Hello all
I'm new to the forums.
I am trying to troubleshoot an old LGA775 Asus P5P800-VM. This MB comes from an old 35mm Cinema sound processor which I am collecting for nostalgic reasons! As this is a custom system, it may work with another board or it may not. If I cannot fix it I'll get another identical one on the internet but for now I am doing my best to fix it.
Little history.
- System failed to boot intermittently. I thought it was one of the expansion boards but in the end it failed to boot altogether even with nothing connected.
- Inspecting the board on the bench I found that the 12V CPU line is immediately drawing 8A as soon as the PSU is switched on. The VRM mosfets gets ludicrously hot, the CPU gets warm but it takes several seconds. The current is so high that I can hear the chokes whining.
- If I remove the CPU, there is no activity and the Mosfets stay cool.
- I have to power on the PSU manually as the Power On pins don't work.
- I ordered a new CPU (Intel Celeron) but issue is unchanged
- Pins on the CPU socket seem ok.
- I have a thermal camera, I do not see anything getting hot besides two of the mosfets - the other 6 (three phases) are still getting fairly hot though.
There are also four 4Ohm SMD resistors close to another mosfet in the middle of the board which are getting very hot - considering they are tiny. But I read the correct resistance when I test them. I removed the mosfet which is nearby to test but nothing changed.
- From my understanding the mosfets are not shorted. On the CPU VRM ones I can read approx 1V on each phase. Voltage is not exactly the same on all the phases though.
- I have tested all other mosfets for shorts and nothing seems to be out of the ordinary, and none of them is getting hot.
- I do not read a short on any power line. But If I understand correctly the resistance I am reading is a bit low.
- I have tested a different PSU.
- I have replaced some of the capacitors on the VRMs. Those are KZG caps and I read around they may go bad without bulging. They tested ok once removed and indeed new ones did not help.
- I checked the phases with my scope. I do not know much on the subject but I thought they were supposed to be staggered so that each phase would only work 1/3 of the time. On my 4-ch oscilloscope they indeed pulse but they run simultaneously.
So in short my VRM gets really hot only when the CPU is in. But the CPU is not bad. When the CPU is plugged in, there is something drawing 8A@12V (96W) that's pushing the mosfets to 100+ degrees C in a matter of seconds.
I am a bit short of ideas here. Would anybody have any suggestions of what to try next? I appreciate any input you may have!
Thank you!
Tony
I'm new to the forums.
I am trying to troubleshoot an old LGA775 Asus P5P800-VM. This MB comes from an old 35mm Cinema sound processor which I am collecting for nostalgic reasons! As this is a custom system, it may work with another board or it may not. If I cannot fix it I'll get another identical one on the internet but for now I am doing my best to fix it.
Little history.
- System failed to boot intermittently. I thought it was one of the expansion boards but in the end it failed to boot altogether even with nothing connected.
- Inspecting the board on the bench I found that the 12V CPU line is immediately drawing 8A as soon as the PSU is switched on. The VRM mosfets gets ludicrously hot, the CPU gets warm but it takes several seconds. The current is so high that I can hear the chokes whining.
- If I remove the CPU, there is no activity and the Mosfets stay cool.
- I have to power on the PSU manually as the Power On pins don't work.
- I ordered a new CPU (Intel Celeron) but issue is unchanged
- Pins on the CPU socket seem ok.
- I have a thermal camera, I do not see anything getting hot besides two of the mosfets - the other 6 (three phases) are still getting fairly hot though.
There are also four 4Ohm SMD resistors close to another mosfet in the middle of the board which are getting very hot - considering they are tiny. But I read the correct resistance when I test them. I removed the mosfet which is nearby to test but nothing changed.
- From my understanding the mosfets are not shorted. On the CPU VRM ones I can read approx 1V on each phase. Voltage is not exactly the same on all the phases though.
- I have tested all other mosfets for shorts and nothing seems to be out of the ordinary, and none of them is getting hot.
- I do not read a short on any power line. But If I understand correctly the resistance I am reading is a bit low.
- I have tested a different PSU.
- I have replaced some of the capacitors on the VRMs. Those are KZG caps and I read around they may go bad without bulging. They tested ok once removed and indeed new ones did not help.
- I checked the phases with my scope. I do not know much on the subject but I thought they were supposed to be staggered so that each phase would only work 1/3 of the time. On my 4-ch oscilloscope they indeed pulse but they run simultaneously.
So in short my VRM gets really hot only when the CPU is in. But the CPU is not bad. When the CPU is plugged in, there is something drawing 8A@12V (96W) that's pushing the mosfets to 100+ degrees C in a matter of seconds.
I am a bit short of ideas here. Would anybody have any suggestions of what to try next? I appreciate any input you may have!
Thank you!
Tony
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