Has anyone ever fixed a broken power supply?

Joined
Sep 30, 2022
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I wonder how difficult this will be. Purchased a "untested" one from ebay. I assume either the seller tested it and it did not work or he legit never test it. But thought it fun to refine my engineering skill I left in college and see how far I can get with a multimeter and soldering iron.
Anyone ever have experience with fixing a power supply?
 

pendragon1

Extremely [H]
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Oct 7, 2000
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yes people have, but not me. drawing a blank on the user name here though, im sure theyll comment when they see the thread...
 
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I was looking at the 10 year old ps thread and people brought up capacitors that needs to be eventually replaced. I have a bunch of vintage receivers that still work, and figure one day when I retire I'll recap them. Better start working on my circuit skills now!
 

GiGaBiTe

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I've repaired many PSUs, I don't recommend it without proper equipment.

At bare minimum, you'll need an isolation transformer. You'll be working with rectified mains (375v-ish depending on the supply.) One slip with a probe can end you, or your supply and cause catastrophic damage. I would additionally recommend a variac to be able to bring a questionable supply up from 0v in steps, reduces the risk of heavy damage in the case of a component fault. When a MOSFET detonates, it can blow a hole clean through the board and vaporize traces. The resulting metal cloud can cause cascading component failures, as it builds a path for electrical current to nearby components.

If the supply has more complex faults, you'll need an oscilloscope and/or logic analyzer to figure out what's going on. A simple multimeter isn't going to be sufficient. You also need to be proficient with them so you don't blow them up, if you use them incorrectly on mains powered equipment with no isolation, they'll blow up.

tl;dr, a "recap special" doesn't always fix a SMPS.
 
Joined
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I've repaired many PSUs, I don't recommend it without proper equipment.

At bare minimum, you'll need an isolation transformer. You'll be working with rectified mains (375v-ish depending on the supply.) One slip with a probe can end you, or your supply and cause catastrophic damage. I would additionally recommend a variac to be able to bring a questionable supply up from 0v in steps, reduces the risk of heavy damage in the case of a component fault. When a MOSFET detonates, it can blow a hole clean through the board and vaporize traces. The resulting metal cloud can cause cascading component failures, as it builds a path for electrical current to nearby components.

If the supply has more complex faults, you'll need an oscilloscope and/or logic analyzer to figure out what's going on. A simple multimeter isn't going to be sufficient. You also need to be proficient with them so you don't blow them up, if you use them incorrectly on mains powered equipment with no isolation, they'll blow up.

tl;dr, a "recap special" doesn't always fix a SMPS.

OMG...thank you. I'll do more research!
 
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So I got the 1300W EVGA PS and did the pin test it won't power on. Looks like clean and almost brand new in/out except someone obviously tried opening it which voided the warranty. I've been watching this guy

and a few other video. At the 36 minute mark, he connects a light bulb in series to the power supply to prevent a short. Would that be sufficient in the absence of an isolation transformer which seems pretty pricy.
 

bigshell

n00b
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Aug 24, 2021
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You can check a few things when troubleshooting a broken power supply, like voltage and amperage on the output, capacitors, connections, and wiring. Many people have found success by following troubleshooting guides or replacing components, don't be discouraged; you can fix it! On Systems might have some helpful information to help you troubleshoot the issue.
 
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daglesj

Supreme [H]ardness
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May 7, 2005
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5,679
I've replaced fans but that's it. Not worth the risk unless you REALLY know what you are doing.
 

GiGaBiTe

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So I got the 1300W EVGA PS and did the pin test it won't power on. Looks like clean and almost brand new in/out except someone obviously tried opening it which voided the warranty. I've been watching this guy

and a few other video. At the 36 minute mark, he connects a light bulb in series to the power supply to prevent a short. Would that be sufficient in the absence of an isolation transformer which seems pretty pricy.


Adding a lightbulb in series (most specifically a tungsten incandescent bulb) doesn't get rid of the short, it limits the current through the short to stop the shorted device from drawing infinite current and melting down or exploding. If whatever component is shorted can't tolerate 60W going through it, it's still going to get roasting hot and burn.

And no, it is not a substitute for an isolation transformer. An isolation transformer isolates you from the mains, reducing the risk of electrocution causing you to die. The purpose of such a device is to galvanically isolate the device under test from the mains and earth, so if you accidentally touch the energized equipment, you would have to complete the circuit to be electrocuted. Since the device is isolated, you'd have to complete the circuit within the energized device, versus unprotected mains where it's looking for any path to ground, so you touch live mains, it goes through you to the earth and you get electrocuted.

But if the power supply doesn't do anything when you force it on, and you can't measure anything on the +5v standby rail, which is always supposed to be on, there's a serious fault with the supply. I would suspect something went wrong on the primary side, because the standby rails on modern supplies are usually on their own regulated supply fed from the primary. I would check the main fuse to see if it's blown. If it is blown, you likely have shorted mosfets, or the driver is dead.

If the mosfets are dead, that's going to be expensive and extensive surgery. And if you don't get it right, it's going to blow up again and ruin your expensive mosfets. At that point, you're better off just getting another PSU.
 
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