well, the fix was easy, finding the time to get measurements was not, but the repair took 3 min to do, after days of looking over pictures i took and schematics, along with this lovely online resource of a full traceable board-view. and some info from this page *of a A4000 audio repair guide.
The problem… a capacitor (C304) leaked on a trace that runs from the top to the bottom of the board, this trace goes from another capacitor’s +12v (C303) line to pin4 of the Op-Amp (U15) and would not power up the opamp with its proper voltage. in fact it was pushing that -12v from pin 11 and gave the opamp input a reading of ~-8v. not good. and im sure would cause issues if left unfixed. so, i jumped from the cap to pin 4 (large overkill yellow wire next to the scope probe) and it works like new. “a’Migamix is reborn”
The picture above is a reading from an output using a kit built DSO150 oscilloscope instead of my siglent scope (maybe more on that later) of one of the channels while i was playing one of my old DSS song modules. You can see the new CF harddrive in the picture too.
It’s fixed, it works great, but i noticed other damage around that area. i will be testing and cleaning up some thru-hole traces if needed some other day after i work on a few other key devices in my “ill get to it one day” pile that is quickly shrinking.
edit, whats next for my 1200, get an add-in card for the video out so i can connect to a modern monitor.
* broken link, will fix when new source is found