[weblacky]

Yo, So I'm no expert and you've had a great deal of success with this, but the coil whine does bother me, this is something that should be addressable and you shouldn't have to live with it. So let me ask the basics...you have a lot off neat meters and instruments. So I'm going to make assumptions that you have...well...everything at your disposal to test with.

1. Have you tested the PSU coils in circuit with either an LCR or a curve tracer to compare them to both each other and to another sister/duplicate PSU you may have? How do they compare? Do they read/compare similar?

2. Have you tried low-voltage Curve tracing (200mV-300mV range) on the Filter Cap groups between "like" PSUs to confirm similarity? I feel like the whine could be a shift in the physical of the caps (even as a collective). Perhaps you measure them as a group first, then if you see a big difference, you can separate one of the pair and measure outside to see if one is REALLY worse than the other.

3. The feedback circuit also bothers me, it's like it's overshooting and undershooting via correcting. Doing a simple online search (my mind goes to Buck COnverters on regulators here) it's this indicative of a failure of a capacitor on the feedback line? I know I've been told that when ESR changes substantially on the feedback circuit it causes this kind of wide under-over shooting. So could you check the feedback circuit you disconnected with a low-voltage curve tracer (again 200mV-300mV) to isolate the cap outside of semi-conductance to check between like-PSUs. I think you'll find you have a change in resistance on the feedback circuit. Assuming there is a management chip (and not a TON of passives doing this) it shouldn't be too hard to trace back from Management IC to find capacitance on the feedback line. This is the only thing I know about output oscillation because of the feedback control on this kind of stuff (limited knowledge).

Great posts and great work, I hope you make more positive progress on this and can continue to dedicate your valuable time to writing these posts up for us!