Hi,

I have two boards which have AT power connectors, and they both show the same behavior: when I use them with an AT PSU they work (i.e. boot, I'm not doing any stability test yet to these machines), but when I plug my test PSU (with a passive ATX-to-AT adapter), it shuts down immediately, like if a protection was kicking in. This same PSU works well with modern boards, I've regularly used it to test socket 775 systems, although it might be a bit in the "old and cheap" side being a no-brand 450W unit.

I've read about the issues with ATX PSUs in AT boards (missing -5V, dummy loads, overcurrent in 5V), but people usually don't describe the same problem I see here. I'm suspecting the boards are at fault somehow, and would like to know if the symptoms are common or point to a particular kind of problem.

The problematic motherboards are: 1) a dual socket 370/slot 1 board with AT and ATX power connector, and 2) a Socket 7 board from a Compaq Presario.

1) boots about half the time with the AT PSU, the rest of the time it stays on but doesn't POST. When using the ATX PSU, either connected to the ATX connector or to the AT connector with the adapter, the PSU immediately shuts down.
Another thing I noticed, that might or might not have anything to do with it, is that the AGP video cards I plug seem to get really hot in the opposite side to the video chip, to the point that I tested a few that now have a brownish area there. This might be normal, the cards might be defective, or getting hot might just be their normal way of operation, but I don't have a lot of experience with them (I should try them with other boards to see if they behave the same)

2) boots every time with the AT PSU, but I had to replace a few SMD tantalum caps that exploded when I plugged it in for the first time. I replaced them with electrolytic caps because that's all I have on hand, but the point is that it might have other issues caused by the caps shorting out.

Any hints?