Repair ATX power supply

Generally they're cheaper to replace than to repair.

Way I do it is to go to the computer store, (or Staples, or Office Depot, or Best Buy, or...) and say: got a power supply?

Since you have not said what was wrong with the supply, I'm sure that will work well for you too.

I have already replaced it, but I want to repair the old one if I can as an exercise.. for all I know it could be a 5c resistor or something that needs to be replaced..

Chris

I have replaced the power supply, and now I want to repair the old one.. I shouldn't have to throw out a $50 peice of equipment because a 25c part is broken.. (spending a day or two finding the problem doesn't worry me)

The problem is that it still supplies +5V standby, but refuses to turn on (I have a load attached and I do have the correct wires for PS_ON and ground).

No fuses are blown, and nothing appears or smells obviously burnt.. I was told to check for open/high resistors near the large filter caps on the high side, so I did and they both read the correct ~220Kohms.. there's two diodes on the high side and both those are ok also.. what else should I test ?

Chris

These are rarely repaired, however often the problem is bad electrolytic capacitors. Occasionally one will grenade in such a way as to cook virtually every semiconductor in it, but often they just refuse to start up. Check the standby power supply, if that's ok then try to figure out why the main one won't start up.

virtually

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I think you may be onto something.. I took a closer look and several capacitors appear to be bulging a bit at the top.. one does have brown stuff around the edge, but I figured this was rust or crap that collects like dust, etc.. (but none of the others have this).. all the capacitors in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..

As for standby power, yes I get +5VSB, it just refuses to turn on and deliver the rest.. is this common ? I'm guessing there is a seperate circuit for standby voltage ?

Chris

"Skeleton Man" schreef in bericht news:TO6dnapev-MlAdXanZ2dnUVZ snipped-for-privacy@wightman.ca...

Long time ago I wrote the following scheme:

I repaired several PC supplies for a hobby, but if your time is valuable, buy a new PS.

First of all read the sci.repair.faq. Especially the parts on safety and SMPS.

Most of the times the fault is found between the mains connection and the transformer(s).

1. In the most simple cases only the fuse is blown. After replacing this fuse, connect the PS to the mains using the serial lightbulb trick. - If the bulb burns brightly, you know that the old fuse had a good reason to quit, so the case is not simple anymore. The first thing you have to do now is to find the short circuit. The most suspected components are the mains rectifier, the filter capacitors and one or more of the power transistors. Use eyes, nose and an ohmmeter to find scene of the crime. Remove and check the suspected components. Replace defective components except for the power transistors at this time. It makes no sense to continue until you fixed the short circuit - If the fuse is good but the PS still dead, you can start to check the voltages.
2. Check the voltage between pin 3 and pin 9 of the ATX-mainboard connector. This should be 5V. - If not you have to check the voltages on the mains side. Otherwise it will be wise to check the voltages on the mains side as well (3-5). Then continue reading up to point 12, not to miss some explanation. Continue at 12.
3. The AC-pins of the mains rectifier should show the mains AC voltage. - If not you may have an interrupted trace or mains filter.
4. Between plus and minus of this rectifier you should find about 310V DC or 325V DC depending on your mains voltage. I call it the primary power voltage. - If not you may have a faulty mains rectifier. - If the voltage is much lower (analog meter) or jumping around (digital meter), the large filter capacitors (p.e. 470 muF, 200V) are also suspected.
5. Both filter capacitors mentioned above are in serial. The midpoint should be at half the primary power voltage. - If not, the mains rectifier, the filter capacitors and the parallel resistors (parallel to the capacitors) may be defective. Another suspect is a third capacitor (p.e. 1muF, 250V) that leads from the midpoint to a transformer.

Explanation: ATX-PS's usually has three power transistors at the mains side. One connected to a small transformer, the other two connected to a larger transformer. You can recognize the pair of transistors best by finding the emitter of one of them connected to the collector of the other. First you have to deal with the one transistor and the small transformer. (Go to 8 if you removed this transistor already.)

1. Check the voltage on the collector of the transistor. - If this voltage is zero or very low there may be an interruption between the collector and the primary power voltage. - If this voltage is below the primary power voltage or jumping, there seems to be switching activity. You can check this with an AC voltmeter on a secondary coil of the transformer. The reading will not be correct, but if you find an AC-voltage you have to continue checking the secondary rectifier and regulator. - If this voltage is the primary power voltage the transistor does not conduct.
2. Check the voltage on the base of the transistor. - If this voltage

That looks excellent, I'll have a read through and see if I can find the problem..

A quick question about transformers... mine has 4 of them in total, a small one near the big capacitors, then two more small ones and a really huge one (like 4x the size of the rest) near the power transistors.. I was comparing model numbers with a photo of a simmilar supply, and they both seem to have the following numbers in their model:

Smallest transformer: 16 (WIN-16LA) Small transformer: 19 (WIN-19L) Really big transformer: 35 (WIN-35P)

Are these numbers for one of the voltages or what ?

Here's the pic I was comparing with - different brand, but the layout of components is almost identical to mine:

Regards, Chris

On Sat, 24 Nov 2007 16:55:23 -0500, "Skeleton Man" put finger to keyboard and composed:

Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies

- Franc Zabkar

I just tried the supply again (had been sitting a while since it first stopped working) and this time the fuse blew..

The rectifier appears ok, but the two power transistors (2SC2625) are giving zero resistance between any of the pins.. does this mean they're fried ?

Chris

Further to my prev message, I tried testing power transistors on diode setting, and nomatter what combination of pins/tab I connect to, it always reads zero volts..

Chris

Since you also said there aren't any burn marks or bad smells, my guess is the caps are bad. That could certainly account for the symptom.

If it's the right vintage, you may be a victim of this:

Do you have any understanding of basic circuit theory ? If you don't even have that then forget it.

Graham

If the caps are bulging, they're bad. It was a real common problem for a few years.

They may have shorted from failing capacitors, but more likely they just appear shorted because of other things in the circuit. You usually have to remove them from the circuit for testing.

Replace ALL the larger caps even if they aren't bulging. Looks like you have a PSU made with 'bad caps'.

NB - the caps on the secondary side MUST be 'low ESR' types made for smps use.

Graham

In or out of circuit ?

Graham

I did remove both of them from the circuit for testing..

Chris

And you get '0' on a diode test between any combination of pins ?

They're stuffed in that case.

Graham