Keith's Electronics Blog

This wretched miserable piece of notworkingness has been causing me a fair bit of grief in the last couple of weeks.

This is the power supply from the Dell Poweredge 750 (yes, that’s 750, not R750) server from which, if it is still near the year 2021, you are being served this web page. A couple of weeks ago, it developed the new behavior of powering up perfectly well to start booting the server and start running the BIOS tests and then shutting off again.

Being in a bit of a hurry at the time, I grabbed my spare ATX power supply and hooked it up to get the server running again with the lid open and deal with the original power supply later. But with the new power supply connected, the server wouldn’t boot at all, because

what is this??? Oh, for a larff, let’s move the purple 5V standby wire from the ATX mother board connector to the PCI power connector, har har! So with the spare power supply connected, the server wasn’t receiving 5V standby [where it wanted it] to run the On button to turn on the power supply and the server.

I got it running temporarily by jumpering the /PS-ON pin to ground. As one does. And thus sits the server.

I shopped for a replacement power supply but found only one direct replacement, which appeared to be old stock, and by old stock, I mean this server is circa 2003 and so I believe was the power supply for sale; so I didn’t have a lot of faith in its longevity. I found no alternate power supplies with ATX connectors that would fit in the 3.5″ x 11.5″ x 1U space this power supply has inside the server case.

I had a look at the original power supply last week; checked the electrolytics with the Capacitor Wizard (they are, surprisingly, fine); and then noticed that one of the two fans was completely seized. And they’re three-wire fans. Oh, says I, perhaps the power supply is giving both fans a while to spin up and when it detects that one hasn’t, it shuts down to save itself. Very reasonable.

I removed the fan and did my trick of peeling back the foil sticker and putting one drop of oil at a time onto the shaft while spinning the blades to work in the oil, with which method I’ve got years of additional use out of seized fans. It worked superbly and soon the blades were spinning freely and striking the fan housing. Yes. The only thing previously keeping the broken fan’s blades from hitting the housing was that the fan was seized.

I ordered a replacement 40x40x28-mm fan, which arrived today; as well as a JST-PH connector set, since that’s what’s in the power supply and the fan came wired with what I know as a TE CST-100 II connector on it.

I had poor luck crimping the JST-PH pins (2.0-mm spacing) with this crimper that works well on JST-XH pins (2.5-mm spacing) — it squashed the barb meant to keep the pins in the housing — so I surrendered and spliced the old connector tail onto the new fan. Alas, the power supply still has the same behavior as before, albeit louder now that it has two fans running enthusiastically.

So. Unless another idea arrives via personal brilliance, comments, or email, I’ll be ordering a “fully modular” ATX power supply (with all of the cables detachable from the power supply), modifying the new power supply’s cables either to reroute the “green” 5V standby wire over the motherboard’s PCI power connector or to jumper the “purple” /PS-ON wire to ground, preconnecting the cables inside the 1U server case and running them out the hole in the back where the original power supply mounted, closing the case, and connecting and sitting the ATX power supply on top of the server in the rack.

This entry was posted on Saturday, November 13th, 2021 at 06:38 pm by Keith Neufeld and is filed under Repairs. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.