"SA EH750 "

That uses a RSN311W64 six channel IC with all inverting inputs. Grounds are pretty important.

Tell you one thing I noticed Technics does is to save copper by using the metal chassis as a ground. First thing I would do is to tighten all the screws that hold down the circuit board. Loosen them a bit first, and see how tight they are. some will not be critical but some will.

I don't really like how they design but there audio isn't all that bad. I'll pass though because those SVI series chips run up into the hundreds to buy.

At any rate, if this is affecting all channels it is "global" so grounds are it. That chip looks like it is getting about five supplies, one set called for at like +/-59 volts.

Another thing about amps designed like this, when you hook a scope to the speaker you also use the speaker ground as a ground. If you use the chassis you might incur oscillation with a 1X probe. not that it happens all the time.

This is all assuming you ran ESR on a bunch of the caps. Look for those little damn 4.7uFs.

If this problem is as stated and I haven't fcked up reading it, I would think about taing a cap from an input to a local ground, inputting the signal to another channel and see if the bypassed channel produces ditortion.

For example, connect a ten uF between pins 26 and 24. Run the same signal through it and see what you get at pin 2, which is where the amp fed from pin 25 feeds.There is only one more ground on that chip at pin 10. try bypassing to that. If you get any oscillation at pin 2, simply confirm clean supplies, and if it has clean supplies it is the chip itself. We took the ground local to make sure of that. The ONLY other thing can be unbypassed Vxxes. Looks like about five of them or so at first glance, but I just got the print five minutes ago.

Let me ask, are you running this into speakers or unloaded ? If it does it unloaded and you are checkiong it unoaded you can just short instead of use the capacitor. With a load no because if the chip has any DC offset it oculd at most be fried, or at best throw off your results. We need clean results.

These power rails also ned to be checked while the distortion is being produced. Don't just scope them with nothing going through there. Doesn't show ****.

Let us know what you find.