AMD AM2 motherboards Ram Shorted

[NxB]

I have 2 AMD AM2 motherboards that have been confusing the heck out of me. In both something is shorted in the ram section. I checked out the mosfets and the caps but short remains.

One is a TA790GXB... capacitor went out near the ram and I replaced it. Worked fine for a little bit and then poof. Cap appears fine, I changed a few others but no dice. Also changed 2 mosfets but issue remains.

The other is a M2RS485-BTX.. no obvious caps but short throughout 5v. Caps in almost the same spot as the TA790 are shorted as are mosfets in the same section but the component thats actually out is somewhere else. I've pulled most of them off the board and they tested good.

Anyone else have ram issues on AMD AM2 stuff? What did it end up being?

[NxB]

I got the second board working... I changed 2 caps near the 5v to 3.3v regulator. It still looks shorted tho... but system works. I don't get it.

[mockingbird]

The Biostar board has a bunch of OST caps near the RAM slots... Did you change ALL of them?

[NxB]

Not ALL of them. I should hit that board next when I find the CPU. But why would caps appear shorted and have 0 ESR and then be fine out of circuit?

I fixed the HP board because another suspect cap appeared open in circuit then puffed up during testing.

[Uranium-235]

cause unless you trace the other components to that cap, you never know what the total resistance is going to be 'in circut'. Theres tons of components put together that can create a reading like that

[severach]

Caps supplying a northbridge or CPU or other high power chip often look shorted when the component is in place.

[b700029]

Originally Posted by severach

Caps supplying a northbridge or CPU or other high power chip often look shorted when the component is in place.

I second this. All chips have protection diodes too, which can cause the readings to be different if you reverse the multimeter probes.

Modern CPUs draw over 100A peak at 1.x volts; that is an effective resistance of ~10mOhm. This is when they're actually operating since CMOS circuits ideally only draw power on clock transitions, but in CPUs the leakage current through the MOSFETs even when idle with the clock stopped is not small - quoting Intel on the "8-9W" idle power consumption of the recent i7s, that's 5-6A at the 1.x volt supply, meaning the CPU has a static resistance of < 1 ohm.