On Apr 22, 2024, at 2:55?PM, Nick Stanley <rosewardxiii@...> wrote:

?Hi all, figure a good old repair attempt would be a good 1st post.
I bought a broken E4404B SA in hopes to repair it. The seller said it has a dead PSU, but doesn't know if it has any other failures.

I took the PSU out to test it. I decided to test all the MOSFETs and the 2 optocouplers, all tested good.
Then I realized since this PSU can take DC input, I can just supply 12VDC and see if the DC-DC converter secondary side work. And it DOES. All voltage rails (+28V, +15V. +5V. -5V, -15V, fan) are all present. Draws about 5-6W which seems alright.

Which narrows down the failure to the AC mains sides. I checked the mains side, the 2 bulk caps hold a total of about 310VDC, which matches with the peak of rectified mains AC.
When I force the supply on while connect to mains, I hears a small click every second. Measurements of the DC rails shows them varying greatly. It's like the PSU is trying to start up but can't.
Will update with any progress and pictures soon.

Also, as I was testing the PSU, one of the RIFA caps exploded, filling my room with smoke and cover the PCB in tars. Horrible