Help! ATX PSU failure!? - Page 1
Forum section: electronics / repair

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 17, 2017, 08:17:00 am
content: Hi everyone,I accidentally power my psu which is set to 110v to 220v...and as you may imagine that made one hell of a bang!! Both storage capacitors that are after the full bridge rectifier blow up, with two C2335F transistors and a fuse, so far I was able to replace the two caps and install a fuse holder with the appropriate fuse but for the transistors I have only one C2335 transistor spare...So can I replace the other one with E13007F2 transistor?

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 17, 2017, 08:28:34 am
content: https://drive.google.com/file/d/0B5vW-k7HbsL4OGVYeV82UXRadjA/view?usp=drivesdkhttps://drive.google.com/file/d/0B5vW-k7HbsL4dWdJVm13V2c2R2c/view?usp=drivesdkhttps://drive.google.com/file/d/0B5vW-k7HbsL4TTdXS2NYdmREMUE/view?usp=drivesdkhttps://drive.google.com/file/d/0B5vW-k7HbsL4cm1HRUZwX2dicUU/view?usp=drivesdkAbove are images for the blown transistors and datasheets for the old and replacement transistors.Note:Replacement transistors are one C2335 and E13007F2

author: Gyro
author_rank: Super Contributor
timestamp: March 17, 2017, 09:49:01 am
content: My personal opinion, don't waste your time, buy another one. If it got to the stage of blowing the caps and transistors then it will have overstressed or damaged every component on the primary side, the bridge rectifier, the NTC thermistor, smps driver IC etc.It's not worth the effort of part by part swapping parts as you find them, some of them will blow again due to other failed components.Irrc, you've been in a similar situation before.

author: janoc
author_rank: Super Contributor
timestamp: March 17, 2017, 11:56:36 am
content: Definitely, what Gyro said.Your life is worth more than $80 or so for an ATX supply. Also if you have to replace so many blown parts, the new supply will likely end up cheaper anyway, especially if you factor in the troubleshooting time and labor.ATX supplies are really worth fixing only if there are some bad caps on the secondary side, often not even then because a good set of caps will be as much as a new supply unless you buy in quantity.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2017, 12:13:51 pm
content: I would also suggest to buy a new supply. Having a SMPS with still BJTs is odd. I would have expected that in before ATX times only. ATX supplies are usually really cheap - maybe except if in an exotic form factor / case.A repair might be also worth it if it is only the startup NTC blown (could happen with fast power off - power on many times).

author: alm
author_rank: Super Contributor
timestamp: March 17, 2017, 12:42:04 pm
content: Pretty sure it is also a pretty long time ago that I have seen a decent quality ATX PSU with a 110/220 V switch. Pretty much all of the non-crap ones use active PFC, which will usually work from 90 up to 240 V or so without switch. So based on that, I would imagine that this is either a very old or a very cheap power supply. So I can't see a repair being economically feasible.In addition, repairing a high-power switcher is quite dangerous (many parts directly connected to mains), and it is easy to blow a bunch of just replaced power transistors in the process of troubleshooting. So I would argue this is not a good learning project either.

author: bktemp
author_rank: Super Contributor
timestamp: March 17, 2017, 01:08:02 pm
content: Quote from: Kleinstein on March 17, 2017, 12:13:51 pmThe typical TL494 driven half-bridge circuit using BJTs was typical in AT and ATX power supplies from between early 90s until people started to care about power consumption and efficency a couple of years ago. Almost every 200-300W ATX power supply built before maybe 2010 used BJTs. Smaller ones (~150W) often used single transistor forward converters using a single mosfet.The BJT circuit is cheap and reliable. Failed BJTs in AT/ATX power supplies are rare. If haven't seen any, but dozens of power supplies with bulged capacitors or blown up standby power supplies.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 17, 2017, 04:28:43 pm
content: This power supply is from 2001 or 1999 I'm not really sure, also u mentioned NTC thermistors, amazing that this power supply dose not have one! And in this I'm not being sarcastic...It is really weird, and as u pointed out I replaced the TL494 with the pin to pin compatible KA7500 I had sitting catching dust on my bench, also I have checked almost all passive components and replaced faulty ones...

author: Gyro
author_rank: Super Contributor
timestamp: March 17, 2017, 05:30:31 pm
content: Well, if you have already replaced the TL494 and once you have checked everything else (including all the passives and probably replacing the rectifier) then the E13007F2 doesn't look a too bad a replacement for the C2335. Its gain is lower (10-20 'ish at 0.1-3A) versus the 30-60 at 0.1A (no curve) for the -O binned C2335. It is also a little slower, but not by much. All other ratings look the same or higher. I'd say it is close enough to try without it going bang.You need to make sure that you load test it and monitor output voltages and temperature (Safely!) for a while before trusting it on anything more valuable.

author: ali6x944
author_rank: Frequent Contributor
timestamp: March 18, 2017, 03:35:09 am
content: Is it natural that this psu don't have a ntc thermistor?

author: janoc
author_rank: Super Contributor
timestamp: March 18, 2017, 10:26:24 am
content: Quote from: ali6x944 on March 18, 2017, 03:35:09 amIt may not need it, especially if it is an old, low power supply - the inrush current could be low enough. It could be also the case of cheap, bottom of the barrel product where every possible penny was pinched and the manufacturer just didn't care to put one in (costs extra $$$!).

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 18, 2017, 10:27:39 am
content: I have a similar power supply problem. No ac getting past the ac + input, the footprint of the board has a ntc thermistor. I put in a jumper to restore the power and it blew the first ic on the heat sink in a smokey loud fashion, but it did not blow the circuit breaker in the house. I think to answer the question, yes it ntc is necessary. Check the top of the board to see if its there. Putting one in can't hurt.

author: janoc
author_rank: Super Contributor
timestamp: March 18, 2017, 11:30:02 am
content: Quote from: joseph nicholas on March 18, 2017, 10:27:39 amI think what Ali6x944 was alluding to is that the board doesn't have a thermistor at all, not that the one on the board is bad. If there hasn't been one populated from the factory, I will hazard a guess that it isn't needed.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 18, 2017, 01:20:43 pm
content: Yes, I think you are right. In my case the footprint for the ntc was there but the component was missing. I buzzed it out but without the ntc the power was not there at all. I found the power supply in a skiff and it looked like maybe someone had removed the ntc so I bodged in a jumper link to complete the ac circuit to the smoothing capacitors. It blew the transistor so I think the ntc does limit the inrush current and without it the transistors maybe at risk. As to if the ntc is essential I don't think it is, but without it the circuit doesn't behave itself.

author: janoc
author_rank: Super Contributor
timestamp: March 18, 2017, 06:58:15 pm
content: Quote from: joseph nicholas on March 18, 2017, 01:20:43 pmI think that in your case that power supply was bad as such. It is very likely the original NTC has failed because of a short/failure in the circuit, the original owner removed it and then junked the supply for some reason. You have bridged it and the supply blew up because the original failure is still there, but now there is no thermistor there to act as a fuse.

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2017, 07:08:30 pm
content: Quote from: ali6x944 on March 18, 2017, 03:35:09 amGiven that the PSU was probably working fine until your voltage selector accident, I would say that it isn't needed (inrush current not high enough). It would have been another blown part that you would have had to replace, I doubt if it would have saved anything else.Once you have the PSU working again, you could maybe consider adding one (its main effect would probably be to improve switch life and allow a lower rating for the fuse you have added). That's as long as you can do it safely, without compromising the insulation and safety clearances.