Electronics > Repair

Slot 1 PC mainboard fried with non-standard ATX power supply

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canadaboy25:
I had an old slot 1 Compaq built PC in storage that I recently was trying to put to use.  The board would power up but not post.  No beep codes or anything.  I tried swapping, RAM, reseating CPU, and then decided to try a different PSU.

I plugged in my test PSU, and immediately upon applying AC to the PSU, I heard it make a *zip* noise which indicates that its overcurrent protection had been tripped.  After some troubleshooting I discovered that the PSU in the original Compaq case was using a non-standard pinout  :palm:.  The ATX connector was keyed identical to a standard one and I did check a few of the wires to make sure before plugging the new PSU in, but it turns out the Compaq PSU only 90% conforms to the standard ATX pinout.  There were a couple wires that weren't standard.

Most notably, the Compaq PSU has a 3.3V standby wire where the standard ATX supply has +12V...  Upon plugging the Compaq supply back in, the board would no longer power up.  I traced it to a short on the 3.3V standby pin that received the 12V.  Providing the 3.3V rail with my lab power supply draws 500mA and the southbridge chip gets hot.


I have the equipment required to replace the southbridge, but I have two questions.  What is the likelihood that every other chip on the board is destroyed as well?  Or is it possible the southbridge died first, and overloaded the PSU protecting the other components?

The southbridge is an Intel FW82801AA.  (See attached image)  However, it also has the markings SL3MA on the third line.  Most replacements I can find online seem to have SL5WK marking on the third line.  I did find one used chip with the correct SL3MA marking.  The full Intel datasheet is available online, but it does not describe the package labeling information.

Would any FW82801AA chip be likely to work, or would it be necessary to buy the SL3MA marked part?

wraper:

--- Quote ---However, it also has the markings SL3MA on the third line.  Most replacements I can find online seem to have SL5WK marking on the third line.  I did find one used chip with the correct SL3MA marking.  The full Intel datasheet is available online, but it does not describe the package labeling information.

Would any FW82801AA chip be likely to work, or would it be necessary to buy the SL3MA marked part?
--- End quote ---
There are different stepping (revision) chips. Most likely will work too. SL5WK is not FW82801AA, it's FW82801BA

--- Quote ---I did find one used chip with the correct SL3MA marking.
--- End quote ---
All of them are used.

canadaboy25:

--- Quote from: wraper on December 09, 2024, 07:21:42 pm ---There are different stepping (revision) chips. Most likely will work too. SL5WK is not FW82801AA, it's FW82801BA

--- End quote ---

You're right, I didn't realize the SL5WK was FW82801BA.  However I have found FW82801AA with SL3Z2 revision.  So it would be likely that these would work fine?



--- Quote from: wraper on December 09, 2024, 07:21:42 pm ---All of them are used.

--- End quote ---

Digikey has FW82801AA listed, but the minimum order quantity is 19 which totals $305.  So I won't be going that route.

Buying chips like this on ebay marked "new" always makes me nervous.  Perhaps I would have better chances buying a used one in case the "new" ones are just relabeled junk in a similar package?

wraper:
Thankfully these are low risk chips so reballed used chip most likely will be fine. I would not touch 2006-2008 Nvidia chipsets and GPU with a 10 foot pole though.

canadaboy25:

--- Quote from: wraper on December 10, 2024, 12:30:44 am ---Thankfully these are low risk chips so reballed used chip most likely will be fine. I would not touch 2006-2008 Nvidia chipsets and GPU with a 10 foot pole though.

--- End quote ---

Yes, that is true.  Faking these chips likely wouldn't be a very profitable exercise.  I will probably try ordering one of the cheap "new" ebay ones and see.  The used one is twice the price and I would have to spend the time to reball myself.

Unfortunately the CPU was installed in the board at the time, so I guess I will find out what else has been burned once I replace this chip.

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