Author Topic: IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice)  (Read 937 times)

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Offline stormyTopic starter

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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/
1219536-0

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.
1219522-1

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)
1219532-2

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 volt

Any advice much appreciated!
 

Offline shakalnokturn

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Re: IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice)
« Reply #1 on: May 15, 2021, 09:22:21 pm »
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 [edit] low in value (big yellow one maybe) next for a fault condition that would be holding a low pulse width or hiccup mode.
« Last Edit: May 16, 2021, 05:15:59 pm by shakalnokturn »
 
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Offline stormyTopic starter

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Re: IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice)
« Reply #2 on: May 17, 2021, 01:54:27 pm »
@shakalnokturn thank you for helping me :) yes 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 :)
 

Offline stormyTopic starter

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Re: IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice)
« Reply #3 on: May 18, 2021, 07:14:32 pm »
Having low voltage on secondary I'd check for a large film capacitor on primary side that may have gone [edit] low in value (big yellow one maybe) next for a fault condition that would be holding a low pulse width or hiccup mode.

Just 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.
 

Offline shakalnokturn

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Re: IPS PEC4074 PSU Repair for SGI INDIGO2 Computer (Help / Advice)
« Reply #4 on: May 21, 2021, 12:23:13 pm »
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.
« Last Edit: May 21, 2021, 12:27:07 pm by shakalnokturn »
 
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