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Old motherboard, new PSU problems

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First post, by homebrewcpu

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Hi - I’m looking for some help or advice. I’m doing a build using an ASRock K7S41GX with an Athlon XP and it’s mostly going well - except for power supply weirdness.

I was under the impression that I could just use any modern ATX supply, but things aren’t working out the way I expected...

I’ve been testing with three PSUs:

1. Super cheap $18 “Coolmax 400”. Reliably boots every time, but this thing feels so cheap I don’t trust it at all and will not use it.

2. EVGA 500 BR. This is the supply I intended to use. However the machine only boots about 1 out of ten times. I’ve got a POST-code card installed, and most of the time it doesn’t begin posting - just 4 “-”’s. However, I discovered that I can get it to consistently boot if I turn it on, wait a few seconds, turn it off and then turn it back on just as the LEDs from the POST code card start to fade. If I can get it to start POSTing, then all is well and the machine runs great (until I fully power it down).

3. Corsair RM 650X. This is the supply I’d like to use (modular). However I’ve never gotten the machine to boot with this one and the turning off & on trick that works with the EVGA doesn’t work here.

I thought it might be related to the power good signal, but testing shows all three supplies asserting it with about 240 to 350 ms. I also thought it might be bad caps. When I got the board, it had one bulging cap which I replaced before starting. However, I’ve since done a full recapping and there is no change.

I recall that years ago, switching power supplies needed a minimum load to regulate properly, but I thought that isn’t the case with modern PSUs. Perhaps it’s an issue that I’m not drawing much from some of the rails?

I’m fairly new to building PCs, so I'm probably missing something obvious. Any advice would be greatly appreciated. Oh, and the reason for this build is that I want a machine that can natively handle 2 floppy drives: a 360k 5.25 floppy and a 720k 3.5 floppy. The K7S41GX supports that configuration. (And, of course, I want to play some games from the late 90’s to early 2000’s).

Thanks,
…Bill

Reply 1 of 3, by Solo761

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It's almost the opposite especially with these Athlons😬. Modern PSUs are not a good match.

Computers of this era used mostly 5 V rail, that's why, for example, 400W PSU of that ERA had 30+ A on 5 and only 10-15 on 12 V rail.

On modern PCs most power comes via 12V rail and that's why for example 600W PSU has 40 A on 12 V and 10-15A on 5 V and 3.3 V rails.

Here's an example of one PSU I pulled from Athlon XP PC, it's only 300W, but it has 20 A on 3.3 V and 30 A on 5 V. Though, not at the same time, it's limited to 160 W. Now compare with specs of some modern 500+ W PSU 😀

The attachment PSU.JPG is no longer available

For Athlon (XP) systems people recommend PSUs that have at least 24+ A on 3.3 V and on 5 V rails. Some older Seasonic (made cca 4-5 years ago) models have beefier 3.3 V and 5 V rails and they're still available in stores. And some LC-Power ones (though they're not known to be the best supplies).

EVGA one is "on the edge" when it comes to these requirements, it has 20A on 3.3 and 5 V rails. Although they seem to be joined. Specs says their combined output is limited to 120 W so it's not that it can provide 20 A on each, just that 20 A is maximum that each can give. Maybe that's why it has problems.

Corsair is a bit better, it ha 25 A on 3.3 and 5 V rails, but again, combined output is limited to 130 W so it can't give out 25 A on each rail. So potential problem...

As for Coolmax, I see there are two I-400 and V-400, both are again combined as EVGA and Corsair, but I-400 is rated for 177 W combined, V-400 is more similar to EVGA and Corsair with 130 W combined.

Last edited by Solo761 on 2022-04-01, 06:11. Edited 1 time in total.

Reply 2 of 3, by Sombrero

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The 2018 models of Corsair RM series have 25A 3.3V/5V rails (I think 650W models and down had ~130W total power output for 3.3V/5V while 750W and up have 150W ) which is enough for Pentium 3 but I don't think it's enough for the fastest Athlon XP CPU's out there, I remember someone saying that might be ok up to XP 2600+ but not 100% sure about that. And if your PSU model is not the 2018 then it probably has less than 25A 3.3V/5V.

Reply 3 of 3, by homebrewcpu

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Ah - thank you all! That makes sense - I hadn't even considered the distribution of power across the rails. I'll start looking around for something beefier on the 3.3v/5v rails.
Thanks again,
...Bill

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