Apple Duo Dock SMPS low voltage

[elemenoh]

I have an Apple Duo Dock with a switch mode power supply that has low voltage on the main 12V and 5V rails. 5V is reading about 1.3V. 12V quickly ramps to about 3V and falls back to ~0 in less than a second. It has separate 20V and 5V rails for the laptop inject/eject motor and accompanying switch. Those are okay.

All of the electrolytic capacitors have been replaced.
The optoisolator has been replaced. It worked briefly after replacement from a known-working PSU, but later went back to the original symptom. The opto works okay in the known-working PSU.
The SCR, MOSFETs and voltage regulators tested okay.
All resistors tested okay.
All diodes on the secondary side tested okay (with a DMM diode test).
I tried heating it up with a hair dryer. There was no change.
I haven't found any shorts yet. I compared resistance across a bunch of points to the known-working PSU and they were all the same.

All of my testing has been with the PSU off and disconnected. I have a DMM and ESR meter. I have a scope, but don't have a differential probe.

I'm assuming the issue is on the secondary side somewhere since the 20V/5V rails work okay for the eject motor. But I'm at a loss for what to check next. Any advice?

Here's an image of the top side of the PSU.