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enseitankado 04-24-2018 10:36 AM

Vcore leak/spike at MSI P4M900M2 motherboard
4 Attachment(s)
I'm working on faulty MSI P4M900M2 mainboard.
First, I accept that it is outdated board, the repairs forced me so much that it turned into a personal challenge.
Because I am trying to record an educational video for my students. I think I spent almost 15 hours working. Of course I also agree that I am not an expert on motherboard repairing.

At first, there was not a Vcore and was short-circuit on power input of CPU. I checked alot of components and inspect short-curcit at front-end of CPU PWM controller and power mosfet at phase of Vcore. Also open-circuit 2.2Ohm smd. I changed all of them. Now, mainboard can start but Vcore mosfets and the CPU itself overheating in 30secs and enters thermal protection.

When I was look Vcore's wave shape detect a spike/leak square waveform. I attached it the post.

After this tested each of the Vcore phase groups (3xMosfet) separately (via desoldering each of groups), but result not changed.

I could not detect a fault component around the CPU pwm controller and vcore mosfets. ie short-circuit or open-circuit elements.

I need your advice and experience in this regard. Where else should I focus. Thanks already for your help.

enseitankado 04-26-2018 06:28 AM

Re: Vcore leak/spike at MSI P4M900M2 motherboard
Is there any idea to help? :(

piernov 04-27-2018 03:47 PM

Re: Vcore leak/spike at MSI P4M900M2 motherboard
The waveform shown on the oscilloscope is typically measured between the high-side and low-side MOSFET, before the inductor. VCore must be measured after the inductor (marked R25 on the picture), or across the output filter capacitors.
The waveform is indeed spikes of 12V. The MOSFETs switches on and off to output 12V then 0V. The inductor and the capacitors filter the signal to get a linear voltage, average of the previous waveform which should be 1.2V to 1.4V. This is the actual VCore sent to the CPU.
If VCore is lower than 1.5V (it should be) and the CPU is overheating, something is wrong with either the CPU (it should be replaced) or its cooling system, ie. not making good contact with the heatsink or bad thermal coupling (bad thermal paste).

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