Hey, whilst I know it's largely irrelevant to your post I can report that i have now fixed that particular AVG PCB. Basically it turned out to be a duff 74LS161 at 1D on the vector timer. The troubleshooting diagnostics showed a waveform that was wrong: test 4 (figure 2 page 6 of troubleshooting guide) was inverted. I spent ages blundering around on this one with a scope and logic tester but eventually I traced it to this part of the circuit by accidentally and briefly shorting the VCTR test point to 'PR111' which caused the waveform to switch to the right phase and was reproducible, funny how things turn out.
Suffice to say the vector instructions tests that can be selected by the option switches are very useful for generating repeatable waveforms when testing against a known good board. I compared the vector test point in demo mode with a good board and the only discernible difference was the intensity of the signal between high and low and I don't think I would have spotted it.