I'll have to read it all the posts later but this seems worth pursuing.
The three pairs of outputs are in parallel. For testing you need only fit one pair, any combination, just one NPN and one PNP.
If the output stage had a short that should have showed in the earlier tests.
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I would fit one pair of outputs and reconfirm your latest problem is still present.
If that happens then apply a short (bit of wire) across C17. This feeds both drivers from the same point and as such no DC fault should be able to draw current in the output stage.
The exceptions to that are if there is a DC path from the output node to ground or the amp suddenly bursts into very high frequency oscillation and then there is a low impedance path via the Zobel network.
@pwdiya12 ,
Your measurement was not what I was asking, because I wrongly wrote resistors, instead of R37/38 it should be R57/58 and R62/63, but already I think that you find the problem. Molly sugested that you make short on place of C17, after that you don't have any bias and any of the transistors shouldn't be conducted, if you have something on mid point like you said, than you have leaky some of the output transistors. If it is when you put output PNP transistor into circuit and you have negative voltage on mid point, obviously you have problem there.
When you detect which PNP transistor it is (remove it) and if there are no DC voltage on mid point, you should decide will you replace all of the transistors with new one, I think that they have to be similar gain (paralled device should share similar amount of current).
For sure I would replace C17 with new one.
Your measurement was not what I was asking, because I wrongly wrote resistors, instead of R37/38 it should be R57/58 and R62/63, but already I think that you find the problem. Molly sugested that you make short on place of C17, after that you don't have any bias and any of the transistors shouldn't be conducted, if you have something on mid point like you said, than you have leaky some of the output transistors. If it is when you put output PNP transistor into circuit and you have negative voltage on mid point, obviously you have problem there.
When you detect which PNP transistor it is (remove it) and if there are no DC voltage on mid point, you should decide will you replace all of the transistors with new one, I think that they have to be similar gain (paralled device should share similar amount of current).
For sure I would replace C17 with new one.
@Mooly and @pitbul
thanks for the analysis.
I will short C12 as suggested and test with a single NPN and PNP.
I have replaced all the electro caps (with the exception of the main power supply caps which test OK),
My simplistic view is that if the relay closes with Q31/Q32 installed (the originals were replaced- the main difference being the Ft of the originals is 200Mhz and the replacements are 30Mhz), and the NPN's (2SC) also close the relay, then the issue has to be on the -ve power rail to the outputs under load, that is with the output devices installed.
One observation is that originally the current limiter had the 2SC installed rather than the 2SA (now rectified), so the current limiter did not work and thus the fried resistors, again predominately on the -ve rail. Consequently something on that rail is failing, so given I have replaced Q21/Q22/Q26/Q30/Q32 and the output stage has previously "worked" , I will replace Q28 and pull Q23 and test out of circuit.
I will report back after doing this in (my) afternoon
Thanks again
Peter
thanks for the analysis.
I will short C12 as suggested and test with a single NPN and PNP.
I have replaced all the electro caps (with the exception of the main power supply caps which test OK),
My simplistic view is that if the relay closes with Q31/Q32 installed (the originals were replaced- the main difference being the Ft of the originals is 200Mhz and the replacements are 30Mhz), and the NPN's (2SC) also close the relay, then the issue has to be on the -ve power rail to the outputs under load, that is with the output devices installed.
One observation is that originally the current limiter had the 2SC installed rather than the 2SA (now rectified), so the current limiter did not work and thus the fried resistors, again predominately on the -ve rail. Consequently something on that rail is failing, so given I have replaced Q21/Q22/Q26/Q30/Q32 and the output stage has previously "worked" , I will replace Q28 and pull Q23 and test out of circuit.
I will report back after doing this in (my) afternoon
Thanks again
Peter
Possibly. This is what you need to prove.
With the outputs removed the drivers (Q31 and Q32) will function like output transistors. The feedback loop is maintained in this state and the DC output voltage should be zero volts.
The voltage across R65 (is it R65, bit hard to make out, its the 33 ohm to the emitter of Q32) will determine if the PNP outputs will conduct or not. You should have almost zero volts DC across the two 33 ohm resistors with C17 shorted (the cap across the bias generator). If you see approx 500mv or more then you are at the point the main output transistors will conduct.
Sadly with one of the 2SA installed the bulb glows. I let it run up to about 0.7A and the emitter of the output device was +ve 1.5v, the base was -ve 0.7 (and rising).
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The 144mV across R65 is concerning. It's only about 4.4mA, but why/where is the current flowing? It shouldn't be there in post #66.
I would remove the 2SA and take more data: Voltages at bases and emitters of Q29 though Q32 re ground.
I would remove the 2SA and take more data: Voltages at bases and emitters of Q29 though Q32 re ground.
Ok, very strange.
Are any of the six output transistors present?
Would you report voltages at collectors of Q20 and Q22, bases of Q29 and Q30, bases of Q31 and Q32, emitters of Q31 and Q32, TP16.
Are any of the six output transistors present?
Would you report voltages at collectors of Q20 and Q22, bases of Q29 and Q30, bases of Q31 and Q32, emitters of Q31 and Q32, TP16.
no outputs are present
Collector Q20/Q22 0v
Base Q29 183mv Q30 -11mv
Emitter Q31 0.9v Q32 -0.9v
TP16 0v
Collector Q20/Q22 0v
Base Q29 183mv Q30 -11mv
Emitter Q31 0.9v Q32 -0.9v
TP16 0v
Thanks.
None of the readings make any sense to me.
I have to think on this and return with some sensical questions.
P.S. Would you report collector voltages on all the above transistors?
Thanks!
None of the readings make any sense to me.
I have to think on this and return with some sensical questions.
P.S. Would you report collector voltages on all the above transistors?
Thanks!
In you post #74, you omitted voltages for bases Q31 and Q32. Would you report what you measure?
Thanks.
Thanks.
I'm going to say that the next step has to be to use a scope and to check that what you see here is DC or.......
If it is AC (such as oscillation) then that will show inaccurately on a DVM. We need to know what that voltage is.
I don't know if this will prove much but you could also try measuring voltage across R89 and R90 which are the 10 ohm in the Zobel network. Use the most sensitive DC and AC voltage ranges on your meter. You should see 0.0000 volts on both DC and AC ranges. If you read anything at all then that suggests AC is present on the output (instability).