Working on Gigabyte GA-8IR533 mainboard with short to ground on 12V \ VOGONS

Working on Gigabyte GA-8IR533 mainboard with short to ground on 12V

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First post, by lowlytech

Posted on 2020-02-10, 00:32

lowlytech Offline

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Had a P4 I rescued. Didn't know anything about it's status. Before powering on I opened it up and there were 3 caps (1500uf 16V) that were bulging (marked in Green). The one cap on the far right was actually popped off it's base and hanging on by a couple of threads, almost like it attempted to blast off. Well got the caps in the right way (triple checked) and I connected up a good power supply and plugged it in and about a second later a huge pop and flash occurred in the yellow square area. After this, when plugging in you get a small CPU fan jerk and nothing. The CPU 12V seems shorted to ground. Didn't notice the 12V on the main ATX harness being shorted to ground, so just something on the CPU side. I looked at this thing for 10 minutes in that area where the spark happened and I don't see any burns or any signs that anything even occurred.

Where is the best place to start to hunt for a short to ground on the 12V cpu line?

Reply 1 of 3, by lowlytech

Posted on 2020-02-10, 00:56

lowlytech Offline

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Additional photos, I took the heatsink off the chipset to make sure there were no craters there. The CPU installed at the time was a 1.7GHz Willamette P4.

Last edited by lowlytech on 2020-02-10, 01:39. Edited 1 time in total.

Reply 2 of 3, by lowlytech

Posted on 2020-02-10, 05:37

lowlytech Offline

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Think I found the short at Q151. When removed from the board pin 1 and 3 measure 0.51 ohms. After this is removed my 12V short to ground is gone. My next problem is I don't know what to put back there. The chip is somewhat unreadable but I can make out what looks like PHD66NO(orQ), 03LT. However I am unsure why this blew up in the first place, so I figure if I do find a replacement, it may just happen again. The 2 caps closest to the Q151 are rated at 3300 that measure appox 3700 but still have an ESR of 0.23 ohm but I will get these swapped out as well. The other caps along the CPU socket do measure their rating.

Any ideas on Q151 and what to use?

Reply 3 of 3, by Deunan

Posted on 2020-02-10, 10:34

Deunan Offline

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lowlytech wrote on 2020-02-10, 05:37:

Any ideas on Q151 and what to use?

MOSFET powet transistor, for the 12V to whatever-CPU-needs converter. Seems like there's 4 of them, the other 2 are probably identical (cheaper that way) so maybe one of them offers a better readable markings.
The one you have most likely died due to heat - you can see the PCB discoloration and the poor state of the solder under the pad. Possibly bad airflow, O/Ced CPU or just poor mobo design.

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