I am working on another pair of L08M's , one of which has some fried resistors. In checking the transistors around these, I removed Q25 and Q27 only to find they are both NPN, Q25 being a 2SC1279 and Q27 being a 2SC2378, both of which are identified as such in the tranistor listing of the circuit.
However, the schematic of the circuit shows an NPN/PNP pair in a symmetrical arrangement, with the other pair of Q26 being a 2SC2378n and Q28 being a 2SA1023.
Am I missing something here?
Peter
btw circuit attached
However, the schematic of the circuit shows an NPN/PNP pair in a symmetrical arrangement, with the other pair of Q26 being a 2SC2378n and Q28 being a 2SA1023.
Am I missing something here?
Peter
btw circuit attached
Something sure is wacky.
I would trust the transistor sexes indicated in the schematic; an NPN installed in the Q25 site would never conduct.
With PNP in Q25, there's symmetry and the circuit at least appears sensical.
Good luck!
I would trust the transistor sexes indicated in the schematic; an NPN installed in the Q25 site would never conduct.
With PNP in Q25, there's symmetry and the circuit at least appears sensical.
Good luck!
if your real product doesn't work and it also doesn't match the schematic, doesn't that point to the problem?
@mlloyd1 - may well be, though I have had schematic and circuit conflicts before, including with these amps where some circuitry is not on the schematic. I will certainly try replacing Q25 and that may solve the problem.
@BSST- that was my conclusion, and I did not check Q25 on the other amp, which does work, and is mainly reassembled, to see if was configured the same and as they are a PITA to get the boards out from, so I am avoiding that at present
thanks
@BSST- that was my conclusion, and I did not check Q25 on the other amp, which does work, and is mainly reassembled, to see if was configured the same and as they are a PITA to get the boards out from, so I am avoiding that at present
thanks
Though I do believe Q25 should be PNP, I'll be pleasantly surprised if changing Q25 fixes the problem.
Follow-up thought. I think Q25 is part of over-current protection and don't believe it would work anyway as currently stuffed. You might consider removing Q25 as a test.
Follow-up thought. I think Q25 is part of over-current protection and don't believe it would work anyway as currently stuffed. You might consider removing Q25 as a test.
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BSST, I agree with you, Q25 should be 2SA1023, complement of the 2SC2378, also service manual of the amp doesn't have any details of the q31 and q32, so maybe checking in part list is the option to verify these mistakes.
Thanks, pitbul. I really appreciate having extra eyes scrutinizing this issue.
The OP claimed that both the found installed part and documentation specified the NPN transistor, and that only the schematic indicated PNP in the Q25 position. Your comment prompted me to double-check.
I confirm that I believe pwdiya12 is correct--- both the tabulation alongside the schematic and parts detail from the service manual specify the NPN 2SC1279.
Extrapolating from my own habits (perhaps a dangerous assumption), I think engineers think in schematics, and documentation is a tedious chore, especially in the era when this design occurred. So I imagine the schematic is correct, but the supporting build data erroneous. I appreciate the review.
Best,
Steve
The OP claimed that both the found installed part and documentation specified the NPN transistor, and that only the schematic indicated PNP in the Q25 position. Your comment prompted me to double-check.
I confirm that I believe pwdiya12 is correct--- both the tabulation alongside the schematic and parts detail from the service manual specify the NPN 2SC1279.
Extrapolating from my own habits (perhaps a dangerous assumption), I think engineers think in schematics, and documentation is a tedious chore, especially in the era when this design occurred. So I imagine the schematic is correct, but the supporting build data erroneous. I appreciate the review.
Best,
Steve
As others have said, the diagram is correct, it can't be any other way...
The 09 is similar but has the correct info.
The 09 is similar but has the correct info.
Thanks @BSST , @pitbul and @Mooly - my assumption (always dangerous) was that the circuit was correct, however these units are original, in that there does not appear to have been any work done on them, and the 2SC2378 was installed-which prompted me to reach out to the guru's.
I do not have any 2SA1023, so would a 2SA992 suffice?
I will remove Q25 2SC2378 and see if makes any difference to my current problem (which I suspect is unrelated to this)- The three heat stressed/fried resistors which were R65-open, R57-open and R52-1k- still OK but heat stress marks. Currently I have -ve 40v on the collector of Q20 and subsequently on the output. All the main outputs and drivers test good and are disconnected
Thanks again.
Peter
I do not have any 2SA1023, so would a 2SA992 suffice?
I will remove Q25 2SC2378 and see if makes any difference to my current problem (which I suspect is unrelated to this)- The three heat stressed/fried resistors which were R65-open, R57-open and R52-1k- still OK but heat stress marks. Currently I have -ve 40v on the collector of Q20 and subsequently on the output. All the main outputs and drivers test good and are disconnected
Thanks again.
Peter
It very occasionally can happen that a product is fitted with an incorrect part. The stand out one for me was some Toshiba TV's fitted with a 1N4148 instead of a ZTK33B (a precision temperature compensated 33 volt reference for varicap tuners). Needless to say they all had tuner drift.
I would test your amp with those transistors not fitted but do use a bulb tester to limit current.
I would automatically replace the drivers and also Q29 and Q30. Check ALL the low value resistors in the output stage including the 4.7 ohm base stoppers. Turn VR2 to minimum resistance before testing... and use a bulb.
D8 and D9 (these are packs of series diodes) are often under suspicion in many makes and models.
I would test your amp with those transistors not fitted but do use a bulb tester to limit current.
I would automatically replace the drivers and also Q29 and Q30. Check ALL the low value resistors in the output stage including the 4.7 ohm base stoppers. Turn VR2 to minimum resistance before testing... and use a bulb.
D8 and D9 (these are packs of series diodes) are often under suspicion in many makes and models.
Thanks @Mooly - Will do as you suggest and report back- and all with DBT inline
All the 4.7ohm are OK, as are all the other low value resistors. My suspicion is D8 or D9, as I removed D20 to disconnect that side of the 71v rail and still -ve voltage on Q20 and R51, however will replace Q29/Q30
Peter
All the 4.7ohm are OK, as are all the other low value resistors. My suspicion is D8 or D9, as I removed D20 to disconnect that side of the 71v rail and still -ve voltage on Q20 and R51, however will replace Q29/Q30
Peter
So- D9 and D8 test OK. If I remove D9 and D12 there is still -ve rail voltage on Q20.
Q29/Q30 have been replaced.
Q25 is now a KSA992
Q22 collector should be -ve 1.8v and is at -ve rail voltage. Q19 collector is around -ve 8v and Q19 collector is around +ve 8v, should be 5.3v for both and both transistors test OK out of circuit.
A wise man once said when looking at rail voltage where it should not be, "it will be open or closed", that is something is either open of shorted.
Q29/Q30 have been replaced.
Q25 is now a KSA992
Q22 collector should be -ve 1.8v and is at -ve rail voltage. Q19 collector is around -ve 8v and Q19 collector is around +ve 8v, should be 5.3v for both and both transistors test OK out of circuit.
A wise man once said when looking at rail voltage where it should not be, "it will be open or closed", that is something is either open of shorted.
well progress- @pitbul kindly helped me track down a relay problem on another pair of L08M's with this advice
"
I would remove Q25 and Q26 and pull up R51 and R52 one leg from the board.
After that you know for sure where the problem is."
Having now done this, the relays now work, however there is -ve rail voltage on Q20 and Q22.
"
I would remove Q25 and Q26 and pull up R51 and R52 one leg from the board.
After that you know for sure where the problem is."
Having now done this, the relays now work, however there is -ve rail voltage on Q20 and Q22.
You can link these out for testing. The amp should work normally (apart from increase in distortion) due to no bias current.
Do not remove them and try and power the amp up as that will cause massive output stage and driver current. Short them out and get the amp working.
There are some quick and dirty tests we can do on the output stage. You can lift the left hand end of R51 and R52 so they are isolated from Q20 and Q22 and then join those free ends of R51 and 52 together. Now connect that junction of the two resistors to ground. This applies zero volts to the driver stage input and you should see close to zero volts on the output line before the speaker relay.
If you see a rail voltage on the output line after doing that then it looks like you still have an issue around the driver/outputs.
Thanks, however I am now confused.
I am testing the section prior to the output stage, with Q25/Q26 removed and R51 and R52 lifted. The relays click in and there is 0v on the output. The drivers ans output transistors are not connected. The rail voltage is around 62-64v rather than the stated 71v.
I have replaced Q19/Q20 and Q21/22, however I still have -ve 53v on the collector of Q20, which should have +ve 1.7.
So I am trying to work out where and why the -ve voltage is appearing on the positive side (Q20)
Q2/Q3 and Q9/Q10 measure correctly as does Q12/Q13. However Q15 and Q17 both have -ve 7v on the emitter and -ve 7.7 on the base (the correct figures are -ve2.7 and -ve3.3 respectively).
On Q14 and Q16, both emitters start with +ve 0.4v and then change to -ve 2.4v and on their bases, they start at +ve 1.1v and then swing to -ve 1.7v.
I have tested the zener diode (D7) out of circuit and it is fine. D5/D6 measure ok in circuit. and R88 is getting warm
So two questions- why is there an additional 5V or so on the emitters and bases of Q15/Q17?
and why does Q14/Q16 go form positive to negative?
Does this mean something is not getting turned on?
Thanks for the assistance
Peter
I am testing the section prior to the output stage, with Q25/Q26 removed and R51 and R52 lifted. The relays click in and there is 0v on the output. The drivers ans output transistors are not connected. The rail voltage is around 62-64v rather than the stated 71v.
I have replaced Q19/Q20 and Q21/22, however I still have -ve 53v on the collector of Q20, which should have +ve 1.7.
So I am trying to work out where and why the -ve voltage is appearing on the positive side (Q20)
Q2/Q3 and Q9/Q10 measure correctly as does Q12/Q13. However Q15 and Q17 both have -ve 7v on the emitter and -ve 7.7 on the base (the correct figures are -ve2.7 and -ve3.3 respectively).
On Q14 and Q16, both emitters start with +ve 0.4v and then change to -ve 2.4v and on their bases, they start at +ve 1.1v and then swing to -ve 1.7v.
I have tested the zener diode (D7) out of circuit and it is fine. D5/D6 measure ok in circuit. and R88 is getting warm
So two questions- why is there an additional 5V or so on the emitters and bases of Q15/Q17?
and why does Q14/Q16 go form positive to negative?
Does this mean something is not getting turned on?
Thanks for the assistance
Peter
Hi Peter,
Q14 through Q17 form four constant current sources. If you find about 6VDC between the base of Q14 and the base of Q15 (i.e. 6V across the string of D5,D6,D7), that's loose evidence the current sources are probably working. The annotated base and emitter voltages around those four transistors don't have any direct reference re ground, but relative difference voltages between bases and emitter should be consistent.
I like Mooly's technique of severing the amplifier loop by lifting R51 and R52--- and the fact you don't find any large output stage currents is encouraging. However, one drawback is that bias from amp output back to the gate of Q4 is inoperative because the feedback loop is open. The disabled loop may be cause of the stuck Q20 collector. As a workaround, I suggest tacking a feedback resistor (about 12k to 15k) from Q4 inverting gate (i.e. junction of Q4, R30, etc.) to the collector of Q20. If the front end is working properly, Q4 collector should bias to about 0V. If it doesn't bias as hoped, the technique should provide a debug facility to trace the frontend defect.
If Q20 bias doesn't then show about 0V and you get stalled, report Q20 and Q22 collector voltages, and input and feedback gate voltages of Q4.
Q14 through Q17 form four constant current sources. If you find about 6VDC between the base of Q14 and the base of Q15 (i.e. 6V across the string of D5,D6,D7), that's loose evidence the current sources are probably working. The annotated base and emitter voltages around those four transistors don't have any direct reference re ground, but relative difference voltages between bases and emitter should be consistent.
I like Mooly's technique of severing the amplifier loop by lifting R51 and R52--- and the fact you don't find any large output stage currents is encouraging. However, one drawback is that bias from amp output back to the gate of Q4 is inoperative because the feedback loop is open. The disabled loop may be cause of the stuck Q20 collector. As a workaround, I suggest tacking a feedback resistor (about 12k to 15k) from Q4 inverting gate (i.e. junction of Q4, R30, etc.) to the collector of Q20. If the front end is working properly, Q4 collector should bias to about 0V. If it doesn't bias as hoped, the technique should provide a debug facility to trace the frontend defect.
If Q20 bias doesn't then show about 0V and you get stalled, report Q20 and Q22 collector voltages, and input and feedback gate voltages of Q4.
Just quickly looking at this
All voltages in the front end will be miles off with the output stage disconnected because you have no DC feedback. All we were trying to do with a quick check on the output stage was to veryify that there was no obvious issue with outputs and drivers.
Not quite sure what you mean by:
They must all be fitted and connected. With outputs and drivers fitted and the base of the drivers ground referenced (by lifting those resistors and connected as I suggested) you should see zero volts at the output. If you get that far then we can reconnect the output stage (the two resistors) and carry on fault finding.
All voltages in the front end will be miles off with the output stage disconnected because you have no DC feedback. All we were trying to do with a quick check on the output stage was to veryify that there was no obvious issue with outputs and drivers.
Not quite sure what you mean by:
They must all be fitted and connected. With outputs and drivers fitted and the base of the drivers ground referenced (by lifting those resistors and connected as I suggested) you should see zero volts at the output. If you get that far then we can reconnect the output stage (the two resistors) and carry on fault finding.
@pwdiya12, look at similar theme L08M relay problem, my explanation about contacts of the switch (post #60), after cleaning the switch there are no more problems with this amp. Maybe info about output voltage (or read it there is no 0Vdc on output) and correction try to force output to be 0V.
So I will direct you to clean S2 switch (if you don't have sum sensor wire from the speaker).
Good luck.
So I will direct you to clean S2 switch (if you don't have sum sensor wire from the speaker).
Good luck.