IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice) - Page 1
Forum section: electronics / repair

author: stormy
author_rank: Contributor
timestamp: May 15, 2021, 07:42:35 pm
content: Hi guys/gals,I'm new! Sorry to begin with such a large ask for advice & help. I'm currently trying to bring 'back to life' this PEC4074 PSU which is designed for a Silicon Graphics Indigo2 machine. These aren't standard ATX at all, also as far as things go these PSUs aren't known to have been repaired by *anyone* before, there are even people with 30+ broken units hoping to fix them one day. I honestly hope I might get some good pointers here, so thanks in advance.Link to specifications and pin-out of the PSU 'non-impact model':https://forums.sgi.sh/index.php?threads/indigo-2-power-supply-basics.111/Initial observations:* Unit *should* automatically power up as soon as AC is supplied, but doesn't.* Nothing on 'standy power' (should be 5v)* Nothing on 5v / 12v rails* Load resistors were used & also a molex hard drive, to try and load the psu.What I have just finished doing:* Re-capped the *entire* unit* Replaced some mosfets in an area which was 'burnt' / 'darkened'* Replaced a crumbled resistor in the same area* Replaced the optoisolator in same area (Photos provided)New observations after what I've done:* Now getting 6.6v on standby voltage* Fan visibly 'twitches' like it wants to go, before it didn't* Tiny amount of voltage on 5/12v rails now, about half a voltAny advice much appreciated!

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 15, 2021, 09:22:21 pm
content: Looking at your photos I assume it's a half bridge topology.Most of the secondaries seem to be at least monitored for correct voltages, some also for over-current, all this checking will be fed to the LM339's on the control board then likely diode OR-ed to be able to stall the UC3825 if a fault condition is met.I'm also guessing that at least some of the secondaries may have dedicated "mag-amp" regulation. (Are IC201/202 on small PCB TL431's?)Having low voltage on secondary I'd check for a large film capacitor on primary side that may have gone lowin value (big yellow one maybe) next for a fault condition that would be holding a low pulse width or hiccup mode.

author: stormy
author_rank: Contributor
timestamp: May 17, 2021, 01:54:27 pm
content: @shakalnokturn thank you for helping meyes IC201/202 are TL431's. I have found a large yellow cap as described, it is an : IC M W F .22K 630. I do have a multimeter and an oscilloscope but I'm not sure how to measure it for being high/low as you explained. The codes on the cap are a little cryptic to me, could you please help me pick a modern replacement on mouser? Thank you

author: stormy
author_rank: Contributor
timestamp: May 18, 2021, 07:14:32 pm
content: Quote from: shakalnokturn on May 15, 2021, 09:22:21 pmJust wondering if there is anything else worth checking/replacing while I do a mouser order for this yellow cap? I know it is a bit of a 'shotgun' method, but I just really want to get it working. Thx again.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 21, 2021, 12:23:13 pm
content: I'd expect the yellow film capacitor to be 220nF.You could try doing a (hand triggered unless you have a function generator) slow RC charge timing to estimate the value on your oscilloscope.Even if it takes a little longer I wouldn't replace the yellow capacitor just for the sake of it, there are many things that could cause the low output voltage.Consider investing in one of the numerous devices mentioned here:https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/They're a small investment and very handy.Other things you may want to check are bad electrolytics and shorted secondary diodes. I wouldn't consider this an easy repair for a beginner, more of a case that suits some SMPS experience, an oscilloscope and HV differential probe.