Slot 1 PC mainboard fried with non-standard ATX power supply - Page 1
Forum section: electronics / repair

author: canadaboy25
author_rank: Regular Contributor
timestamp: December 09, 2024, 07:06:36 pm
content: 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. 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?

author: wraper
author_rank: Unknown
timestamp: December 09, 2024, 07:21:42 pm
content: QuoteThere are different stepping (revision) chips. Most likely will work too. SL5WK is not FW82801AA, it's FW82801BAQuoteAll of them are used.

author: canadaboy25
author_rank: Regular Contributor
timestamp: December 10, 2024, 12:26:05 am
content: Quote from: wraper on December 09, 2024, 07:21:42 pmYou'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 pmDigikey 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?

author: wraper
author_rank: Unknown
timestamp: December 10, 2024, 12:30:44 am
content: 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.

author: canadaboy25
author_rank: Regular Contributor
timestamp: December 10, 2024, 12:46:18 am
content: Quote from: wraper on December 10, 2024, 12:30:44 amYes, 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.

author: wraper
author_rank: Unknown
timestamp: December 10, 2024, 12:51:28 am
content: This is not so much about the price but that these barely ever failed. While Nvidia chips I mentioned died like cockroaches due to flawed design and reheating them usually resurrected them for a short while, so you can guess what you'll be getting.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 10, 2024, 03:14:26 pm
content: on the 3.3v you could have many many parts .... would maybe become expensive to repair it ??can you find another one with the right psu ??

author: canadaboy25
author_rank: Regular Contributor
timestamp: December 10, 2024, 03:37:52 pm
content: Quote from: coromonadalix on December 10, 2024, 03:14:26 pmYes I think that's very possible. However, I'm holding out hope that everything isn't destroyed since it was the 3.3v standby rail, and not the main power rail that received the 12v. I could find another board, there's nothing particularly special about this one, but just for the principle I'd like to see if it's repairable.

author: macboy
author_rank: Super Contributor
timestamp: December 11, 2024, 04:08:17 pm
content: Start by pulling the fried South bridge, then apply power again and check for more hotties. You might also probe around to see if you can find other devices which connect to that 3.3V standby rail. You might have also damaged capacitors on that rail, especially tantalum ones (if there are any).

author: magic
author_rank: Super Contributor
timestamp: December 12, 2024, 10:51:48 am
content: Nasty.Practically everything has a data connection with the SB and was at risk.Even if not completely blown, something could have its I/O circuits damaged.

author: canadaboy25
author_rank: Regular Contributor
timestamp: December 27, 2024, 05:31:20 pm
content: I have removed the shorted southbridge and the board no longer overloads the power supply when connected. Obviously it does not power up as the southbridge was responsible for the power on signal. The 3.3V standby rail now draws 50mA. I was able to find a few discrete logic chips connected to the standby rail. Nothing tests obviously shorted. The ram that was installed during the overvoltage still works in another board so I am hopeful the damage is not catastrophic.I have ordered a replacement southbridge from china, so I will see what the board does once I receive and install the new chip in a couple months...

author: canadaboy25
author_rank: Regular Contributor
timestamp: February 12, 2025, 02:16:59 am
content: The used (but reballed and in reel packaging?) southbridge arrived from china and I soldered it onto the board. The board booted right up with the same processor and memory that was installed during the incident.I found out that the original no POST issue was apparently caused by my PS2 keyboard. The keyboard works fine on every other system I have, but when it it plugged into this one, the processor will not run. A standard USB keyboard works fine with this board.It seems the southbridge failed and overloaded the PSU before anything else was killed. From now on I will be checking for non-standard PSUs disguised as ATX...