Get some of the needle point leads that connect to the multimeter. They'll fit in there easily. Also, to test the ripple voltage, you'll need to have the power supply loaded -- the easiest way is to connect a dummy load to the molex connector (if there is one) sized for 50% wattage of the power supply's rating.

Open it and look at the inside.
A visual inspection would be the first thing to
do before firing up such an old device.

make sure the black cables from both plugs are on the inside and the red cables on the outside when plugged into the mb, just the way you have it in the picture. You probably know this but doesn't hurt to mention.

and yes as mentioned open it up, clean the dust and inspect all the capacitors etc

Thanks for all the replies! So I did as advised and opened up the PSU and blew air through it to get all the dust out and did a quick visual inspection and didn't see anything abnormal ... plugged it in and it will power up about half the time I push the power button. The other half of the time it does as it did before (tries to start but dies quickly i.e. ... keyboard light flashes ... fan begins ... fan stops/powers off ... after about 1-2 seconds). Seems to run fine when running. It's running right now with a keyboard, monitor and floppy disk attached ... and I can get to the BIOS and boot from a floppy disk that has a little Linux kernel on it. I'll attache a HDD next time I power down. I've moved the PSU while it's running to see if it's a short. So far it has not powered down when I move the PSU while it's running. So ... I'm not sure what may be causing the PSU to not start half of the time. This is about as far as I've ever gone trouble-shooting a PSU. Anyway ... if anyone has a suggestion for testing/trouble-shooting the PSU for its failure to power up part of the time ... I'd be willing to try. Otherwise I guess I might try find someone who has an older PC that might have another PSU that matches this motherboard. Might be kinda' hard though since the PSU connecters are different from what I'm used to. Again ... thanks for all the replies. Much appreciated!

Does your AT PSU have a thick, line-type power cord running to the switch and four wires connecting the cord to the switch? This is the most common setup, although some soft switched AT PSUs do exist.

If so, mains power is mechanically latched on at power up. If the PSU doesn't power up and doesn't catastrophically fail, it's being interrupted by either the power switch or one of the protective devices in the PSU itself (and those don't generally reset). There's no status checkout as with an ATX supply that we have to worry about.

I'm wondering if the switch might be dirty and intermittent.

gdjacobs ... thanks for the reply. Indeed the switch on my PSU looks identical to your photo. And I'm finding the more I use the switch today ... the less it's having trouble powering on. So I think you're correct ... that it's a dirty switch. Can I use a tad of Deoxit on a switch like that? Anyway ... since it's getting better at powering up now ... maybe I just needed to limber up the switch.

On a side note ... I'm trying to get this Zenith motherboard to recognize an old IDE Conner CFS270A 270mb HDD. The BIOS (PhoenixBIOS V 4.03) will not recognize the Conner HDD on either IDE motherboard controller (IDE0 or IDE1). BIOS says "Failure Fixed Disk 0". The BIOS IDE Adapter 0 Master field was set to Auto (and wasn't detecting) ... so I input the HDD specs manually ... and after that the IDE Adapter 0 Master field within the BIOS detected the drive as "C: 270mb" ... which is correct. But when I save and reboot ... I still get "Failure Fixed Disk 0". The HDD is jumpered as Master/Single and works on another PC's IDE controller (boots into DOS) ... so I know the HDD works. I tried two different IDE cables that I know work ... with the pin connectors in the proper orientation. What am I missing? Sorry ... I just realized I should start another thread since this question isn't related to the PSU. Anyway ... thanks again for the reply!

obcbeatle wrote:

gdjacobs ... thanks for the reply. Indeed the switch on my PSU looks identical to your photo. And I'm finding the more I use the switch today ... the less it's having trouble powering on. So I think you're correct ... that it's a dirty switch. Can I use a tad of Deoxit on a switch like that? Anyway ... since it's getting better at powering up now ... maybe I just needed to limber up the switch.

That's what I'd do. Shoot it full and see if it helps.

I have 1 at psu as a tester and don't have a external switch, what I do is plug those prongs together on the psu cable (the 4 from the power switch) then plug the power cable into the wall socket and the psu will switch on, just make sure u connect the right prongs together like on the case power switch, this will emulate the power button being constantly pressed in, good way to diagnose if your button is broken

Remember, that switch is MAINS, we've got used to not having nasty voltages in a PC case since ATX, so treat it with respect.... I tripped a lab RCD when I thought I'd unplugged it, but had unplugged something else

yes I forgot to add make sure u have some insulation between the 2x2 prongs connected together because if all 4 or the wrong 2 make contact together there will be trouble, a piece of cardboard between them is what I do.

good point

Hello ... thanks for all the replies! I ended up shooting some Deoxit into the switch and the PSU seems happy now. I am going to return to this thread for later use re: a PSU tester as I have another PSU with some issues that I plan to get to soon. Thanks again for the help ... and thanks to all who replied!