What is your current PSU? 25A on the 5V line should be more than enough for a Coppermine/Tualatin, even if it's stuffed with expansion cards. I ran a 70W TDP Athlon XP for years on a PSU that delivered up to 30A on the 5V rail, so a 30W TDP P3 should require considerably less power. I suspect there may be a problem with your PSU, maybe a crossloading issue?

Regarding your options: I would take the StarTech unit, I don't know much about the ATX ones, but the AT variants are not bad at all (I have a couple of them), and as I said before 30A should be plenty for your system. At the very least their power output ratings are honest, unlike that "Kentek" modern PSU (high 5V/3.3V output seems dodgy, all decent modern units should have a heavier 12V rail).

And you definitely shouldn't spend hundreds of dollars on a 1600W PSU to run a P3, unless money is not a concern for you.

Current PSU is a SolidGear SDGR-550E: https://www.newegg.com/Product/Product.aspx?I … =1HU-008G-00027

I have multiples of this model new-in-box and I've already tested with a second identical unit to rule out the first one beginning to fail. Behavior is identical so I feel confident it's not a problem with the PSU in that sense. However thanks to your comment I just learned what crossloading means: http://www.playtool.com/pages/psucrossload/crossloading.html

+5 runs low, 4.5-4.8v and +12 runs high, 12.8-13v so I think that speaks to a crossloading issue if I understand correctly. That also suggests favoring the StarTech unit in that the article mentions "older PSUs were designed to regulate voltages best when most of the load was on 5 volts. New PSUs like ATX12V 2.0 models are designed to work best when most of the load is on 12 volts."

I think I'll order the StarTech unit! I'll just have to paint it black for looks 😀

try loading 12V rail a bit

You have practically no load on 12V.
Instead of buying another PSU, just install a hard drive, preferably one which pulls more than 1A at 12V.
Then the voltages will probably come back to spec.

I use a Corsair TX850W that is rated for up to 30W on the 5V rail, and it has a bunch of molex connecters unlike many modern PSUs. I have a 1,1Ghz PIII installed right now that runs fine with various other devices plugged in GF4/HDD/DVD/FDD/Vortex2. I have a 1,4Ghz Tualatin sitting on my desk, but I have been pushing off putting it in as the cooler I am using is nearly impossible to get off. The PSU ran me about $35 used from an auction site locally (Japan), but prices might be different in your location.

A strong +5V rail is not the only requirement for an older system. You'll also need a PSU that's capable of handling the extreme cross loads. From my experience the better PSU's (Seasonic or be quiet! starting at around € 85 and upwards) don't use load balancing but have a separate rail that converts +12V into +5V which does the job. Yes they are only 100-120W on +5V but that's enough for an average PIII pc.

Yeah, try adding a hard drive. the 12V load of a Pentium III system with only ssds is going to be very very low.

Given your 550W PSU is really rear bottom of the barrel in terms of design and probably construction, I will echo other people saying that your system is suffering from extreme crossload. More expensive PSUs which handle crossloads well work better. You get more stable operation of adding more 12V load.

My 5V heavy PSU is an Antec truepower 380W that I've going ahead and recapped in full. Not cheap, but a very versatile PSU once recapped. Handles both 12V and 5V computer loads well.

Don't buy off brand PSUs at all. They are usually the cheapest chinesium crap you can imagine, they skimp on component quality, particularly on the caps, they have terrible construction, questionable regulation at the best of times and worst of all, they lie about their specs

If there is a single component to never "save" money on it is the PSU. There is not a single component that can fail with more expensive consequences than this and if you are exceptionally cheap and unlucky, it might even burn your house down.

Good luck finding that Startech PSU. 😉 They seem to have lost interest.

There are still quite a few decent early ATX PSUs available on Ebay though. Of course, they're no longer young, and ideally, you should re-cap them. But if you inspect them every now and then for signs of cap failure, you can probably get away without it for some time.

Conclusion: I received my StarTech PSU and I'm very happy with it. Actually I'll admit I purchased both PSU's I mentioned in my original post. I had already purchased the KenTek one before posting here because I'd initially thought it would be the better option. OMG, it was so bad guys!! The KenTek housing was so thin I could deform it by pressing hard with my thumb and they took some wild shortcuts internally too. Despite having a claimed 680w capacity, the transformers were half the size of those in the 300w StarTech unit and instead of soldering the individual output wires to the board like every other PSU I've ever opened up, this one had a single thin metal clip for 3.3v, one for 5v, one for 12v, and two for grounds. Then the wires were twisted together and clasped within these little clips. I could probably have tugged and pulled wires loose if I'd tried. The 120v input wires were so thin too! Like half the thickness of the same wires in the StarTech box. Even if we suppose that everything would have worked out OK with the KenTek unit, it squealed something awful when powered-up, even with no load! The sound was like that of a failing PSU, but the loudest I've ever heard. I would never have used it, just on that fact alone. The sound would have driven me crazy!

Back to the StarTech unit, its construction is nice and heavy (twice the weight of the KenTek) with a nice thick metal housing; really solid! The output voltages are right where they should be too - I've got 5.0v on the nose under full load, and +12 is always between 12.0-12.2, never the 13.0 I was reading on my old modern unit. Also resolved are the quirks I was having with the PSU I replaced - having to sometimes press the power button more than once to fire up the system, and not being able to restart is fixed too. The voltages used to momentarily drop too low during a restart and some components would not initialize properly, so I always had to power down completely and then back on after a few seconds.

One last note: I originally thought the StarTech box was new-old-stock because it's clearly such an old design but the PSU I received does have a sticker on the metal housing that indicates it was manufactured in June of 2018 so I can really recommend this product as a great option for anyone looking to build a retro rig. Given that I opened mine up to examine the guts, I can say that everything looked really well-made and there was even a little temperature sensor board drilled into the larger of the two heatsinks, which is something I cannot recall ever having seen on these models back 20 years ago, so the design may have even been updated with modern safety concerns in mind. It really did look out-of-place like something that was patched into an existing design as an update. Anyway, the NewEgg link is in my original post if anyone else wants one of these. Thanks again everyone!!

Try to run 3DMark 2001SE maxed out and see whether the new PSU can handle it.