DEAD MOBO - Did a PSU Cause This??

So here is my story. I obtained an Emachines desktop from a friend..about 3 years old. It just wouldn't turn on. The little green indicator light on the MOBO was on, but other than that, the computer was dead. I tested the PSU with a tester, it tested perfectly (+3.3V, -12V, PG, +5VSB, +12V, -5V, +5V). All of these voltages lit up, so I determined it was the MOBO. I purchased a brand new MOBO, pretty the same one that came out of the machine in fact. I installed it, plugged the PSU in, hit the power button and nothing. Checked the front panel button configuration, that was correct. Everything else was plugged in correctly. Checked the PSU again, tested fine. Figured it was a DOA and I sent it back.

I then received a brand new replacement board, but I DID NOT used the same PSU. Used a different, known working PSU and the computer functioned fine. At this point i still thought that lat MOBO was a DOA. So I kept that old PSU that tested fine on my tester. I then put the PSU in a known working computer, and it would not turn on. Same symptoms as that "DOA" board. The little green light came on, but it was dead. Now this is in a P4 Dell. I then plugged in a know good PSU, and the computer is still dead...so now I have a dead MOBO in my Dell that was known to be working as of yesterday.

Now, I am almost positive it is this PSU blowing these boards out, but why? If the PSU tests perfectly (meaning it is putting out all the correct voltages), what else would cause it to blow out motherboards? Now unless this tester is not accurate, which I feel that it is, I can't figure out why this PSU is blowing thee out?

Those PSU testers don't tell you anything about the quality of the PSU's various outputs, particularly when loaded. If merely swapping in that PSU caused a previously good system to croak, unless you find a grounding/shorting problem, chances are the PSU killed it.

OLd PSUs have lots more amps on the 5v rail and todays tech has very little 5v amps and high 12v amps. It could be your not getting the right amps on the right rail.

Well it seems I have learned the hard way that these basic PSU testers are basically useless. Is there any special...more expensive meters than can actually measure quality, spikes, and regulation of voltages?

Not unless you want to spend a few hundred $ for a test bed (oscilloscope,etc)