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750 Watt PC Power Supply Repaired

By on May 27, 2016
tl341 diagram










Recently my friend Frans dropped by with a very big PC Power Supply of not less than 750 Watts!! (and to give an example how much this is: just a little over 1 horse power!

750 watt power supply repair

His son apparently had tried to replace the old Fan by another 12Volt Fan inside, after which this SMPS PC Power Supply didn’t work anymore. And I got all the parts disassembled in a plastic bag.

So because this was a great opportunity to see such a big PSU for the first time from the inside, I was very happy to examine it.

Of course I used the Power Bulb trick to be safe against unnecessary short circuiting the 230 VAC Mains. I used a 60 Watt Bulb. It burned while being modulated. So I checked for Short circuits. But because this is a very big Power Supply I will come back on that later!

On the Power Board there were a FAN4800 chip, a FSEM311z chip, and a EST7610 chip. Plus 3 Opto couplers and four TL431 shunt regulators. I replaced the FAN4800 which didn’t change anything, so that chip probably was fine. The other 2 chips I didn’t have so I couldn’t replace.

Next I checked the Opto couplers with my universal Digital Tester. They all checked out fine being the 817 four pins Opto coupler version. And I checked the (Q10) A1015 and both (TQ1 and ATC) C1385 transistors. No problems there either. I also checked all TL431 regulators with following test circuit:

TL431 Voltage Monitor Circuit – Resistor Values

tl431 ic diagram

Above is shown the standard circuit diagram for a TL431-based voltage monitor. The aim of the monitor is simply to light up an LED when a target voltage is reached – perfect for a solar battery charger to let you know when the batteries have reached full charge.

high limit=vref

For both R1 and R2 I used 10Kohm, and for R3 a 470 ohm Resistor, and for R4 a 1Kohm. This way the Led (red) just starts to burn at a minimum of about 5.10 Volt. Vref is always 2.5V so calculating the desired setup is easy peasy. See here for a datasheet about this TL431 chip:

And all four TL431 tested fine!

And next I checked all semiconductors on the secondary and primary Cooler Plates. All were fine!

Therefore I checked the biggest T1-SP-8801-LA Transformer inside this Supply, but removing it didn’t change the always burning 60 Watt Light Bulb (measuring about 214VAC over the Bulb at about 0.25A). Also removing all Diodes on the secondary cooler plate didn’t make any difference whatsoever. Only removing the second largest Transformer with code EL EEL-18A-CE 72-9901-800A stopped the Bulb from burning. The Blue Ring tester showed that at least the primary Transformer sides were okay. However if the Transformers still worked on the Secondary Transformer side (attached to the Primary safe side of the Board) was a bit of a mystery. (maybe I should design a universal Transformer Tester circuit special for these SMPS Transformers ?).

Next Photos shows the Semiconductors on both Primary Cooler Sides.

atx schottky diode

The voltage on the primary Buffer E-cap was okay measuring 312 V DC.

Measured at IC U4 EST 7610 pin 7 there was a voltage of 5V (5VSB). And according to the datasheet of this chip

there also should be a voltage of 12 Volt. All other pins measured only about 0V. At circuit around Transistor Q10 just above U4 was a voltage of about -1.16V. Around smd Resistor R47/100 (10 ohm).

IC FSEM311z measured about 14.4 V DC on pin 2 which is okay and this needs to be above 5 Volt (8-12V according to datasheet to start).

Diode D15 turned out to be a simple 1N4148 Diode. And Zeners ZD4 and ZD5 were okay too but only ZD4 had the previously mentioned voltage of about 14.4V to pin 2 of the FSEM311z. ZD5 was attached to pin 3 of that IC but only measured a small negative voltage.

While testing the Power Supply I noticed that sometimes the Modulating stopped working, when the 60 Watt Light Bulb had been active for several minutes. But after about 4 minutes of rest the Unit worked again.

And yesterday after removing and placing back all components I noticed that the modulating had stopped completely. It turned out that a Zener ZD3 next to Capacitor C53 had become a short circuit (with marking C15). After I had replaced it by a new 15V Zener the Supply Modulated again. Still my ATX Tester did not work!

atx power supply circuit board

Above Photo with the now removed bad ZD3 Zener. After I replaced it with a new 15V Zener the Supply Modulated the Light Bulb again, still without working ATX Tester.

I tried if maybe the Modulating 60 Watt Light Bulb would stop burning when switching a second Light Bulb parallel to it of 75 Watt. But they both turned out being modulated at the same 214 VAC. And only briefly the connected ATX tester occasionally produced some sound signals and a working display. While the Modulating Light Bulbs kept working.

So next I tried if maybe a Halogen Light of 300 Watt could share some light on this Modulating thing that occured. After all the Power Supply is rated on the Label for 5A (!) input at 230 VAC.

But the strangest thing happened. After switching in really nothing worked!?! No Halogen Light was working, neither was the Power Supply.

Because I wanted to explain what happened I decided to test my ATX 410-212 410 Watt Power Supply with my 60 Watt Light Bulb. And I measured 74 VAC over a weak burning Light Bulb while the ATX Tester worked splendidly!

So next I tried switching 2 extra Light Bulbs (old Disco Light Bulbs of about 75 Watt) to my already attached 60 Watt Light Bulb (giving a Light Bulb of 210 Watt). And now all Light Bulbs worked together being almost fully Modulated at about 214 VAC by this 750 Watt Power Supply Unit. More about the conclusion after next photos.

power supply repair

Next Photos show all Semiconductors on the secondary (Safe) side and the Board after the Cooler Plates of Both Primary and secondary Plates were removed:

(All Semiconductors on both Coolers were Fine!! So I placed them back afterwards)

And the Photos following also show the Controller Chips used on this 750 Watt Board.

750 watt atx repair

750 watt atx repairing

750 w atx repairing

Conclusion: because I couldn’t find any real problem or bad components afterwards, and although all 3 Light Bulbs still heavily Modulated, I also noticed that my ATX-Tester now finally worked!!

Better yet : All Voltages were present and fine !

And I also noticed that the Light Bulbs not kept burning indefinitely but stopped after a short period, or even didn’t burn at all after the Supply had been switched on. They only burned – if they were burning – for 1,2 or up to about 6 seconds.

So this time I was really thinking to try if the 750 Watt Supply runs WITHOUT any Light Bulbs.

But to still be a little bit safe and smart I still used a safety measurement by using a cheap 5 Amp Car Fuse in between the Input Lines. See below the Car Fuse Box I used for a single 5 A Safety Fuse (the Orange one).


I know it is actually not designed for 230VAC but cheap and better than no Fuse at all (not counting the original Fuse on the Supply Board, which I do hope will be intact after my test without any Safety Bulb).

And guess what: Everything works fine without any Light Bulb. And next Photo is proof of that. I also replaced the old Fan by a new 12V Fan. And the Fan only works after the Unit heats up controlled by the ATC circuit with a C1385.

I checked if the Fan worked by heating up the ATC circuit (see next Photos) with using my Hot Air Solder Station.

All works Splendid again. At no cost at all (Again!).

atx power supply repair

atx power supply repaired

So if you ever try to repair such a big 750 Watt PS Unit, do what I did and first use at least about 210 Watt of Light Bulbs as safety precaution.

PS: This Supply also had a 8 Pole Connector but that shorted on the ATX Tester! Only after I turned it into a 6 Pole Connector by removing the 2 Black wire Connector part, the 6 Pole also worked fine!!

Fixing this 750 Watt supply worked out fine, and it only caused me unexpected complications. But that did make it more interesting too. So I hope you enjoyed it as much as I enjoyed it!

Until next time.


Albert van Bemmelen, Weert, The Netherlands.

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Note: You can read his previous repair article in the below link:



  1. Robert Calk

    May 27, 2016 at 9:02 pm

    Good job, Albert. It probably had some cold solder joints.

    • albert van bemmelen

      May 28, 2016 at 4:06 am

      Hi Robert. I couldn't find any cold solder joints.
      And I guess that the failure had something to do with the Fan
      connector that didn't make any real contact after my friend's son
      probably had used Super Glue that got into the connector holes.
      Because the 2 pole connector was made electrical isolated
      unattendidly in the process. Something that Super Glue tends to
      do with metal contacts. So the Fan could never have worked anymore.
      That's why I soldered both 12 Volt Fan wires straight onto the board
      Fan1 position. It is possible that the not attached Fan was the only
      reason why this Power Supply failed to work. Because everything else
      tested Fine!

      • albert van bemmelen

        May 28, 2016 at 4:16 am

        All components were fine, except the 15Volt Zener ZD3 that
        probably only failed lateron while I was testing with the Light
        Bulbs. Because previously I am almost certain that I had measured
        a good ZD3 Zener Diode with my Digital Meter on Diode/Beep test.

  2. Albert van Bemmelen

    May 28, 2016 at 7:08 am

    PS: I noticed that the other way to test the TL431 Shunt regulators with a Universal Digital Meter on Diode Beep Test gave different results on assumably TL431s made by different Manufacturers.

    On several TL431s the Meter Display gave 1200 as value, while on others it was only 600 (mV?). But in the testmethod as described in my article they all tested the same !

  3. radley kakubo

    May 29, 2016 at 8:39 am

    Hi. I've a problem with my lg dvd home theater ,when I switch on ,on display its only displaying please wait;

    • Albert van Bemmelen

      June 22, 2016 at 3:11 pm

      Sorry for my late reply Radley, but that is because the reply button before just didn't work. I guess the best thing you can do is dig up any circuit or service-manual you can find for your LG that explains what to repair or replace.
      Maybe you can find more on any forum on the subject that someone has solved concerning your device. I guess if its the laserhead that has gone bad you need to replace your DVD player inside the LG Home theater set? Good luck.

  4. Parasuraman S

    May 29, 2016 at 9:04 am

    Another wonderful and very useful article! Many thanks for sharing!

    • albert van bemmelen

      May 30, 2016 at 2:41 pm

      Thank you dear Parasuraman S. The Power Supply tested fine on the ATX Tester, and all Voltages were present as shown in the article. But there is still something odd going on that needs to be examined as it seems that the Mainboard of my friend's son (A big tower of course) a few days later did not start yet. Only the green Mainboard led was on. And I previously already ordered the other controller chips of this 750 Watt Power Supply in case the EST7610 or the FSEM311z need to be replaced. And I ordered another ATX Tester that also gives a decent Load through Bigger resistors to be sure the PSU works under regular circumstances. See this link:
      And today my friend will bring his son's Big Tower to examine the problem more closely!
      So maybe the used ATX Tester is not thrustworthy enough in these special cases? And in these circumstances above new ATX Tester needs to be ordered.

  5. Humberto

    May 31, 2016 at 1:46 am

    Hi Albert, another good article, you had to undertake a deep investigation. Congratulations.

  6. Chris

    June 3, 2016 at 6:59 am

    Hi Albert!
    That's a nice article with lot of investigation and declarations.
    I always love to read your article because of the way as you describe the problematic and all the road until you reach the goal.
    As I read a good crime book such as Sherlock Holmes.
    But, exactly this is why I love electronic and cybernetics science.
    Great job and you are a great master of electronic repairing.
    And! I'm always frustrated when I see a thumb down without explanation.
    My best regards.

  7. Paris Azis

    June 13, 2016 at 2:55 pm

    Interesting case, Albert and nice job as well.

  8. albert van bemmelen

    June 13, 2016 at 6:22 pm

    Thanks all for your kind replies! I have however to explain something
    unexpected and wierd that occured after the Supply was Tested on a Real
    PC Mainboard lateron. And keep in mind that the ATX 750 Watt Supply tested
    okay on the ATX LCD tester! And it also did on another new 20-24 pin ATX
    Power Tester!
    But as soon as the 4 pole FAN 12Volt connector (the 2 Black and 2 Yellow
    wires) was connected to the Mainboard the PSU stopped working. So then I
    tested the Supply on my friends PC Mainboard without using the 12V
    connector. And the Desktop PC started but in fact didn't do anything
    because it couldn't be stopped by holding the Power Button for 4 seconds.
    Another 500 Watt PSU was then attached to the same Mainboard also
    connecting the 12V Fan Mainboard connector. And now the Desktop PC really
    worked !!
    So this means two things: 1. The 750 Watt Unit still must be defect in one
    of the controller chips that I haven't replaced yet, the FSEM311Z or the
    EST 7610 chip. Because everything else seems to have tested fine!
    2. The ATX TESTERS are not capable of detecting this defect because the
    four Pole 12V connector tested well! However maybe the not working 8 pole
    connector shorting the ATX Supply on the LCD ATX Tester is an indication
    of this defect, as I wrote at the end of my article?
    As soon as my friend brings his 750 Watt Supply, and both already ordered
    Chips are recieved I will be able to answer this strange unexpected

  9. Albert van Bemmelen

    June 21, 2016 at 2:28 pm

    Please Discard my previous post !!
    I never had any problems with the LCD ATX Tester myself.
    But because some readers apparently wrote about some concerns they had that it probably not always does test Power Supplies satisfactory because some Mainboards were not operating after a completed ATX LCD Tester test, I thought I had the same problem here.
    It wasn't the case, so I only can tell anyone that I guess the comments about the ATX Tester not always working correctly were not supported by real existing facts. And my last post above can completely be erased as redundant or obsolete.
    I rather think some readers made the same error I had made by just connecting one of the 12V 4pin (FAN) connectors wrong onto the PC Mainboard! Because my friend Frans told me yesterday that he noticed that the 4 pin connector was previously connected the wrong way onto the Mainboard while quickly testing on his son's Big Tower PC Board. Which should not have been possible but somehow can be done!
    Creating the result that looks like : 1. the ATX Tester test was at Fault,
    and 2. the Power Supply was still defect. Which of course was not the case whatsoever !!
    I therefore leave my fine working ATX LCD Tester just the way as it is. And I am glad to tell any reader of my article that my 750 Watt repair article was correct at every written text character!
    The 750 Watt Supply was completely fixed after my quick repair!

  10. Ulises Aguilar Pazzani

    March 14, 2017 at 10:11 am

    Mr Bemmelen very good testing very technical fix Sir


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