amplifier lost one channel

My old amplifier has lost the right channel signal. Could anyone help me how to trace where it goes wrong? My amplifier signal sequence: pre -> tone control -> volume -> power amplifier. I tried to change different inputs but the problem is still there. Bypassing the tone control didn't work out so i think the problem exists in the other sections.

It would sure be handy to know what the heck your working on. Make/model etc.There are hundreds of different "amplifiers". And I for one, am not psychic.

^^^

Yep, give us more details.

The first thing I would check would be the fuses. Did the channel fail while you where playing music? Was it like the flipping of a switch (sudden)?

I have found one loosen cap in the pre section, resoldered it and now the right channel sounds. But I found another problem: if i turn on the loudness function, the left channel sounds louder than the right one, othewise in the normal mode both channel sound the same. Does anyone know about this problem?
Here is the loudness schematics:

And we still don't know what we're working on, the loudness switch is probably dirty.

Craig