ATX power supply repairs

On Mon, 23 Nov 2009 16:42:30 -0600, bob put finger to keyboard and composed:

This may help you understand the inner workings:

- Franc Zabkar

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"bob" schreef in bericht news:hef33l$mi0$ snipped-for-privacy@aioe.org...

Back in 2002 I wrote the following posting and I did so again in 2007. I did not many repairs ever since. The components required for the repair became more expensive then a new supply.

Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W ATX supply on the net. This can be a great help though a little bit late for me. Just Google for it.

Be aware that the schematic is an example of an ATX supply. You may find a lot of differences out there though the general idea will be the same. At least most of the times.

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

Some cap failures. Recently one client had a batch of four identical

230V units die the same day. These were auto-sensing (supply voltage) types, and during brownouts they think they are on 110V rather than 230V, then the voltage rises and phattt! The pair of 1000V 1A ultrafast diodes went short on all four (same brownout!).

On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter" put finger to keyboard and composed:

You may like to know that DTK's original circuit diagram is here:

More DTK PSU circuits are here:

- Franc Zabkar

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Clarifying that, these were protection diodes.

00W

neither URLs worked for me

On Thu, 26 Nov 2009 08:07:30 -0800 (PST), Robert Macy put finger to keyboard and composed:

here:

Sorry, it appears that the links are now dead.

I found them at the Wayback machine, though:

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.