Well, I unsoldered the flyback to inspect it, and also because I was thinking about temporarily swapping the one from my platinum Plus into here so I could stand to use it (the high-pitch ringing was very intense). I decided to solder it back in first just to see if anything changed, and it has not made the ringing sound since! Maybe it really was bad solder joints. I didn’t initially reflow the flyback because it looked fine, but maybe it wasn't. I did find it odd that I could occasionally make it stop by pushing-in on the analog board side of the Mac.
Time will tell If this is a lasting fix. I’ve also still got that early analog board on the way with the smaller flyback, which I’m still quite curious about. Either way, having an extra analog board never hurts!
Yeah I certainly had to fiddle with the phase to make the background stop shimmering. Did get it near-perfect, though.
Yup. Fortunately you can tell them they're not talking to a TV-type monitor, and they get a lot better behaved
Is it the same error every reboot/reset? This link (https://udcf.gla.ac.uk/~gwm1h/Error_Codes/Sad_Mac_Codes.html) provides a breakdown 68k error codes, with some specifically for the Portable further down the page. If I'm reading it right, the error you show indicates a RAM test failure at Bank A.