Hi all,

I could really use some help figuring out why I am unable to get my Compaq Deskpro EN PIII 450 MHZ motherboard (SP#: 178919-001) to boot with a modern ATX power supply - EVGA SuperNOVA 750 https://www.evga.com/products/specs/psu.aspx? … 95-109c09fa0c01. I've tried doing my own research and searching for various pinouts. I came across https://pinoutguide.com/Power/compaq_deskpro6 … 00_pinout.shtml which looks correct. It looks like the non standard Compaq ATX Power Supply - Part No. 334112-001 should be close enough to the ATX spec that swapping it out for a modern ATX power supply should just work but that doesn't appear to be the case.

The obvious differences being the Compaq uses a Fan Off signal in place of PWR OK (pin 8 ) and the new EVGA power supply is missing the -5V rail and thus no pin 18. Neither of these are an issue though as I removed both pins from the Compaq power supply connector and the motherboard still boots fine.

As a bit of an additional quirk, though it doesn't really affect the issue at hand, the power connector for the motherboard isn't actually on the motherboard itself, it's on the riser card:

Here's the motherboard:

The motherboard boots fine using the Compaq power supply but will not boot when using the modern EVGA power supply.

Compaq Series PS 2013 Part No. 334112-001

The Compaq power supply Part No. (334112-001) matches the Part No. for the pinout guide I linked above. The wire colors all match EXCEPT for pin 7; the pinout guide lists the color as black (ground), mine is grey. That being said, doing a continuity test on pin 7 still appears to be ground for me so I'm not sure why the wire is grey? It must have some significance given all the other grounds are black.

As a last ditch effort, I measured the voltages of both the Compaq PS and the EVGA PS by forcing them on (not connected to the motherboard) to compare values and that's where I've become really confused. Let me explain:

The label on the Compaq power supply has the usual values for an ATX power supply
+5V
+3.3V
+12V
-5V
-12V
+5V AUX

When I force the Compaq power supply on by connecting pin 14 (white On/Stdby) to pin 9 (green +5V AUX) and measure the voltages I get values like:
+3.8V
+12.5V
+5.54
Notice how they seem to be roughly half a volt over spec BUT (and here's the part I find really confusing) when I connect the power supply and boot the motherboard and then measure the voltages at the riser card, the values show much closer to spec:
+3.35V
+11.96
+4.96V

The same voltage drop happens with the EVGA power supply too. The difference here being when I measure the voltages by the forcing the power supply on (disconnected from the motherboard) they are very close to spec (not roughly half a volt over spec like the Compaq which of course is what I would expect). Yet when I connect the EVGA power supply to the motherboard, power it on and measure the voltages at the riser card the same .5 voltage drop occurs with the values roughly half a volt under spec which I suspect could be why the motherboard isn't booting?

I'm really confused as to what's happening here and would appreciate any ideas of what else I could try and/or test.