Cougar C500 4 distorted audio on 3rd channel

[Mote]

I received this amp with blown output transistor on 3rd channel. Replaced them both TIP41C and TIP42C. One of the drivers was also shorted (C1815), replaced that as well. Tried it, but audio was very distorted. Found that driver (A1015) for the other output was leaking. Replaced that as well.

Now audio on 1st 2nd 4th channel is good, amp does not draw current, nothing gets hot.

But audio on 3rd channel is still distorted, higher frequencies not so much, but bass is quite bad.

What Ive done so far:
Replaced all opamps associated with 3rd channel
Changed outputs one more time
Replaced emitter resistors
Checked 100uF caps, all read near 100uF
Measured resistances and voltages in audio section of 3rd channel and compared results to 4th channel. Results were similar.
Looked for burnt traces bad connections. Resoldered some jumpers and legs of components.
Resistance from RCA shield to non bridging speaker terminal is 0 ohms.

Distortion is still there and still the same.

What to try next?

Picture of audio sections of 3rd and 4th channel. 3rd is with 5w emitter resistors (temporarily installed to see if emitter resistors were causing the distortion).

[Perry Babin]

With a sine wave driven into the amp (50-100Hz), do you have the same AC voltage on pins 1 and 7 of the op-amp indicated in the attached image?

[Mote]

Using 50Hz sine wave from tutorial CD and meter set to 200V AC (I have only 600V and 200V AC to choose from) I get the following results:
Pin1: 1,2V
Pin7: 0,0V

[Perry Babin]

The problem would appear to be in the preamp section but let's make sure that the amplifier stage is OK. There are two 1k resistors passing the signal from pins 1 and 7 to the inputs of the two power amplifiers (ch 3 and 4?)? Swap them so that pins 1 and 7 feed the opposite channel. If the distorted channel plays perfectly, you know that the problem is in the preamp section.

[Mote]

There are 1k resistor and 12mm jumper near the opamp marked with yellow arrow. I made the switch there so that jumper connects to channel 4 and resistor connects to channel 3.
Things got very strange: 4th channel plays perfectly but not at the same volume at what other channels play although position of gain pot is the same. 3rd channel plays as loud, distortion is still there but now there is an added bonus. If I switch the channel to LPF mode, it does not play at all, just pushes the speaker cone out (sorry did not measure DC on speaker terminals). Speaker cone stays pushed out as long as amplifier is turned on.

Does that make any sense at all? That should indicate that there are problems in both sections. Starts to remind me of the Auna amplifier which had an issue with opamp.

[Perry Babin]

Check all of the op-amps to see if any have DC on their output pins. If none have any significant DC in full range mode, try it again in low pass. If you find one with DC, post the DC voltage on all of its pins.

[Mote]

Voltages on opamps in full range wihtout input:
Starting from the edge of the board (as siin on the picture)
-0,57
-0,51
-0,45
-14,6
0
0
0
14,46

-4,11
-4,11
-4,56
-14,33
0
0
0
14,38

0,87
0,02
0,02
14,40
0
0
-0,34
14,43

0,02
0
0
-14,48
0
0
0
14,50

In LPF mode two of the opamps one with yellow arrow pointing to it on the picture and the one next ot it towards the edge of board have 13V on their ouput (pin1).

[Perry Babin]

The voltage on the second one seems to indicate that pin 3 is being driven with DC but it also appears that the op-amp is not in good condition due to the difference in voltage between pins 2 and 3.

You need to post the voltage when you have 13v on pin 1.

[Mote]

I had created a solder bridge between legs 3 and 4 of one of the opamps, cleaned that up. Now they have no significant DC on their input/output legs. 3rd channel is still distorted.
Second opamp I probably will replace tomorrow anyway.
Maybe I should replace all of the a1015 and c1815 transistors in 3rd channel, they dont cost much and I recently repaired another amp like this, it had blown outputs, blown drivers and 2 or 3 a1015 and c1815 were leaking bad.
I did not find any leaking ones on this amp but that way I could be sure.

[Mote]

Never mind, one of the small transistors was still leaking. Replaced it and at the moment it works great.
Will test it thoroughly to see if some of the transistors fail again but so far so good.

I see that a lot of these amps fail because output transistors fail.
There is a version of this amp with more powerful outputs and more powerful drivers but rest of the schematic is the same. Would I be able to make the amps more durable if I replaced the TIP41/42s with these bigger transistors. There are predrilled holes in the board.
Power supply has also predrilled holes and tracks to add 2 more irfz44ns and the necessary resistors.