I started to test Q31..Q38 C and E as @BSST suggested, although with Q25 and Q26 removed. I also looked at B-E for Q31..Q38.
Unsurprisingly Q31..Q38 all have -ve 34v, Collectors are all at (reduced) rail voltage and B-E 0.6v
What was surprising was that the dim bulb started glowing with the current draw rising from 0.2A to 0.5A in a cycle. This only started to occur when I started measuring these voltages to ground (TP23).
I also checked the output of Q4 at the junction of R13 and there is +ve0.9v, whereas it should be 0v
Unsurprisingly Q31..Q38 all have -ve 34v, Collectors are all at (reduced) rail voltage and B-E 0.6v
What was surprising was that the dim bulb started glowing with the current draw rising from 0.2A to 0.5A in a cycle. This only started to occur when I started measuring these voltages to ground (TP23).
I also checked the output of Q4 at the junction of R13 and there is +ve0.9v, whereas it should be 0v
That seems to indicate the current in R52 is passing via Q26 into the output stage. Q25 and Q26 should be 100% off. The only way to turn them on is via the volt drop across R74 and R75... and there should be zero volts across those 0.47 ohm emitter resistors with it in this no bias state.
So removing Q25 and Q26 just takes all that of the equation. We can come back to those. Leave them out.
When you lifted R51 and R52 and grounded those resistors we had virtually no offset, just a few hundred millivolts. That shows the output stage isn't shorted in some way, if it were you would have a very high offset.
We need to fault find in the normal way now. Make sure the amp power up OK, the bulb should be dim or out. If the bias in the other channel is causing it to light then turn the bias on that good channel to zero.
So it should power up, you should have reasonable voltage rails.
What DC offset do you see in that condition?
If I could write my opinion which I didn't post, I was surprised with voltage of 0.88V on mid point of the amp when R51 and R52 are lifted and grounded.
To me it is some of the transistor is conducting, do not know which one, because I don't understand what measuring were done, with reference to the GND or the mid point of the amp. So I suggest that should be mentioned with measurement.
Also I suggest that emitter resistor should be checked out of the board, there is parallel 0,47 ohm resistor across them, so maybe there is no balance on the output stage.
It will be good to take measurement on R37 and R38, also R62 and R63, if you are not sure which side of the resistor is on the mid point, simply measure across each one and make a note, it will tell us which side is conducting. I assume that one of the side is not providing current to the mid point because of the failed component (maybe emitter resistor).
For sure it will be good to know which polarity is on the base - emitter resistor, side which is connected to the base should be on NPN side positive (red on base side), and on PNP side will be negative with red on the base side of the resistor. After that we will know what is going on.
To me it is some of the transistor is conducting, do not know which one, because I don't understand what measuring were done, with reference to the GND or the mid point of the amp. So I suggest that should be mentioned with measurement.
Also I suggest that emitter resistor should be checked out of the board, there is parallel 0,47 ohm resistor across them, so maybe there is no balance on the output stage.
It will be good to take measurement on R37 and R38, also R62 and R63, if you are not sure which side of the resistor is on the mid point, simply measure across each one and make a note, it will tell us which side is conducting. I assume that one of the side is not providing current to the mid point because of the failed component (maybe emitter resistor).
For sure it will be good to know which polarity is on the base - emitter resistor, side which is connected to the base should be on NPN side positive (red on base side), and on PNP side will be negative with red on the base side of the resistor. After that we will know what is going on.
Good thinking and you're absolutely correct 👍 and this was my thinking 🙂
The output node should be totally isolated and cut off in that state and at zero volts. I'm taking a calculated gamble at that point that the 0.88 volts is coming via the feedback route and lifting the now floating node and I'm also gambling that if there was some conduction somewhere in the driver/output stage that it would generate much more of an offset.
These are quick and dirty tests just to try and get a handle on what is going on.
When you're done, I'll have an L1000M and a KA1000 to entrust to you, I'm too lazy to stick my hands in it to redo the eight billion solder joints 😆
@pitbul - measurements as requested.
R37 (end that connects to R32/R35) -43v/(end that connects to Q15/Q17) -3v
R38 (end connected to Q17) -2v/(end connected to Q16) +2v
R62- I replaced R62 as it did not measure (ohm) correctly and split in two when I removed it both sides measure -34v
R63- I think you mean R65- and both sides measure -34v
I lifted each of the 0.47ohm resistors and the 4.7ohm resistors and all measured as they should.
On the outputs with the red probe to base the PNP all measured negative and the NPN positive- however as soon as I measured the first transistor the overall current rose form about 0.3A to 0.45A with the bulb glowing, even with the probes removed this continued with a gentle rise and fall between 0.3A and 0.45A, about 1 per second.
@Mooly - I checked the protection diodes and all measure correctly. I have not removed one, but can do so
I hope this assists, for the diagnosis. I do not understand why the power draw is pulsing, unless something is being turned on and off and being slowed by the current limiting of the bulb.
On R62- I did measure it in circuit before, and it did measure OK.
Peter
R37 (end that connects to R32/R35) -43v/(end that connects to Q15/Q17) -3v
R38 (end connected to Q17) -2v/(end connected to Q16) +2v
R62- I replaced R62 as it did not measure (ohm) correctly and split in two when I removed it both sides measure -34v
R63- I think you mean R65- and both sides measure -34v
I lifted each of the 0.47ohm resistors and the 4.7ohm resistors and all measured as they should.
On the outputs with the red probe to base the PNP all measured negative and the NPN positive- however as soon as I measured the first transistor the overall current rose form about 0.3A to 0.45A with the bulb glowing, even with the probes removed this continued with a gentle rise and fall between 0.3A and 0.45A, about 1 per second.
@Mooly - I checked the protection diodes and all measure correctly. I have not removed one, but can do so
I hope this assists, for the diagnosis. I do not understand why the power draw is pulsing, unless something is being turned on and off and being slowed by the current limiting of the bulb.
On R62- I did measure it in circuit before, and it did measure OK.
Peter
I have been checking and redoing joints as I go, but will do all of those I have not already done
pwdiya12, I need to write down your results on schematic to see what is going on. I am in Europe, so, now is too late to work...
Tomorrow I will have some conclusion, if Molly is faster, maybe he will give you the answer.
Tomorrow I will have some conclusion, if Molly is faster, maybe he will give you the answer.
@BSST- no, all the measurements are taken with R51 and R52 installed and Q25 and Q26 removed
I replaced Q31 and Q32 (MJE15034/35)- this made no difference
In then replaced D4, D5, D6 and the 5.1v zener D7.
This has resulted in TP16 measuring -61v rather than -34v before
I also check Q17 out of circuit and on the transistor tester it is fine
I replaced Q31 and Q32 (MJE15034/35)- this made no difference
In then replaced D4, D5, D6 and the 5.1v zener D7.
This has resulted in TP16 measuring -61v rather than -34v before
I also check Q17 out of circuit and on the transistor tester it is fine
I'm not sure that would have been apparent to Mooly and pitbul--- I wrongly thought they were still grounded.
I believe the cycling bulb tester behavior is because of interaction of front-end time constants with the thermal time constants of the light bulb--- their interaction must form sort of a relation oscillation. I believe if you returned R51 and R52 to the grounded configuration, you wouldn't see this behavior and the fault would be more "stable" and apparent.
Sorry, but I recommend repeating your measurements with R51 and R52 grounded.
I believe the cycling bulb tester behavior is because of interaction of front-end time constants with the thermal time constants of the light bulb--- their interaction must form sort of a relation oscillation. I believe if you returned R51 and R52 to the grounded configuration, you wouldn't see this behavior and the fault would be more "stable" and apparent.
Sorry, but I recommend repeating your measurements with R51 and R52 grounded.
@BSST - no problem- The current state is that Q25/Q26 are removed
I just replaced D3 and with the drivers (Q31/Q32) installed and no output transistors- Relays click!!- progress!
I have just installed the 2SC outputs, one by one- and the the relays click!
When I plug in the 2SA- I get the bulb coming on- so there is a short in the output section involving the PNP transistors
Thanks for your perseverance
Peter
I just replaced D3 and with the drivers (Q31/Q32) installed and no output transistors- Relays click!!- progress!
I have just installed the 2SC outputs, one by one- and the the relays click!
When I plug in the 2SA- I get the bulb coming on- so there is a short in the output section involving the PNP transistors
Thanks for your perseverance
Peter