Personally, I'd replace the mosfet. That's a nice motherboard, it's worth a trying to repair it in my opinion.

I doubt that's the original mosfet - the solder work tells me it was replaced at one point... I'm no expert of course, and after googling some pics of your board, the original mosfet is indeed made by CET (can't make out the model from the pics).

Mosfets are a common failure point on any motherboard. I believe that replacing it should fix the board.

Thank you for your assessment! I think I'll try to replace it.
I thought that what looks like a sloppy solder job might indeed be the result of the MOSFET getting too hot and melting the solder by itself...

Since I'm not too experienced with this type of repair, do you think this would be a suitable replacement?
Vishay SUM90N03-2M2P-E3 (Datasheet: https://www.vishay.com/docs/74342/sum90n03.pdf)

Edit: I have now removed the defective MOSFET which was pretty straightforward and dug out an already dead donor board (A7V266-E). This one has a couple of GFB75N03 MOSFETS which I guess I could use. The voltages match (VDS = 30V, VGS = +-20V) and the max. drain current is higher (80A) while the RDS(on) is a bit lower. Datasheet: https://www.digchip.com/datasheets/download_d … number=GFB75N03

Be careful with that K6-2, chances are that it was also killed when powering on the board. Or it was the cause of the cracked/dead mosfet. If a K6 dies due to overvoltage, it can cause a short-circuit on its power supply pins Vcc2 and Vcc3 to GND. This in turn will also kill the voltage regulator on the board.

Don't think the mosfet had become so hot the solder melted. The melting point of 60/40 solder is around 370 deg F and the lead free ones even higher at around 420 deg F. It looks more like a sloppy solder job.

The mosfet shouldn't be cracked. If you have a multimeter, you could measure the input and output voltage and check if it is working properly. The leg that is connected to the coil is the output (you can just measure the voltage at any leg of the coil against ground). Also measure all other mosfets on the board, including those for the CPU and RAM. The CPU mostfet output should match the Vcore of your CPU (with CPU in socket). The RAM mosfet output should also match the RAM voltage.

pyrogx wrote on 2020-07-15, 08:38:

Be careful with that K6-2, chances are that it was also killed when powering on the board. Or it was the cause of the cracked/dead mosfet. If a K6 dies due to overvoltage, it can cause a short-circuit on its power supply pins Vcc2 and Vcc3 to GND. This in turn will also kill the voltage regulator on the board.

Thank you for the heads-up! I took out the CPU and checked the continuity between some of the VCC2 (AN9..19) and VCC3 (AN21..29) pins and VSS (AM8..30). And yep, short. So I will definitely use a different CPU and throw this one away.

kalohimal wrote on 2020-07-15, 08:46:

Don't think the mosfet had become so hot the solder melted. The melting point of 60/40 solder is around 370 deg F and the lead free ones even higher at around 420 deg F. It looks more like a sloppy solder job.

The mosfet shouldn't be cracked. If you have a multimeter, you could measure the input and output voltage and check if it is working properly. The leg that is connected to the coil is the output (you can just measure the voltage at any leg of the coil against ground). Also measure all other mosfets on the board, including those for the CPU and RAM. The CPU mostfet output should match the Vcore of your CPU (with CPU in socket). The RAM mosfet output should also match the RAM voltage.

Okay so maybe the original MOSFET died and killed the CPU as well, then it was replaced (sloppy solder job) but due to the CPU pins being shorted, the replacement died immediately as well... 😉

I do have a multimeter but since I have already removed the dead MOSFET I don't think there's much to measure at the moment. But thank you for the detailed explanation, as soon as I have a suitable replacement component, I will check everything.
First, I will have a friend of mine (who is an electrical engineer) check the datasheets of the dead MOSFET and the one I sourced from the dead Asus board to see whether I can use it.

Tbh, I'd say it's unlikely that the MOSFET was primary cause here. Yes, it probably killed the CPU, but something else killed the MOSFET. I'd check for additional shorts. Sometimes bad caps might do that, but with electrolytics that's unlikely (but possible: I once saw an MS-6309 where some electrolytic caps failed short and the MOSFET that failed because of that actually melted its way through the PCB 😮 ).

Okay, so my friend said that while not really meeting the specs, the replacement from the Asus board would be close enough to give it a try.
So I soldered it in place (slightly less sloppy solder job than before):

I put in a known-good Pentium 166 MMX, fired it up, and yes, it's alive!

So this is already a great success. Although I agree with dionb, it's hard to imagine that the MOSFET itself was the primary cause. I will do some tests under load later and see whether the system gets unstable or any other components get too hot. Also, I will probably hunt down a proper replacement for the MOSFET that matches the specs of the original one better.

Huge thanks to all of you, especially to @pyrogx who has pointed in the right direction with the faulty CPU!

I have a Matsonic MS6260S (Aladdin V PCChips clone) that had a similar problem. It died while rebooting, running a K6-2 500. The MOSFET looked like it melted the solder and lifted off the board- but it’s possible it just wasn’t soldered down well in the first place, and the heat sink wasn’t touching the pad on the board - so it probably went into thermal runaway and died.

Anyways, I found an exact replacement mosfet online and replaced it - fired right up and the k6-2 500 that was there was completely unharmed, still running today.

At least your MS-5196 board appears to have better caps than my (rev 4.0) did, mine was straight out of the Taiwanese capacitor plague era, and had a few leaking 1000µF caps. I ended up recapping the board, and stability improved quite a bit.

I'm surprised by how different the mosfet and capacitor layout around the cpu socket is between the two board revisions.

kepstin wrote on 2020-07-16, 04:58:

At least your MS-5196 board appears to have better caps than my (rev 4.0) did, mine was straight out of the Taiwanese capacitor plague era, and had a few leaking 1000µF caps. I ended up recapping the board, and stability improved quite a bit.

I'm surprised by how different the mosfet and capacitor layout around the cpu socket is between the two board revisions.

Yes, caps are all looking fine to me.

I have now flashed the latest beta BIOS (v. 3.83) and successfully tested with a K6-2+ 550 MHz, which gets detected correctly 😀
I was a bit hesitant because the later BIOS versions for this board are provided by MSI for PCB revisions 3.0 and 4.0 only, but multiple sources stated that 3.83 beta should be working on revision 2.1 as well - and I can confirm it does. However, this PCB revision won't allow for VCore below 2.1V, which is 0.1V more than the K6-2+ would need, but in my experience that's no problem at all. I would have raised the VCore anyways so the CPU will run stable at 600 MHz. But I will take out the multimeter again and measure if the voltages are actually correct after swapping out the MOSFET, just to be sure. Don't want to ruin a rare K6-2+...

"Yes, caps are all looking fine to me."

It should be noted that caps can look fine, but still be completely damaged 😀 In fact, this is the case most of the time. An ESR meter is the single best way to test them (especially in-circuit).

kepstin wrote on 2020-07-16, 04:58:

At least your MS-5196 board appears to have better caps than my (rev 4.0) did, mine was straight out of the Taiwanese capacitor plague era, and had a few leaking 1000µF caps. I ended up recapping the board, and stability improved quite a bit.

I'm surprised by how different the mosfet and capacitor layout around the cpu socket is between the two board revisions.

My MS-5169 also had at least 1 bulging cap when I handled it a couple weeks ago (moving stuff) and I already suspected a bad batch of caps because I had bought that board a long time ago and ended up never using it because while installing an MII I caused too much stress on one of those socket tabs and it broke.
I was kinda surprised as the caps plague only really started getting noticeable at the sA/P4 era.

gex85 wrote on 2020-07-16, 07:02:

I have now flashed the latest beta BIOS (v. 3.83) and successfully tested with a K6-2+ 550 MHz, which gets detected correctly 😀

Interesting! I ended up not using the V3.83 BIOS on my Rev 4.0 board, since it did not correctly detect my K6-III+ CPU. (I've tried a bunch of BIOS versions, so my memory isn't entirely clear, but I think it ended up running with the on-die cache disabled, so it was extremely slow). Maybe I should try it again with a K6-2+ just to see.

But since everything is more or less working with the V3.50 BIOS (most recent non-beta with K6+ support), I'm reluctant to mess with it 😀

Tetrium wrote on 2020-07-22, 03:08:

I was kinda surprised as the caps plague only really started getting noticeable at the sA/P4 era.

There was actually a bit of overlap between the K6 and Athlon eras - Late K6 stuff was produced into the early 2000s as a low end/value option to the Athlon, which was introduced in 1999 (and Athlon XP in 2001). I guess they ended up being replaced by the Duron lineup over time.

macroexp wrote on 2020-07-15, 12:26:

I have a Matsonic MS6260S (Aladdin V PCChips clone) that had a similar problem. It died while rebooting, running a K6-2 500. The MOSFET looked like it melted the solder and lifted off the board- but it’s possible it just wasn’t soldered down well in the first place, and the heat sink wasn’t touching the pad on the board - so it probably went into thermal runaway and died.

Anyways, I found an exact replacement mosfet online and replaced it - fired right up and the k6-2 500 that was there was completely unharmed, still running today.

hi i also have a matsonic MS6260S motherboard that died while running an unreal tournament test using a radeon 9250 (AGP).

Could you tell me which were the parts that you replaced, I am inclined to think that it must have been either Q2, Q3 or Q51.

Thank you!