate voltages of 12V 3V and the frequency response is tailored by C93 C99 and C97 Selection of F switches TR50 on making C91 effective Selection of F switches TR52 on making C95 effective Selection of F switches TR50 and 52 on simultaneously The mono switch SW 10 parallels the left and right channels by connecting R182 to R183 via C117 Tone control The signal appearing at the output of the high filter stage at T25 is fed into the tone 24 2 8 3 0 3 1 3 2 3 3 control stage This
29. rrier as it is important that this is replaced accurately during reassembly The replacement of sliders is the reverse of operations one and two In the case of sliders having ball click position it will be necessary to lift the ball retaining spring to allow the slider to be inserted If the adhesive stated above is not available an epoxy resin 1 e Araldite may be used to secure the magnet block to the sliders Note It s important that if magnets are removed from their respective holders then polarity must be maintained in the same direction 1 e N S N S N S Testing Procedure If the ACI is returned with a description of the fault then refer to the table of typical faults section iv d In order to check a fault or to diagnose a fault condition three testing procedures are outlined Listening test of the ACI complete with AP1 2 Bench testing of the complete ACI Bench testing of the uncased printed circuit board of the AC I Listening Testing A full listening test when correctly carried out can apart from providing very quickly an indication that all functions of the ACI are working correctly be a very useful technique for diagnosing and pin pointing any fault condition and is the only useful way of tackling intermittent crackles etc This test is best carried out with the ACI connected to a AP1 2 via the usual connecting lead and the listening carried out with the use of headphones connected to the speaker outlets On th
30. t 10kHz f3 dB at 10kHz change to input of 10mV rms 100Hz Bass control should give 10dB at 4 10dB at 4 Set bass controls to O Low filter should give 1dB at f 1 8 Procedure for Listening Test Follow through the tests a z in order a Connect ACI to main supply and to API Connect API mains lead to ACI outlet Connect headphone to API as described above and put on phones b Set the controls as follows API speaker switch to AB ACI input to PUI Monitor off Volume 0 Treble 0 Bass 0 Balance 0 Mode Stereo High f out Low f out phones off Power on Result Background and hum should be inaudible red light indicating operation of power supply to ACI should be alight c Advance volume control to number 8 a fairly loud rumbly hiss should be heard in both channels the level of hiss and rumble need not be the same in both channels Fit a shorting plug to the PUI socket socket pins 3 5 2 and note that the level of noise drops considerably in both channels and that a small amount of hum can be heard Remove shorting plug d Advance treble control to 4 observe correct change in noise character in both channels e With treble set to 4 check that original sound is restored in both channels when low filter switch is set to cancel f Move phone switch to on check that the unit stays in the cancelled mode Move phone switch to mute and check that both channels become silent g Return phone to
31. t mode TR62 65 Open circuit drain source This will not allow the transistor to switch on Typical symptoms would be An input not switching on TR1 16 RIAA equalization on RAD AUXI or AUX2 TR66 67 No output either in cancel on or off TR38 41 Filter failure TR46 47 No output in one of the cancel modes but correct output in the other TR62 66 Oscillating in the off mode This oscillating takes place in the off mode and is of very high frequency gt 10mHz It manifests itself as a loud hiss in the affected channel which disappears when the particular FET is switched on Typical solutions would be Loud hiss on all inputs affected by volume control TR19 20 Loud hiss on all but one input In this case the oscillating FET is in the input switch of that one input Hiss on filter out high or low but disappears in a mode fl f2 or f3 or fl low TR46 47 50 53 Hiss on cancel or out TR62 65 Rumbling in the off mode This is due to the leakage around the FET junction and occurs in the same situations as c above IMPORTANT Before suspecting or changing a FET ensure that the control voltages on the gate are correct as described above 5 2 Light Emitting Diode All the supply current in the left hand channel passes through the LED The common failure mode is an internal short circuit so that the amplifier will work with no visible light 5 3 Reed Switches These sealed switches are activated by magnets carried on the
32. the 7 pin DIN socket on the base plate left line to pin 1 right line to pin 2 and ground to pin 3 2 The sensitivity of the units is such that 500mV rms input will give I 6V rms output APi or 22V rms AP2 3 Check that the amplifiers will produce at least SOW per channel into 8 2 both channels driven APD or 90W into 8 2 AP2 4 Check that the distortion is within the specified limits at 35W for API or 70W for AP2 S Check that the static d c offset voltage at the output terminals of the power amplifiers is below 250mV 6 Check that the output is switched off when pin 7 of the DIN socket is connected to ground this tests the relay 7 Test the speaker switch A AB B Power Board It is possible to run the amplifier in the dismantled state if it 1s laid out as described in section 2 3 but the need for great care cannot be over emphasised Firstly mains voltages are exposed on the transformer and secondly the power supply is powerful enough to do considerable damage if it is accidentally shorted out Some repair work can be done on a power board in this mode but it is not practical to remove and replace the board many times as this is a very time consuming and tedious operation If there is any doubt at all about working on a power board place on a jig which provides the following 4 3 4 4 I Quick release clamps on to the heat tabs to hold the board and remove heat 2 40V supplies 50V AP2 with optional
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