Okay I just took another measurement:
V_C83 = 0.34 V
V_C84 = 1.29 V
V_Q25CE = 1.85 V
V_Q26CE = 2.04 V
So there's quite a difference at the voltages and as you said it's less than 1.2 volts at the cap. I desoldered the cap and measured it: 0.7 nF and infinite resistance. Not quite sure what capacitance it's meant to be, in the schematic it says: "0.068 100", not sure what this means, but I guess it's alright.
I also swapped Q25 with another new one, still the same result.
V_C83 = 0.34 V
V_C84 = 1.29 V
V_Q25CE = 1.85 V
V_Q26CE = 2.04 V
So there's quite a difference at the voltages and as you said it's less than 1.2 volts at the cap. I desoldered the cap and measured it: 0.7 nF and infinite resistance. Not quite sure what capacitance it's meant to be, in the schematic it says: "0.068 100", not sure what this means, but I guess it's alright.
I also swapped Q25 with another new one, still the same result.
Strange result... not sure what to make of that tbh.
One dark thought is that there is some issue with the output transistors and that they are really low gain and so taking more base current than the chip can deliver, but that theory sort of falls down due to there being insufficient voltage in the first place to get the transistors to conduct.
Hmmm... what if the transistors has inbuilt resistors across B and E. Such devices do exist.
Dark thoughts. Ebay and semiconductors isn't a great combination as there are numerous fake parts around.
The only way to prove this would be to swap the outputs with the good channel but that invites causing more issues.
It might be worth doing a critical voltage measurement here. Do you see R83 and R85, the 10 ohm resistors to the base of the outputs. If you measure the voltage across
these resistors you can calculate the base current. Lets see if the bad channel is sucking lots of drive current from the chip by measuring that and comparing to the good channel. If it is then the voltage across these resistors will be higher on the bad channel.
One dark thought is that there is some issue with the output transistors and that they are really low gain and so taking more base current than the chip can deliver, but that theory sort of falls down due to there being insufficient voltage in the first place to get the transistors to conduct.
Hmmm... what if the transistors has inbuilt resistors across B and E. Such devices do exist.
Dark thoughts. Ebay and semiconductors isn't a great combination as there are numerous fake parts around.
The only way to prove this would be to swap the outputs with the good channel but that invites causing more issues.
It might be worth doing a critical voltage measurement here. Do you see R83 and R85, the 10 ohm resistors to the base of the outputs. If you measure the voltage across
these resistors you can calculate the base current. Lets see if the bad channel is sucking lots of drive current from the chip by measuring that and comparing to the good channel. If it is then the voltage across these resistors will be higher on the bad channel.
How does that compare to the good channel ?
0.07 volts is actually 7 milliamps. That may sound nothing, but its more than you would expect and should be biasing the output transistors on pretty hard I would have thought. A typical discrete stage might only have 1ma or less flowing there and yet still have 100ma or more flowing in the outputs.
I'm suspicious of the output transistors at this point tbh.
(It might be worth lifting the 10 ohms on both channels and comparing B-E readings for resistance. You never know what it might show)
0.07 volts is actually 7 milliamps. That may sound nothing, but its more than you would expect and should be biasing the output transistors on pretty hard I would have thought. A typical discrete stage might only have 1ma or less flowing there and yet still have 100ma or more flowing in the outputs.
I'm suspicious of the output transistors at this point tbh.
(It might be worth lifting the 10 ohms on both channels and comparing B-E readings for resistance. You never know what it might show)
Oops, made a mistake. The values on the right are the right (working) channel.
no problem.
One thing you could try in order to get a handle on this...
If you adjust the preset to MINIMUM bias (which is max resistance for the pot) so that the voltage across C83 is at its LOWEST possible value you could then try tweaking R81 (2.4k) to a slightly lower value. That would allow Q25 to turn OFF a little more and so develop a higher bias voltage. You could try 1k8 (but no lower at this point) and see if that allows the bias voltage to be set correctly.
I recommend using a bulb tester while looking for this fault.
If you adjust the preset to MINIMUM bias (which is max resistance for the pot) so that the voltage across C83 is at its LOWEST possible value you could then try tweaking R81 (2.4k) to a slightly lower value. That would allow Q25 to turn OFF a little more and so develop a higher bias voltage. You could try 1k8 (but no lower at this point) and see if that allows the bias voltage to be set correctly.
I recommend using a bulb tester while looking for this fault.
I bought Q25 at Kessler-Electronic.de. Q26 is the same and works. Also the problem appeared before I swapped the transistor so I don't think that thats the problem. I also swapped Q21 and Q23 with those of the other channel. No success. Also tried your trick but theres still no measurable bias voltage...
Does the tweaked value of R81 allow more voltage to be developed across C83. Does it get nearer to the 1.2 volts needed to begin to turn the outputs on ?
So all the semiconductors have been proved OK by substitution including the IC. The next step would be to carefully go around every pin of the IC and look at the voltage present and compare it with the good channel. Look for obvious differences.
Areas to look at would be:
R19 and D3
R75
C59 (not shorted with a blob of solder ?)
Check the print to and from these parts by comparing voltages. Make sure there is no open print around the IC to any of these parts.
So all the semiconductors have been proved OK by substitution including the IC. The next step would be to carefully go around every pin of the IC and look at the voltage present and compare it with the good channel. Look for obvious differences.
Areas to look at would be:
R19 and D3
R75
C59 (not shorted with a blob of solder ?)
Check the print to and from these parts by comparing voltages. Make sure there is no open print around the IC to any of these parts.
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