**Not power supply noise. The power supply is as clean as a whistle.
All electros have been replaced with high quality (Panasonic and
Nichicon) components.
The unit is pure analogue. NO digital stuff to be seen.
There are 25 dual OP amps (5532), a handful of bipolar transistors, 2 X
dual gate MOSFETs, four Vactrol opto-isolators and here's the big one:
27 preset pots. Every single one has been moved from factory settings by
a previous tech. Without access to factory service data, I have no
chance of restoring this thing to anything resembling normal operation.
I have been able to stabilise one channel (though I still have a low
level 2MHz oscillation to deal with). I was able to measure the
frequency response curve and fell confident that I can duplicate the
curve using a a 32 band, digital parametric EQ. I won't have the ability
to use the speaker feedback section, but that should be a concern, give
that the system is designed for sound reinforcement use, but will be
used in a domestic application. The speaker feedback system is designed
to protect the HF drivers under very high power operation. MY solution
should work well and will cost my client much less cash.