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Sony SE SE/30 Power Supply low -12V output

ttb

Well-known member
Attempting to rule out all variables, I recapped the Sony power supply I have for my SE/30. Before this operation, all outputs looked good, but I wanted to be sure (fix it until it's broken and all). After recapping, I noticed it was covered in flux residue (not mine) so I rinsed it in IPA. This is what it looked like at that point:

IMG_1085.JPEGIMG_1086.JPEG

After recapping, the -12V is too low (it's like -9.2V). It's got about 500mV peak to peak high frequency switching noise it it but that shouldn't be an issue. There's no trim pot for that output as far as I can tell. I pulled off the caps that I could clearly see are connected to the -12V side (C259 and C264) and both test fine (e.g. not open, about 500 uF). I've triple-checked the polarity of the caps matches silkscreens on the board, but don't have a photo of the board before I started (of course). I looked at the board with a thermal camera while it was running and didn't see any hot spots that would indicate a load dragging down the -12V (only the power resistor R255, IC251, and the power transistors were warm).

The schematic here (https://museo.freaknet.org/gallery/apple/stuff/mac/andreas.kann/schemat.html) seems to be for the Sony PSU:

SE_Netzteil.GIF

Any thoughts as to what went wrong/what I can do to get that output back down to -12V? Or is -9.2V low enough and it doesn't really matter? I'll probably start by probing up to T151 where it appears the -12V signal originates, but I'm concerned it's an issue with the feedback circuit (e.g. IC253 and IC251 at the bottom). I'm tempted to reinstall all of the original caps but would rather avoid over-reworking the board.

Thanks!

EDIT: Also, this was the capacitor list I used (bought from Digikey): https://68kmla.org/forums/index.php?/topic/54363-macintosh-se30-digi-key-capacitor-list/

 
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ttb

Well-known member
Update: I got another (working) analog board/PSU to help troubleshoot. This was an Astec PSU and it’s -12V was also around -9.5V. I swapped in the subject Sony PSU and the computer appears to work fine. Turning my attention to the AB now.  
 

Unless someone says otherwise my conclusion is that -9.5-ish V open circuit voltage on the -12V PSU output is good enough to run. 

 
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