Under-voltage on the 5v rail.

[DrGamut]

So my computer stopped recognizing/reading my hard drives reliably and became unusable recently. At first I thought it was the HDDs/SATA controllers. But after testing the connectors on my PSU with a multimeter I found that while connected to the motherboard the PSU is outputting < 4.4v on the 5v wires. However if doing a paperclip test with just one hard drive connected I get 4.9v. The BIOS reports even less voltage than this. The other rails are fine.

I think this is the problem, but before I buy a new PSU, is there any possibility the PSU is not at fault? Is there any way the motherboard could be causing a healthy unit to output less voltage? Or is this a cut & dry faulty power supply unit?

Thanks.

This is a PC P&C Silencer 750w PSU, and an Asus P5B Deluxe motherboard by the way. The actual system uses not nearly that capacity.

 

[AnarkAtPlay]

Disconnect everything except a 12V fan from your psu and power it up with the paperclip method. Check your 12v and 5v rails for a baseline. Then connect some hdd's to the psu, power it back up and check again. If the psu drops our of reg on the 5V rail, your probably have some bad caps in your psu that need to be replaced or need to just replace the psu. If it stays within spec then disconnect everything from the psu and just connect you motherboard. If the psu drops out of reg with just the MB connected then you probably have a bad MB. Look for leaky or bulging caps on the MB.

 

[larrymoencurly]

What happens to the +5V rail when you connect more hard disks or optical drives? If the voltage goes up, maybe the problem is that the +5V rail isn't being loaded down enough. Mobos draw a few amps from the +12V and +3.3V but sometimes way less than one amp from the +5V.

OTOH some PC P&C models use Teapo capacitors, which aren't the most reliable.

 

[drescherjm]

But after testing the connectors on my PSU with a multimeter I found that while connected to the motherboard the PSU is outputting < 4.4v on the 5v wires. However if doing a paperclip test with just one hard drive connected I get 4.9v. The BIOS reports even less voltage than this. The other rails are fine.

Forget the BIOS measurement. Those are useless since the sensors are cheap/inaccurate and they are also not calibrated. Also remember (like larrymoencurly said) that power supplies require a minimum load to regulate the voltage correctly.