Kind of you to think of me, but unfortunately I'm nowhere near competent enough at the kind of low-level programming this would require. Aside from some dilettante dabbling, I'm most comfortable firmly in application space
That said, definitely sign me up for a Linux version (modulo financial disaster). I'd love to have a go at getting the "split world" stuff going (someone hacked rootless mode into Basilisk a while back, I was wondering if using the same approach but in reverse might work to allow handing over parts of the Mac desktop to the Linux side to draw...)
A quick update on this: I've spent quite a lot of time trying to get what I think is the release GEMDOS 1.1 running on this using the DR compiler suite. Unfortunately it is very crashy, and adding debug output seems to move the crashes around, which is not entirely helpful. Part of this is about uninitialised memory, but it's quite hard to debug. I'm considering whether to try to build this using the GCC Atari cross-compiler suite thing for MiNT, which can produce basic TOS executables. Nothing else is required for building the GEMDOS as far as I can see, but it would mean that this wouldn't be able to be self-hosting any more, which would be a shame.
Are you measuring the 12V rail with or without load? Maybe something is pulling too much current of this rail, causing the voltage drop. I usually disconnect logic board, FBT and the vertical drive circuit, which all feed of the 12V rail, when troubleshooting this scenario.
Also, if you have access to a benchtop power supply, you can inject 12V directly on the analog board (with the Mac disconnected from the mains) and check current draw and troubleshoot the feedback circuit (see below). Like bibilit mentioned, the opto-coupler is usually a prime suspect, and on rare occassions I have also had to change the LM324 op amp.
Here is the feedback circuit that controls the switching transistor:
And the explanation to the above:
The feedback control circuit monitors the +12V supply voltage through a voltage divider comprising R35, R56 and R38.
This divided-down voltage is compared with a 6.2V zener diode-derived reference voltage (from R34 and CR19).
The system tries to maintain equality of the voltages presented to the input terminals of the op-amp by driving the
switching converter more or less hard as necessary.
An optoisolator (U3) couples the control circuitry, which is all referenced to digital ground, to the core switching
converter, which is referenced to primary ground. Resistor R56 (located just above the speaker) allows fine adjustment
of the output voltage. Stability of the feedback system is assured by C21, R37, C27 and C28.
If the +12V supply voltage is too low, the LED inside the optoisolator is driven with less than normal current. This,
in turn, reduces the current through the optoisolator’s transistor, which ultimately commands the switching core to
increase its output. The opposite happens if the supply voltage is too high.