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Recapped SE/30 crashes/resets when playing sounds


I've just finished recapping an SE/30, and have a puzzling problem: while the boot chime is loud and clear and sounds normal, playing any sound while the system is running causes it to lock up and play a 'stuttering' boot chime for a second or two, and then restart - as if somebody pushed the reset button a bunch of times in quick succession. Other than this fault, the system seems fairly stable - SCSI, ADB and Serial work fine, and other than the sound test, it passes Apple Personal Diagnostics just fine.

There didn't appear to be any corrosion damage to the logic board, just a bit of crusty electrolyte that I cleaned up with warm water and detergent, followed by isopropyl alcohol and then compressed air to dry it off. I recapped using the parts list from https://recapamac.com.au/macintosh-se-30/. I've confirmed that I've got continuity on all the ASC address, data and control lines, and I assume that if I had any shorts there, I'd be seeing more general system instability (since they'd affect the processor bus as a whole) and/or a strange-sounding boot chime.

Right now my working hypothesis is that something is causing the Sony sound chips (UB10, UB11) to repeatedly toggle the reset line when a sound is played - a bad ground or power supply issue, maybe? - but I don't really know enough about their behavior to know for sure - looking at the schematic, the reset circuitry all seems to be on the other side of the board, and untouched by capacitor goo.

I'm going to continue to investigate and update this thread, maybe solder on some flying leads so I can watch the reset line with the system running - but has anybody run into this issue before, or got any tips about how I could isolate the fault?


Some more findings:
  • It will sometimes play sounds successfully, other times it hits the reset loop.
  • The reset button does not function properly - the screen blanks while the button is held down, but when the button is released, the system freezes, with the screen showing what it was when the reset button was pressed. The only way to get out of this is state to power-cycle the machine.
  • Holding the reset button down when the machine is stuck in a reset loop causes the stuttering boot chime to be replaced with clicking from the speaker at approximately the same rate.
  • Once in a reset loop, the time it takes to 'escape' seems to vary - sometimes it resets cleanly after only a couple of stutters, other times it seems to get stuck indefinitely.
I've linked a video demonstrating the problem, pressing ⌘W in the Chooser - note that the first time I press it, it beeps as expected, but the second time it gets stuck in a reset loop (and I kept on waiting for it to come out, but this time it didn't).

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Found the problem - while replacing the audio filter capacitors C3 and C4, I had shorted a pad to an adjacent trace. I can't recall which pin on UB11 that trace went to, but the net result was that any audio signal on the left channel would upset UB11 enough to trigger a reset. I insulated the exposed traecs with some carefully-applied Kapton tape, and now we appear to be all good!

Just got to figure out what's up with the floppy drive now, it seems to read disks just fine, but it can't seem to format successfully.