Upgrade from hell

About two weeks ago, I decided to finally take the time to upgrade an old Ubuntu 10.04 on a home server. This ofcourse involves a couple of upgrades, since that release is too old to bump to the latest and greatest.
The first upgrade step went rather uneventful, albeit slow. However, my weekend plans were quickly interrupted when after booting the new release, the machine decided to insta power off after 2 minutes. *sigh*

Mind you, this machine did give the occasional warning about temperature issues before the upgrade, so my first reaction was to think too much dust had collected on the CPU cooler. I opened the machine and tried to clear the dirt that had settled between the cooler and the CPU heat sink.. however while doing that, I accidentally broke one of the fan supports.. doh!

This was starting to get annoying. Good thing I had some super glue lying around. Couple of minutes later, the support was firmly reattached and I could continue (though the fan wasn't 100% aligned, causing it to scratch the sides a bit, making a wonderful whining noise)

After this operation, I could boot the board again and it seemed to stay up. Just to be safe, I kept it running overnight before continuing my upgrade fiasco.

The next day, still no reboot issues. So I prepared to do the remaining upgrades. It didn't take long for new issues to pop up. One or two minutes after starting the update manager, the machine again went completely dead. However this time, I couldn't power on the machine anymore. The hardware had completely failed this time. Just my luck again...

Now it just so happens that I recently picked up my interest in electronics again. Therefor instead of throwing out the board, I had a look at what I could do or find about what was wrong.

First investigation showed that I could power it on if only the 20 pin ATX connector was attached, but when the 4 pin ATX connector was also connected, it refused to work for more than 1-2 seconds.

That 4-pin ATX connection supplies 12V to the VRM section of the motherboard. When looking closer, I noticed 2 capacitors which were bulging, which is usually a dead giveaway that they are damaged.

This looked promising...

I had some old CD and DVD audio player I wasn't using anymore, so used those to find some spare parts. Desoldered a couple of capacitors from them and put them in the motherboard. However, while turning the motherboard around a couple of times to see what I was doing, I accidentally also turned one of the caps around. Needless to say the repair didn't work. To add insult to injury, I also took the wrong cap for 1 of the replacements, picking a 6.3V in a place that had a 16V cap.

Some swearing ensued...

When I corrected the error, the board still didn't work (maybe it would have, if I didn't screw up the first fix).

Since working without a schematic is fairly hard, especially for a novice in electronics. I decided to just check on google if there was any info to be found for this motherboard (MSI 6728). To my surprise the service manual with a full schematic turned up. Nice!

Anyway in order to pinpoint the issue, I hooked up my lab power supply and put some juice on the 12V circuit. This immediately showed that there was a short circuit on that rail. When I turned up the amps, one of the MOSFETs quickly heated up, while the neighbouring ones which were for the other phases didn't. So I decided to desolder that MOSFET and check again.

With that one removed, the short on the rail was gone and the lab supply showed a steady 12V. To double check, I reconnected the original PSU, but nothing happened. It seemed the PSU had died due to the short, good thing I still had another PSU around and decided to try that one. This time the board booted again!

Next problem was actually finding a replacement part. I could only find the old part on mouser, but it was EOL and out of stock, but they did have a similar part (I made sure the rise and fall times were the same or better, since I figured that was important.. for details, google for multiphase buck converter). The part was only 60 euro cent, but of course shipping was a biatch, but I offset that by ordering some more goodies :-)

Few days later, the order arrived and a couple of minutes later, I was enjoying the fruits of my labor... my board was booting again! Victory!