Desktop PSU dead - Page 1
Forum section: electronics / repair

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 14, 2020, 06:33:53 pm
content: My PC decided not to turn on today for some reason.I took it out of the case and tried the paperclip trick to try to turn it on but no luck.I remember having had the same issue in the past and it turned out to be a bad capacitor (bulging on top).My guess was the same component needed replacement but unforturnalty its capacitance is measuring well.Is there anything else to measure in a capacitor to determine if it's in good condition or needs replacing?There appears to be no obvious sign of damage on the board so I'm not sure where to even start?Mind you this is a very old PSU (15 years old) so maybe it's time it hit the bucket. These days they're at least 10 percent more efficient.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 14, 2020, 06:59:01 pm
content: I have retired many PSU's. Being that old, you really should get a new one. Most of the decent brands EVGA, Thermaltake, have seven and ten year warranties now. I just burned out two PSU's recently and easily used the warranty to replace them.Did you check the fuse?

author: wraper
author_rank: Unknown
timestamp: March 14, 2020, 07:04:55 pm
content: Quote from: vidarr on March 14, 2020, 06:59:01 pmMore like most overpriced brands. FWIW you need to look at particular model. Under single brand you often can find a very good PSU line made by one OEM, and not so good made by another OEM.

author: wraper
author_rank: Unknown
timestamp: March 14, 2020, 07:05:57 pm
content: Not very likely it's simply a bad capacitor. First of all you need to check if there is 5V standby voltage.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 14, 2020, 07:14:27 pm
content: Quote from: vidarr on March 14, 2020, 06:59:01 pmI did check the fuse and it's in working order.I would much rather try fixing the unit before giving up on it entirely...

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 14, 2020, 07:15:52 pm
content: Quote from: wraper on March 14, 2020, 07:05:57 pmHow do I do that? Do I keep the green and black wire shorted and test for a 5V signal on the red and black wires?

author: wraper
author_rank: Unknown
timestamp: March 14, 2020, 07:34:37 pm
content: Quote from: drummerdimitri on March 14, 2020, 07:15:52 pm5V stand by must be always present as long as PSU is connected to mains. It's violet wire. How do you think shorting green wire to GND even works?

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:33:53 pm
content: I found a large diode with SR 540 written on it.Seems to be a schottky diode and upon testing it with my DMM, it has a voltage drop of 0.0003 V or so in both directions.Could this be the culprit?

author: Mr. Scram
author_rank: Super Contributor
timestamp: March 14, 2020, 10:42:50 pm
content: It's probably best to get a new PSU. Modern ones are more efficient and old power supplies can cause all kinds of intermittent trouble. Cheaper units have the potential of damaging other components too.

author: wraper
author_rank: Unknown
timestamp: March 14, 2020, 10:45:52 pm
content: Quote from: drummerdimitri on March 14, 2020, 10:33:53 pmIt's dead if you measure it out of circuit. If in circuit, there is also a little chance that something on power rail (after diode) shorts it to ground. SR540 is 5A 40V schottky diode.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 14, 2020, 11:16:52 pm
content: Quote from: wraper on March 14, 2020, 10:45:52 pmYes I always test components out of the circuit to be sure.My local electronic parts dealer has only the SR 560 in stock which is 60V, 5A. Will this work?

author: vidarr
author_rank: Frequent Contributor
timestamp: March 14, 2020, 11:45:47 pm
content: Quote from: Mr. Scram on March 14, 2020, 10:42:50 pmTHIS.

author: xavier60
author_rank: Super Contributor
timestamp: March 15, 2020, 12:00:58 am
content: Quote from: drummerdimitri on March 14, 2020, 11:16:52 pmYes, the higher the voltage rating the better. If that really is the rectifier for the STB5V, the reverse voltage can be surprisingly high.It will be close to a small transformer if it is.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 15, 2020, 12:02:28 am
content: More like most overpriced brands. FWIW you need to look at particular model. Under single brand you often can find a very good PSU line made by one OEM, and not so good made by another OEM.One man's overpriced brand, is another man's pocket change. There is a reason to buy a good brand -- they have a reputation of being good. Some people, like me, like the ease of mind knowing if it goes bad, there is a ten year warranty. I change out PSU's before they even get that old. When you maintain hundreds of PCs, this is a HUGE Advantage/Plus. I practice this not only with corporate computers, but with my own, personal PC's too.As for the model, it is up to the OP, to pick out the model. He didn't even provide what CPU he is using -- nothing at all about his computer. You cannot properly pick out a model without knowing this. That is for him to decide too based on his budget. If he provides specs on his rig, I would gladly suggest a model.Also, as another poster wrote (in other words), feeding "bad" electricity into his computer is going to take a toll on other components.

author: wraper
author_rank: Unknown
timestamp: March 15, 2020, 12:34:05 am
content: Quote from: vidarr on March 15, 2020, 12:02:28 amThere is nothing magical about EVGA.http://www.orionpsudb.com/evgayou are simply buying HEC, FSP, Super Flower, Seasonic. Which you can easily buy with different label for less. As of their quality, I have repaired some EVGA GTX 1080 SC, and they are the worst 1080 I've seen. They even provided updated bios and free thermal pads to fix VRM and RAM overheating problem.https://www.evga.com/thermalmod/EDIT: BTW they have plenty of products with 1 year warranty. And when you buy something with 10 year warranty, you are seriously overpaying. You often can buy PSU with the same internals and comparable warranty for 30%+ less.

author: m3vuv
author_rank: Super Contributor
timestamp: March 15, 2020, 10:59:59 am
content: has no one mentioned checking the caps esr?

author: Circlotron
author_rank: Super Contributor
timestamp: March 15, 2020, 12:03:36 pm
content: Quote from: drummerdimitri on March 14, 2020, 10:33:53 pmYes.I have had one of the diodes on the +12V rail go shorted and it stopped the whole supply from starting up.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 15, 2020, 02:27:43 pm
content: Quote from: m3vuv on March 15, 2020, 10:59:59 amHow do I do this? Do I need an LCR meter?

author: Rasz
author_rank: Super Contributor
timestamp: March 16, 2020, 05:59:06 am
content: Quote from: drummerdimitri on March 15, 2020, 02:27:43 pmdont bother, replacing caps instead of measuring is cheaper and fasterafter desoldering the diode did you check if this spot is shorted?first thing first would be checking if the controller is being powered at all, picture of the pcb would help to identify what chip is running the show, tl494? something modern? how is it powered?Where was the dead diode?is active PFC working(charged high voltage caps)? is standby supply working?older EEs dont like fixing power supplies, $20 here $50 there means nothing to them, they got fat and lazy, like dealer car mechanic - replace and forget

author: hesam.moshiri
author_rank: Regular Contributor
timestamp: March 16, 2020, 09:32:10 am
content: The problem also could be from the controller chip, I assume it is TL494, especially if you don't have any output voltage

author: wraper
author_rank: Unknown
timestamp: March 16, 2020, 10:56:20 am
content: Quote from: hesam.moshiri on March 16, 2020, 09:32:10 amFrankly it is starting bugging me how people throw some random guesses instead of helping troubleshooting. OP already found faulty diode, and I'm assuming in standby supply rectifier according to part number. TL494 has nothing to do with standby supply. And if it is somewhat decent PSU, not very likely there is TL494. Not that they fail often to begin with.

author: wraper
author_rank: Unknown
timestamp: March 16, 2020, 11:02:02 am
content: Quote from: drummerdimitri on March 14, 2020, 11:16:52 pmShould work fine. The only downside is that it has sightly higher forward voltage. Also you don't necessarily need diode from SR series.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 16, 2020, 10:22:13 pm
content: I found a diode labeled IN5844 in my parts bin.Looks like it's also rated at 40V 5A so should I replace the old one with this diode?

author: Circlotron
author_rank: Super Contributor
timestamp: March 16, 2020, 10:45:06 pm
content: ^^ Looks okay at first glance.

author: wraper
author_rank: Unknown
timestamp: March 16, 2020, 10:52:39 pm
content: Quote from: drummerdimitri on March 16, 2020, 10:22:13 pmPerfect substitute.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: March 17, 2020, 12:20:36 am
content: Looks like it's back from the dead a second timeI had a good feeling about this and it all paid out in the end thanks guys!