Uncreative Labs

Joined: 28 Feb 2006 Posts: 127 Location: LI, NY

One of the power supplies for my XTs decided to die randomly. It used to work fine but now doesn't output any power to the board.

Any tips or ideas where to look first that might be a simple fix? It's definitely the power supply as when plugging in a different one, the computer boots right up (or as quick as it can slowly count the ram Laughing

).

I checked the fuse in the PSU and it's still intact. Nothing appears to be burnt inside and none of the caps are bulging.

Thanks!

Joined: 02 Oct 2004 Posts: 594 Location: Western NC

Just because capacitors don't bulge, doesn't mean they're not bad. Electrolytic caps are the first suspect in any P/S that shows no other signs of failure.

It'd also go a long way for you to purchase a (cheap) multimeter. They can be had for a little over $6 at Harbor Freight. They're still good enough to check for electrical shorts and measuring resistors, diodes and transistors...

https://www.harborfreight.com/7-function-digital-multimeter-63759.html

Joined: 02 Oct 2004 Posts: 594 Location: Western NC

And, FWIW, I repaired a board (for a car-wash) yesterday at work which had a shorted electrolytic cap in the power supply section. It showed absolutely no signs of being bad...it's routine for me to check these since they are on a backplane board that supplies power to other boards.

Joined: 28 Feb 2006 Posts: 127 Location: LI, NY

Sorry for the very late reply. I just finally got around to taking this thing apart and checking things with my multi-meter.

Case taken off:

Large caps... look okay but as you said, they could still be bad.

Left side of board. According to some youtube video I found, the two green caps and the one on the right are for the 5v & 12v output. (If it that source can be trusted) Also "look" okay.

Power input board. This one checked out with the multimeter. Neither fuse is blown and I get 120v to the black & white plug to the main PSU board.

This is the board I'm currently questioning. I see the 120v entering but lose it when it enters the blue block. Not sure when power gets changed from AC to DC. Any ideas on what voltages I should see here?

There's no voltage output to the power plugs or the fan connector. (I made sure to load the PSU with two IDE hard drives to hopefully meet the minimum load required to switch it on.)

Joined: 02 Oct 2004 Posts: 594 Location: Western NC

After draining all potential voltage sources (large capacitors), check to make sure primaries and secondaries of all the transformers are not open. Wouldn't hurt to use a (clean) paintbrush and compressed air to dust off this before proceeding. I'm guessing you've checked all the solid state (diodes, transistors, voltage regulators) for shorts?

Does your multimeter do capacitance? If your desoldering skills are good, then it wouldn't hurt to remove the electrolytics to see what they measure (an in-circuit reading isn't reliable). Just make sure polarity is correct when you replace them.

Posted: Thu Oct 22, 2020 12:20 am

That blue block is a relay. Relays have two parts: A coil and a switch. You'll have to look at the data sheet to see if the 120V enters the coil side or the switch side. I bet it enters the switch side.

Joined: 01 Oct 2006 Posts: 36 Location: USA

Is it for sure the power supply and not a shorted tantalum cap on the motherboard? The power supply has a safety feature where it will not do anything if there is a short anywhere in the system.

Joined: 02 Oct 2004 Posts: 594 Location: Western NC

Hmmm, that site looks very similar to minus zero degrees (which I know to be legit). I'd be somewhat cautious about any downloads from there...

http://minuszerodegrees.net/5150/problems/5150_known_problems_issues.htm

Joined: 01 Oct 2006 Posts: 36 Location: USA

You're right, looks like someone just completely copied the site. I removed the link just to be safe.

Joined: 28 Feb 2006 Posts: 127 Location: LI, NY

Skyfrog wrote:

Is it for sure the power supply and not a shorted tantalum cap on the motherboard? The power supply has a safety feature where it will not do anything if there is a short anywhere in the system.

Yep, positive it's the power supply. I have a different power supply from my 286 in there now and the computer works fine with it.

Sorry for the lack of updates on this.. things have been pretty busy lately so I haven' had time to really tinker with anything yet. I'll be sure to update this thread once I make some progress.