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Fluctuating 3.3v line

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Seath

Registered
Joined
Sep 1, 2002
I have a cheap 400w power supply I picked up a couple months ago, and now it's developed this wonderful little problem. The 3.3 v line fluctuates from 2.9-3.7v almost constantly, causing instability and graphical errors. I was wondering if anyone had any suggestions on how to fix this, as I plan on opening it up in a couple days and taking a look at it.

I was also wondering if throwing a few caps on the 3.3v line might help leveling out the voltage, as I have a few lying around from another failed project.
 

larrymoencurly

Member
Joined
Jul 14, 2002
Could it be a bad connection in the wiring? Some supplies include extra wires in the mobo connector just to measure the voltage at the mobo to compensate for this (they may be thinner than normal), especially on the +3.3V and ground, but I've seen no-name PSUs without them.
 
OP
S

Seath

Registered
Joined
Sep 1, 2002
Stedeman: No, I haven't tried running it off of a UPS, though the other power supplies I own have no problem with thier 3.3v line. (the 5v line is another story)

larrymoencurly: It could be a bad connection somewhere. One of the 3.3v and ground connectors both have an extra wire running to them, so it's possible that any corrective feature it may have is malfunctioning.

Now that I have it sitting in front of me, it's actually a 500w robanton power supply.

Anyone have any suggestions regarding how to fix this?
 

larrymoencurly

Member
Joined
Jul 14, 2002
Maybe the connector is loose, either where it fits into the mobo connector or where the wires are crimped. I've measured 0.5V drops across crimps when the load was only a few amps, but one time I measured a couple of volts, and re-crimping the connector didn't help because it turned out that there was corrosion. I had to solder it.

The only Robanton I've seen is a 350W, and it seemed like the worst built supply in stock at Fry's, totally different from the Powmax supplies made by the same company.
 
OP
S

Seath

Registered
Joined
Sep 1, 2002
It's actually a powmax, but I thought it would be kind of odd if a company has two separate manufacturing lines producing the same product. Guess I'll just start playing around with it and hope I can figure out what's causing the problem.
 

ChadP

Registered
Joined
Aug 23, 2001
They might not be the same product at all. Perhaps they're contracting to a different supplier in China, etc. for the cheaper batches.

And since when did Fry's have decent power supplies... without madly overcharging for them? (Maybe they've changed in the last few years, but given their endemic biz practices I really doubt it)
 

skyfish

New Member
Joined
Oct 10, 2016
Dear all, I know this is an old post, but I had this exact problem with a power supply and found the solution.

It was causing my computer to shutdown (clearly to protect itself.) The readings I had were 3.3V-3.76V, up and down all the time. After the fix the voltages reported were 3.312-3.341V only. Much better.

There are two 2200uF capacitors on the PSU motherboard that were damaged. (Think of them as worn shockabsorbers.) You can see it by the top of the capacitor looking a bit bulged or rounded and not flat. I simply replaced them and the voltage fluctuations were stable again.

What is important to understand is that the PSU's are built to be as cheap as possible, and so the component specs are sometimes selected to be quite marginal to the work they have to do. These capacitors are marked as 2200uF, 6.3V. This runs them pretty close to the 5V line rating where they were operating, and so they don't always last well. The ones I used to replace in the PSU are 2200uF, 16V caps, and they will certainly take the job in their stride.

(My disclaimer: Please don't solder on your PSU unless you know what you're doing, have the correct equipment, skill and soldering technique.)

I hope this helps someone out there!
Cheers
Colin