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TheYellowDart
13th June 2006, 12:44 PM
Okay, this isn't actually a thread on arcade repair, but this seemed the most suitable forum.

I found a socket 370 motherboard in the council cleanup for which PinnyP so kindly donated a celeron 700 and some RAM. Finally got around to gathering all the bits I needed last night, and it won't boot, powers on for about half a second then off.

I tested the PSU and it is pretty nasty (the +12v rails only give out 11v and the -12v only gives out -10.8v) so I tried another which is in my current PC and it doesn't even power on for long enough for the LED to light up, a current flashes onto my multimeter and disappears before I can even read it.

I figured there was probably a short somewhere so I checked the resistances between ground (side of my case) and all of the pins in the ATX connector on the motherboard. What I found was that all of the +5v rails are shorted and peculiarly the -12v and -5v have an infinite resistance. I took the motherboard out of the case to make sure it wasn't shorting with the case (it wasn't) and also to check for bulging capacitors which I found several (and some leaking).

So what I want to know is this:

a) is it normal for there to be an infinite resistance between -12v/-5v and ground? (perhaps the motherboard just doesn't use them at all.. I tried reversing the polarity of the probes in case it goes through a diode but got the same result)

b) could leaking capacitors be the cause of the short on the 5v rails?

c) how would I go about tracing where this short comes from if it's not likely to be the leaking capacitors?

Considering the age of this motherboard I could probably replace it for about the same price as some new capacitors so at the moment there isn't really much motivation to fix it except as a learning experience (I've never gotten this involved before in hardware repair). Basically what I'm trying to suss out is whether or not simply replacing the capacitors is likely to fix my problem or whether a much more involved repair is required.

Thanks,

-Dart

Berty
13th June 2006, 12:54 PM
Some atx power supplies wont even start unless they have got a load on the 5v line. You could try to put a crappy/sacrificial HDD on one of your molex's to see if that helps.

The Pinny Parlour
13th June 2006, 01:02 PM
Probably why the MB was in the bin

TheYellowDart
13th June 2006, 02:21 PM
Berty: The power power supply is not faulty as I removed it from the computer entirely and starting it by shorting pin 14 w/ a ground. I then took readings for all of the rails on it and found they all worked (though as I said the +12v and -12v lines were way off). When it is started in this way it is able to remain on for at least 5 minutes (the time it took me to test all of the lines).

While I could put a load on the PSU in my current PC to test with this motherboard in case it's just being picky I'd need you to give me a damn good reason to do so first seeing as I've already located a short on the motherboard between the +5v and ground and don't want to do any more damage.

Pinny: Yeah, I should have expected as much too.. I wonder if davefjedi has any of those socket 370 mobo's left.. :P

[edit: heh wrong name, sorry Virgil, we all know you'd never involve yourself with PC parts]