Ampeg B-15T distortion

[BiBi]

Hello,
A friend has brought me one of these early 90's Ampeg B15T's over to fix. The preamplifier section looks ok but somewhere in the power amp the top of the waveform starts to warble creating an ugly distortion. I've scoped out a few of the transistors but haven't found the source of the trouble and nothing looks burnt or the like.
I don't have a schematic unfortunately although there was an earlier thread here where someone had them but didn't post it. If someone could send me a copy of the schematics that would be great. Otherwise any help on finding the trouble would be much appreciated.

[Enzo]

SCope the power rails, is one going soft?

[BiBi]

Hello Enzo, thank you for your reply.
I had the same thought and it seemed perhaps that the positive rail was indeed "going soft". So I changed out the filter caps both negative and positive supplies. All seemed fine at this point-- the fluttering on the top of the waveform was gone and the amp sounded much better. I let it sit on for a couple of hours and switched it off.
When I came back in the evening, the amp was distorting exactly the same as before, but I can't see it on the scope. My guess is that it has to be in the power amp somewhere as there are numerous outputs on this amp for preamplifier and even headphones. All these are clean.
I've run a signal at various amplitudes and frequencies but I can't see any distortion. Then when I play through any speaker (all double checked with another amplifier) the distortion is certainly obvious. One strange element: I noticed that single notes on the bass are essentially ok up to a certain amplitude while intervals come out completely distorted.
I've thought of coupling capacitors or maybe bias in the driver transistors? I'm getting +/-35 Volts in the power section and +/-16 in the pre.
Thanks again for any help or suggestions.