You say the transistors on the input stage "they all seems OK"....... how
did you check them?
Look carefully at the input stage, driver stage and output stage of the
defective channel .... likely suspects would be leaky and defective
transistors or diodes... ... bad capacitors, resistors, etc.
Since you have only one bad channel you can compare your test results and
meter readings with the good channel.



Thanks for your reply.

I have a transistor tester, a device that can identify if a transistor is
PNP, NPN, short or open.

The amplifier is DC coupled; so a little divagation in DC level will
propagate throughout the whole amplifier.

So if I e.g. turn the potentiometer fully on or off, there does not seems to
be a problem, but if I adjust it towards the middle, the DC level increases
throughout the amplifier, so it is difficult to locate the error.

The amplifier is 28 years old, so it is likely that some of the capacitors
no longer meet their specifications. I have changed some of them in the
input stage, but no changes.



/Tom