View Full Version : Power supply problem??

Billy T
30-03-2004, 03:53 PM
Hi Team

No1 son's homework computer has taken to refusing to start, but unfortunately the problem is very intermittent. Now that I think of it, it has done this a couple of times before over the last two years but I never took much notice because it always started on the second attempt. The PC is a PIII 733, 256MB, 2 HDD with a PC Chips M756LMRT+ motherboard. Cheap as chips I know, but it has been 100% reliable apart from failure of the onboard modem a year or so back.

If it fails at switch-on, there is a brief burst of HDD activity but no post, no beeps, and the monitor doesn't come out of standby. Pushing the reset button does nothing, and I did the usual check for plug seating etc but everything seemed all right.

Power-Good comes up OK, and the only supply voltage that appears to be out of spec is pin 9 (standby) on the ATX psu plug which should be 5 volts but is only 3 with the power off and 3.5 with the power on.

We don't use any standby features on this box so I'm not really sure if this is relevant or not, but it seems remotely possible that the low standby voltage compromises startup from cold.

I have thought about cutting off the standby lead and connecting that pin up to another +5v supply rail, but is there a way to turn that off for good in bios? This computer always keeps its network card live (its Led is alight on the network switch even when all computers are off) so that suggests that I should be able to disable something to kill it.

Any ideas or suggestions before I hack into it or go out and buy a new PSU?


Billy 8-{)

30-03-2004, 04:26 PM
I find those sort of problems are PSU related (ie they go away with a replacement).

Have you tried disconnecting as much as possible. I notice you have 2 HDDs in it which will draw quite a bit of power to spin up.

Do you have another PSU to try in it?

Billy T
30-03-2004, 05:20 PM
The power doesn't sag when switched on so I don't think the two HDDs are an issue. All my computers have two HDDs, including the twin of this one. I'd swap the PSUs over but No.1 daughter would slaughter me If she lost the use of her computer just for my fault finding!

I don't have a spare PSU but I'm willing to buy one as they are cheap and a spare doesn't go astray. My previous spare is in use in another computer after a failure so they have their uses!! :)

I'm hoping to get a pointer here so that I can do it once and do it right.

Anybody else got comments/knowledge on the standby voltage issue? This computer also makes a weird but very faint hissy-warble noise when switched off so maybe the standby voltage source is shonky.


Billy 8-{)

Graham L
30-03-2004, 05:52 PM
The 5VSB line, which is on all the time the computer is connected to the power, is used to energise the "Power" switch on the front panel. If it is replaced by a line from one of the main 5V supplies, which are on only after the "Power" switch has been operated, guess how well it will work ... ;-)

The hissy-warble will probably be a small switchmode unit working from the rectified mains, and producing the (up to 500 mA or so) 5VSB.

It ought to be 5V. Are you measuring it at the main connector? At the power switch it is used as a logic supply, so it will be through resistors for when the Power switch is closed to ground for the active low start signal.

I don't know, but it might not be supposed to hiss and warble. I'd expect a nice steady 30kHz or so pwm (with very low duty) keeping it steady at 5V against the few resistors used to sample the voltage.

30-03-2004, 06:06 PM
a while ago i had a weris problem with a pc not booting due to the standby voltage being to low. i also noticed that inserting the modem caused the voltage to drop.

have you got a modem installed? if the old modem is faulty it may be causeing a few problems (the curse of all in one systems).

also make sure all wake on ring, wake on lan etc type settings are all off.

Billy T
30-03-2004, 09:07 PM
Thanks guys

It sounds like the 5 volt standby line is the problem, and it may just be an open cap in the standby psu (hence the noise) so I'll take a look at the supply waveform with a scope and if it isn't clean DC I'll take the psu out and replace some caps.

After that if it doesn't behave I'll deep-six it and buy a new one.


Billy 8-{)

30-03-2004, 09:15 PM
On modern cpu's I do believe that one of the required voltages is 3.2volts and not 5, so be careful of assuming that it is faulty.

CU Rob.

Billy T
30-03-2004, 09:22 PM
There are two 3.3 volt rails and they are OK, but this pin is labelled 5VSB.


Billy 8-{)