Since the last thing we looked at on the secondary side was the photocoupler, let's see if anything interesting is happening across its output terminals. In a photocoupler, the current going through the output phototransistor is roughly proportional to how much current is used to light up its LED. On the secondary side, we saw that the diode was being driven continuously at more than 100mA. So, on the primary side, we can expect the output transistor to be fully turned on with its two terminals nearly shorted out.
As expected from a photocoupler in the fully 'on' state, its collector to emitter voltage is under 1V. The amount of noise here appears to be too much for my AD629 to cope with, and it ends up outputting a mostly useless waveform. From the way the photo-transistor's voltages pulse together, it looks like there is no source filter capacitor on the primary side of the feedback circuit to tide it over between transformer pulses. A quick look at the circuit traces reveals that the photocoupler does connect directly to the positive terminal of that 22µF capacitor labeled EC3 on the board, so I would expect the collector voltage to hold some sort of value above the 60Hz wave it is riding on, not sharp pulses coincident with the auxiliary supply switching cycle. This looks very similar to the auxiliary supply output waveforms on the SL300 before I changed the auxiliary capacitor. While the 22µF capacitor should have seen negligible electrical load, it could still have gone bust from its close proximity with that scorching-hot zener diode.
Another oddity with the photocoupler's collector-emitter voltage is that once it turns on, the voltage across it on the primary side becomes mostly negative, which is really odd. Time to replace that 22µF capacitor and see what happens.