My apologies, Tom, CarlB, Tysniffen. I confused the issue by not looking closely; I didn't notice that the AVR was also hooked to the Z windings. If the Z winding output is insufficient, it would explain why swapping AVR and bridge diodes, did nothing. Connected as shown on the AVR diagram, this AVR can only LIMIT the output of the Z winding. If the Z winding is bad, output will be too low even though the AVR ouput will be full on.
It does look like a case of bad harmonic windings, as CarlB suggested.The AVR you currently have, Tysniffen, MIGHT work if the 120V stator output was directed through the AVR where the Z windings normally go. One leg of the stator's 120V out should go straight to the bridge, the other through the AVR as your Z winding does now. Under a peak load, the Z winding will put out peak voltages higher than the 120VAC, so it should not fry your AVR.
The CGG regulator that I suggested to CarlB does work in this manner IF the problem is indeed the AVR, as it appears to be. If you will read CarlB's previous thread, you'll see that I suggested using a Variac as input to the bridge diode to confirm that the field and stator windings were all working fine. You don't need external AC power with the variac, as you can set it to the highest output voltage initially to get it started using just the stator AC input, then dial it back once the voltage comes up. Bill Rogers kindly told me of this method when I was troubleshooting my own ST-3 problems.
Again, sorry I confused things.
Bruce