If you have a multimeter you may be able to probe the pins on the AT connector while it's running and measure the current, then you could compare against the working PSU. Perhaps the PSU is mostly working but isn't delivering enough current and that is why the system runs slow, and why the POST beep is lower pitch.

If there is a faulty capacitor, maybe that is why it works briefly and then needs a complete discharge before working again.

Just curious, does it show manufacture date on the PSU? Is it an old stock?

Will wrote on 2023-03-31, 17:57:

I bought an Athena AT40 AT PSU from Amazon for a 486 build I'm working on and it doesn't seem to be working.

I have 2 of these Athena AT40 AT PSU's and they are somewhere deep down in a box and I will never use them because their quality is... scatchy, to say the least.

Not only that, AT power supplies can be dangerous as hell.

I would highly recommend to get a better quality ATX PSU and use an adaptor cable to power an AT motherboard.

Will wrote on 2023-03-31, 17:57:

I've tried it with 3 different boards, and all of these boards work fine using a really old 2o0w AT PSU.

This is a good way to kill all your precious old hardware.

Edit: To answer sangokushi question. I don't remember dates on those PSUs. I think they might have been manufactured in the mid to late 2000's. So, not exaclty NOS, just a bottom of the barel very late production run.

Is this just a bare board connected to PSU or are you remembering to connect an old drive to it to put some load on 12V ?

sangokushi wrote on 2023-03-31, 18:51:

Just curious, does it show manufacture date on the PSU? Is it an old stock?

It's brand new from Amazon.

Turns out I think the problem was not enough load. When I was testing I only used a board and a video card just to see if it would post. I added two more cards and connected a cd-rom drive to power as well, now it works everytime. 400w is overkill for a 486 system, but it was actually cheaper than the 300w when I bought it.