Has anyone taken apart a 557 to the point of being able to replace components on one of the amplifier PCB?

I have removed the top, leaving the bottom, front and back panels and the two headsinked sides.

I can almost envision removal of a heatsink side panel and attached amplifier PCB. How do single wire connectors detach? Do you unattached supply and speaker wires before or after heatsink side is loose? Separation of heatsink from PCB looks straight forward. Reassembly of power transistors to heatsink looks like a nightmare.

Amplifier problem - works correctly for first 10 minutes, then I can hear distortion and a 4KHz squeal from speaker. After another few minutes, total silence from that amplifier.

Troubleshooting - when I hear squeal, I can also see 500 KHz, 1v RMS oscillation on output (yes, I see this oscillation on oscilloscope - of course I am not hearing it). Reducing AC Power from normal 115vac to 105vac will revive amplifier. All internal supply voltages are normal. No signal output on speaker terminals is 27mV.

200, 500, 501, 504, 505, 506.20, 508.20, 508.24, 541, 551, 555, 556, 557, 561, G55 and Allison IC-20