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750 Watt PC Power Supply Repaired
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!
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
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.
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:http://engineering.dartmouth.edu/courses/engs032/datasheet/lm431acz.pdf
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.
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!
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.
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.
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!).
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: