Any PC power supply experts out there? I'm just trying to repair a friend's PC with a dead PSU. Well I assume it's dead - it does produce the +5V standby rail OK and when the PC's front panel power button is pressed the PS_ON signal line is pulled low, but nothing else happens - none of the main power rails come up. IOW the PSU is getting the command to turn on, but appears to be failing to do so.
Anyway, a nice simple job I thought, trotting down to the local Chinese computer take-away (MKC) to buy a new PSU. "Oh they all come with a 24-pin motherboard connector now, not 20-pin like that" (pointing at the old one I'd taken along). "Don't worry, it'll be compatible, you'll just have 4 unconnected pins on the connector. Line the latching tabs up and it'll be OK."
Tending not to trust anything anyone in a shop says, ever, I have carefully verified that if the new PSU connector is inserted into the mobo connector pin 1 - pin 1 then all rails and signals do correspond, and the four new pins only duplicate the +3.3, +5 and 0V rails. The latching tabs don't align in this position, but that's immaterial, other than confirming my mistrust...
But there is one possibly significant difference. The old PSU provides a -5V rail on pin 12, but the new one doesn't - there's no connection to the corresponding connector position (now pin 14 in 24-pin land).
Hmm, time to RTFM. Does the motherboard - an Asus A7V8X - actually need a -5V rail? The manual (user guide) from their web site shows the 20-pin connector pin-outs, including -5V, but says nothing about what it's for or whether it's needed. Only slightly more helpfully, the new PSU's installation guide - it's a CoolerMaster RS-380, by the way - says this:
"This power supply does not support –5V for the motherboard for new ATX form factor operation. Please update the BIOS setting of the motherboard."
Now the questions:
1. DoeS aNyBody know whether the Asus A7V8X motherboard needs a -5V rail? Will it be harmed if powered-up with this rail missing (o/c). What is/was this rail used for anyway?
2. What have BIOS settings got to do with this? How can changing a software (firmware) setting compensate for the lack of a power rail?
3. What is the line labelled PG (pin 8 on both version) for? (This question is for interest only.)
4. Can you still get ATX PSUs with 20-pin mobo connectors and -5V outputs, if this turns out to be needed?
5. Is there anything else I should know, before powering-up.
The Joy of Standards? - There are just sooo many to choose from...