Hi one channel is having only top half of sine wave and lower part missing. If I increase gain beyond clipping then the wave form is as shown in the picture. Good sine wave is reaching the output section. Lifted R308 and it had good sine wave. All the transistors are looking good.
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From what I see, R308 is the input to the channel and the amp wouldn't have any output with it out of the circuit.
Yes that's true. I just lift it one side to see if I am getting clean signal upto that point. The picture I attached is from the output terminal after I connected it back.
It sounded like you were getting a good output signal when the resistor was lifted.
Does the bias pot have any effect in that channel?
Does the bias pot have any effect in that channel?
I tried that but did not see any change in the sinewave. I always uses power supply with limiting resistor. When I turned the pot fully I just noticed slight increase in current draw. 0.8A to 0.9~1A.
No, other channel changes much more than this. On further checking I found bios transistor bad. MPS6521. Is there any substitute I can use?
I know of no substitute. If you try something else, you need to test at the full range of operating temperature (up to thermal shutdown) to ensure that the bias holds. Do you have any junker boards that you can salvage one from?
I got the transistor and the problem still exists pot is not making any difference when rotate all the way.
There was no output on one channel. I replaced one of TL072 and got clean output. Another channel there was no change in frequency no matter which side the selector switch was. I also changed another TL072. Now the frequency changing and also getting clean signal until that resistor I mentioned. That's it. But I only gets top half of sine wave bottom sinewave missing.
Is the bias voltage any more effective as you adjust it clockwise (with the diodes out of the circuit)?