enermax eg851ax-vh(w) pc psu repair - Page 1
Forum section: electronics / repair

author: 2takt
author_rank: Contributor
timestamp: October 04, 2019, 07:51:17 pm
content: hi,this psu came broken to me. the safety unit switched it off after i powered it on. with my DMM i found a short between 5v red and ground. after disassembly and probing with multimeter i thought it was a transistor but after desoldering, the short was still there. did some more desoldering and finally found a cap that is shorted.befor i started desoldering there were a lot of shorts. i think this is normal and a cause of the shorted cap, is this right?the 2 coils with the letters on top, i dont know if there are shorted. one has all 6 "secondary" outputs shorted and the other has 1 and 6 shorted; and 2,3,4,5 shorted. i can make pics and label the shorts i found, if needed.what components should i do further testing?i have no wiring diagramm for this psu. it has 2 pcb's, on the first (ac to dc board) i did not find any shorts. there is no visible damage to the psu except 1 leaking cap.greetings daniel

author: 2takt
author_rank: Contributor
timestamp: October 04, 2019, 07:51:53 pm
content: after desoldering

author: wraper
author_rank: Unknown
timestamp: October 04, 2019, 08:01:57 pm
content: What do you mean by "short"? Actual resistance? If it's something like 10 ohms, most likely it's fine, just resistor connected between the rail and GND for ensuring minimum load.

author: 2takt
author_rank: Contributor
timestamp: October 04, 2019, 08:07:10 pm
content: i measured 0.2ohms where the cable go in the board. the cap i found is also in this value(desoldered) and does not rise in resistenceedit: i normaly use the continuity setting of my DMM

author: Rasz
author_rank: Super Contributor
timestamp: October 05, 2019, 12:50:02 pm
content: are you sure you found a hard short on electrolytic cap? never seen thatusually blown diode is most likely culprit

author: 2takt
author_rank: Contributor
timestamp: October 05, 2019, 05:17:54 pm
content: i think so. on the cap the continuity test give me a steady beep and when measered ohms it goes up and down between 0.1 and 0.4 ohmthere is still a bit under the cable where i cannot see the parts. when i desoldered the cap the short in the board was gone. so assume i found the error.could a faulty diode make the capacitor bad?i dont know if this matters but my esr meter give me 0.007ohms like the other same rated caps, so it didnt detect it.my other multi tester give me the conclusion "resistor 0ohms"

author: TheMG
author_rank: Frequent Contributor
timestamp: October 05, 2019, 09:12:05 pm
content: Quote from: Rasz on October 05, 2019, 12:50:02 pmI have. But it is quite rare.Quote from: 2takt on October 05, 2019, 05:17:54 pmIt could, but it would most likely make it overheat and vent catastrophically, that is if the power supply doesn't shut down first (capacitor would be almost a dead short to AC).Extremely rare for the schottky diodes to fail. They're quite robust. Never seen a PC power supply with this failure mode to date.

author: 2takt
author_rank: Contributor
timestamp: October 06, 2019, 01:46:07 pm
content: yes the psu switched itself off, on power up. the head from the cap is flat, no sign of heat or leakage.today i removed the 2 diodes i found on the secondary output stage. give me a reading of 0.473 and 0.425v.i double checked the big transistors all in the 0.115v/0.140v rangedid some in circuit testing with the small transistors and the diodes on the primary. couldnt find something suspicious. should i remove this parts from the board for testing?i think the cap is the only fault, not sure about the big coils(no black marks or any sign of damage). would do you guys say?

author: decsystem20
author_rank: Newbie
timestamp: October 08, 2019, 11:16:47 am
content: It seems that you have removed all bigger semiconductors from the secondary side of the power supply, so its a good idea to test them.Get the Datasheet of the component to test, if you are uncertain of how to test a specific part, google and youtube are a good starting point.Remove all components from the circuit that you want to test, it takes some extra time but it will get you more accurate results.Check the primariy side as well.If the Fuse was blown, it may have damaged other parts on the primary side.Are the switching Transistors BPJs or MOSFETS?MOSFETS need a different testing approach than BPJs.Attched is a document that shows the procedure (sorry, it seems as if its available in german only)Check the controller chip too.Desolder the part and check the resistance between VCC and GND.It should read a high value if the chip is still good, if not replace it and check all surrounding parts tooMost of the cases I've come across were like this:- Blown Fuse: Problems on the primary side.- PSU alternating between on and off (restart because of an overload condition) problems on the secondary side (most likely shorted diodes).Take care of your safety!- Discharge all Capacitors before you touch anything!- The big ones on the primary side carry rectified mains voltage, thats about 320V, nothing you would want to get close with!- Don't short them with a screwdriver, use a 2.2k Resistor with 5W, better 10W to discharge them.- Check the voltage across them before you handle the PCB.- Wear protection glasses, components may fail violently, sending out shrapnels.Regards,Jens

author: 2takt
author_rank: Contributor
timestamp: October 09, 2019, 03:48:33 pm
content: i removed all the transistors and mosfets. couldnt measure the mosfets with my DMM but i added some wires to my multitester. so it give me the result of N-E-MOS. i didnt look for the datasheet but the corresponding parts where all in the same range.did some more on the primary but found nothing(fuse is still good)not shure about the controller chip. i will add a pic later.the 2.2kohm resistor for disscharge is a good tip!

author: TheMG
author_rank: Frequent Contributor
timestamp: October 09, 2019, 04:21:40 pm
content: Generally speaking if a MOSFET in a power supply doesn't have a crater blown out of it, and doesn't have a D-S short, it's most likely ok.As for testing the control IC, the trick I use is to apply proper operating voltage to the IC (usually it's around 18V or less, refer to datasheet) and then with an oscilloscope check for a switching waveform at the MOSFET gates. Note for the main switch control IC in PC power supplies there is an opto-coupled enable signal required to turn it on. Testing feedback circuits can be done by back-feeding the output of the power supply with a bench supply, gradually increasing voltage to the normal output and observing a change in the switching waveform.All of this testing above is done with NO AC applied to the power supply, and all components in place. The specific method depends on topology of the power supply in question. It's a good method to find faults without applying AC power and risking letting out the magic smoke on freshly replaced components.

author: 2takt
author_rank: Contributor
timestamp: October 09, 2019, 06:13:43 pm
content: i will check the mosfets for D S shortthe ic is this one on the left, yellow board, right?data sheet i have found:http://datasheetz.com/data/Integrated%20Circuits%20(ICs)/PFC%20(Power%20Factor%20Correction)/296-16718-5-datasheetz.htmli tryed testing the tl431ac voltage reference. but i cant do it.. is there a chance they have gone bad?measured all the resistors on secondary, they are all in their specsthe only thing i havent tested are the octocouplers. should i test them too?

author: decsystem20
author_rank: Newbie
timestamp: October 10, 2019, 07:56:24 pm
content: The part you mentioned (the one in the red square) is the controller for the active PFC, I think there should be another controller on the small PCBs in the picture.There is a part (the one in the red circle) that looks burned.Check google for the chips on the small PCBS.The optocouplers should be tested too. Check the datasheets and look for a tutorial how to test them on youtube.How did you test the TL431? Here is a video on how to test them:How about the MOSFET tests? Did you find any bad ones?Can you take pictures of the bottom of the PCBS?

author: wraper
author_rank: Unknown
timestamp: October 10, 2019, 08:30:03 pm
content: I don't get it. You have pretty clear fault (short on the rail). Quite easy to trace. Yet you desoldered just about everything, 90% of which is not related to particular power rail. Removing transformers is an epitome of doing useless work. At most a few parts needed removal to find the problem. For example, removing inductor connecting 2 parts of the rail to find which one of them is shorted. And freaking wanting to test TL431 which cannot even possibly short the rail as it is not connected to it directly. If any serious repair job would be done in this way, nothing really would ever be fixed in this world. And customers would get equipment back in a bag full of loose parts.

author: 2takt
author_rank: Contributor
timestamp: October 11, 2019, 02:59:11 pm
content: the red cyrcle was flux, i cleaned it no damagei did find the same video(the only one i could understand what the guy talks about). i have no bench supply only my 2 testers a cheap oscilloscop and a DMM. so i cant do that test. the control ic i would need a bench supply too..mosfets no shorts foundgreen pcb bottom is in first post. here is the yellow pcbwraper:i partialy aggree with you. this is my first psu repair and some parts can take other parts with them if they fail. my first post asked what components should i test, since i dont know it, i want to know itafter i found the shorted cap i thought myself problem found but i have no experience with psustransformers where removed befor i found the error.

author: Per Hansson
author_rank: Unknown
timestamp: October 12, 2019, 03:57:40 pm
content: Quote from: TheMG on October 09, 2019, 04:21:40 pmQuote from: 2takt on October 11, 2019, 02:59:11 pmReplace the fuse in the PSU with a 60w or higher rated lightbulb.That way if the PSU is shorted the lightbulb will turn on brightly: but nothing in the PSU can explode because it's current limited by the lightbulb.(Of course you don't need to replace the fuse itself, can put the bulb on incoming phase or even neutral, it is just to get the point across).(Also you can not put a big load on the PSU, but you can power it up in standby and also running with no or very little load).

author: 2takt
author_rank: Contributor
timestamp: October 12, 2019, 11:20:05 pm
content: sounds like a good idea. i will check if i still have a bulb over 60w.