I've got a D12246 Williams power supply board in a Taxi machine with no 5V. (No load it gives 4V and even a small load, pulls of the display drivers, it pulls it down to 3V). The transistor is a ST 2NG057. I desoldered it and I'm planning on replacing it with a 12V to 5V regulator that is rated for 5A. It should be more efficient, and won't use the old comparator circuitry.
I verified the rest of the voltages are rock steady with no ripple issues. (+12V, -12V, +100V and -100V). I found and fixed the shorted coil which toasted the transistor and shorted the 5V supply. Since the 5V is the only voltage that is bad, it seems best to just fix that portion.
Has anybody tried anything similar? It seems like it should work and be significantly more efficient than the previous design. Anything I should be worried about?