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PSU Rail undervoltage

post #1 of 7
Thread Starter 
Hey all

I got myself a Gigabyte Odin 800W, 2nd hand, for a file server I'm building
When I powered up, I noticed only one of my harddrives was detected by the BIOS

I then noticed that, all the voltages, displayed in bios where low
I tested the PSU with a multimeter to confirm and these are the results
12V rail was sitting at 10.25V
5V rail at 4.28V
and 3.3V rail at 2.9V

Now these are really low, especially seeing this is not even under load
I plugged in a spare generic PSU, and all the voltages are perfect, all the drives boot correctly too

Is there a way to fix it?
My electronic skill are okish, but if I where to open it up, I'm not to sure what would be causing the issue to possible resolve this, any ideas?

Or is it a situation of return to sender?
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post #2 of 7
No way to fix it

Only thing you can do is replace it
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post #3 of 7
Thread Starter 
Really? that sucks

What would cause something like that?
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post #4 of 7
Quote:
Originally Posted by N3G4T1v3 View Post

Really? that sucks

What would cause something like that?

The PSU is probably broken in general. You can probably run a file server with a good 500W Gold PSU.
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post #5 of 7
It's probably easy to fix, but don't work on the PSU while it's plugged into the AC because high voltage will be present on a lot of the components even when the PSU is turned off, including turned off by its rear switch. It's not unusual for some of the big heatsinks to be connected directly to 170-340 VDC (ironically, as a safety measure). Fortunately you can do almost all the needed testing with just an ohm meter while the PSU is unplugged from the AC. Also never operate a PSU except with its cover completely reinstalled, including all the screws.

Some old models and server PSUs are designed to need fairly high loads before they'll put out the right voltages, and I have a vintage 1999 Delta ATX that needs a load of at least 3 amps on the +5V rail or else the +12V rail will only 10V. Otherwise when all a PSU's output voltages are low, I'd suspect a problem with the high voltage section's big capacitors or the bridge diodeconnected to the, but the problem could be with the voltage reference in the low voltage section. The latter is probably set by an adjustable resistor or a pair of precision fixed resistors (color may be different than most in the PSU, last color band will be brown or red instead of gold, indicating 1% or 2% tolerance instead of 5%). Adjustable resistors can get dirty or oxidized, but sometimes a bad solder joint around the controller chip will cause trouble. Articles about voltage mods for PSUs may be of help here.
post #6 of 7
Thread Starter 
Quote:
Originally Posted by larymoencurly View Post

It's probably easy to fix, but don't work on the PSU while it's plugged into the AC because high voltage will be present on a lot of the components even when the PSU is turned off, including turned off by its rear switch. It's not unusual for some of the big heatsinks to be connected directly to 170-340 VDC (ironically, as a safety measure). Fortunately you can do almost all the needed testing with just an ohm meter while the PSU is unplugged from the AC. Also never operate a PSU except with its cover completely reinstalled, including all the screws.

Some old models and server PSUs are designed to need fairly high loads before they'll put out the right voltages, and I have a vintage 1999 Delta ATX that needs a load of at least 3 amps on the +5V rail or else the +12V rail will only 10V. Otherwise when all a PSU's output voltages are low, I'd suspect a problem with the high voltage section's big capacitors or the bridge diodeconnected to the, but the problem could be with the voltage reference in the low voltage section. The latter is probably set by an adjustable resistor or a pair of precision fixed resistors (color may be different than most in the PSU, last color band will be brown or red instead of gold, indicating 1% or 2% tolerance instead of 5%). Adjustable resistors can get dirty or oxidized, but sometimes a bad solder joint around the controller chip will cause trouble. Articles about voltage mods for PSUs may be of help here.

Interesting, I never knew that old PSU's had odd voltages at no load, seems odd that the 5V rail would effect the 12V rail voltage, although, I don't think this is the case on a modern PSU

I'll open it up this weekend, and have a look for corrosion, if it is a unhealthy resistor, it would be easy enough to replace, it's just weird that all the rails are problematic
I'll try look and see if the bridge rectifier is giving issues, but I don't have a oscilloscope to check properly, but I'll look for AC voltage on the output of the rectifier

Interesting, I'll see if I can find anything on PSU voltage mods
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post #7 of 7
It's not unusual for a single voltage reference source to be used, even when there are several separate voltage regulators.

I should mention that those voltage mod guides for PSUs often include information about adjusting the voltages by splicing into the sense wires, those thinner wires that are paired to some of the voltage rail wires. Adjusting the voltage that way won't fix a problem PSU.
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