Strange Power Supply Tester Result
Yesterday my desktop power computer user friend brought over 2 of his now defect ‘bigger’ power supply units for repair. One was 650W, and the other one was 850W. I tested both smps units with my special made 75Watt safety bulb power box placed in between the 230VAC power line input to both supplies.
The 650W desktop supply worked at first but the 3.3V line was dead. And I noticed a bit of smoke coming from the middle of the board. I found why this unit was the cause of blowing out his main power line fuse at his home. He had placed the input switch to the 115VAC setting which was totally wrong of course! And I only had tested this unit when the switch was placed back to the correct 230VAC position.
Because I noticed a bit of smoke coming out of the case I took the power board out the case and checked the primary components for short circuits. And I checked the 4 pin PC817 compatible optocoupler, both primary HV transistors (T13009S), and the diode bridge. All checked out fine!
And none of the primary transformers failed when I checked them with the Blu Ring tester. Also both primary e-caps checked out fine! As also all diodes seemed to pass all checks. And none of the other smaller transistors had any short either when checked in the diode beep test of my DMM. Afterwards I soldered back the tested components I had taken out to test them with my Peak Atlas DCA75 semiconductor tester, like the primary cooler plate with both HV T13009S transistors on it (one had an Hfe of 21 and the other just about 18). I also tested the optocoupler which I tested with the small digital tester I also had used and shown in my previous repair. And a few other parts I checked before retesting the power supply that previously showed some smoke and light bulb
problems. But this was when tested with my second PC desktop output voltage tester that also had five 15 Watt load resistors on all voltages to resemble a connected PC mainboard. [ * I will come to this later on!]
Because it was fair to assume that his main power line fuse had protected his power supply from getting any real damage. I therefore started testing the output of this 650W Power supply with my first ‘black cased’ output tester (that only has a smaller internal load). And it showed that all voltages were there without any problem. And my 75W light bulb didn’t show any short currents either.
Above both primary npn HV transistors, and the PC817C that were fine! Below my first Power supply tester that approved all outputs, and the safety light bulb didn’t show any short current issues!
Above photos showed on the right my special safety test Light bulb placed in a metal case of an old defect desktop power supply. On the left the perfectly working 650W desktop power supply when tested with above shown first Power Supply Tester.
So it looks like that this Power supply is just fine.
* And here I come to my conclusion about the testers used! Here apparently the first above shown Power Supply tester is wrong and not at all trustworthy!
Because when I tested the same 650W unit that tested good with the first tester again now with my second Power supply tester with several 15W resistor loads on it, I got completely different results! Now my light bulb is practically completely lit up! And not all voltages are there now! Next photo shows my lit up safety light bulb and the photo after that shows the second output tester that reveals a big problem!
Next photo shows that second Power supply tester with the fewer active output voltages.
Now the previous good tested 3.3V output is gone and a real problem! So there must be some shorting circuit problem in the 3.3V circuit somehow.
If there is a problem in one of the non replaceable parts of the supply, like a partly bad transformer, then fixing this power supply is too expensive and all parts are only good for spare parts in another repair.
So don’t get fooled by the good results when using the above ‘black cased’ Power Supply Tester without the external 15 Watt resistor loads! To complete this article I show a few photos with the IC’s that were soldered in this unit.
Above on the left the special SD6863 IC. On the right the AC input selector switch that was wrong placed into the 115VAC position. On next photo the 16pin DIL UTC 7500 controller is shown.
On next photo on the left the UTC339 is viewed. On the right photo the optocoupler and the SD6863.
Below on the left the 10D241k VDR that apparently caused the smoke before it blew up with BANG!
It was removed from the now empty spot in between both 560uF 200V e-caps in the previous photo on the right. The red wire on the right of the diode bridge is one of the wires from the AC input voltage selector in that photo. So the VDR was definitely damaged by the wrong 115VAC switch position it was set in before. Next photo shows the 3 HF transformers of this 650W power supply.
All 3 transformers look a bit brownish on the outside yellow tape.
Maybe their transformer coils collapse only at higher current energy levels? Like when tested with 15 Watt loads on the output voltages? Next photo shows the solder board side. The red circle marks the spot where the now bad VDR is removed.
Both smps desktop units had some sort of short current issues, because the 75W 230V AC light bulb lit up in both cases but only when the second Power Supply Tester with the five 15W loads was used. However both the primary fuses were still intact! And the cause of the problem is not yet found and maybe also unfixable if it involves parts like coils or the special HF transformers that normally can’t easily be replaced. The transformers tested fine at their primary sides, but it still may be that the secondary coils are damaged and they are normally low impedance. The VDR blew later on with a loud big bang after I kept the power supply running while it was connected to the second Supply tester with the 15Watt load resistors. While the 75 Watt bulb kept indicating something was still wrong.
Maybe replacing the special SD6863 chip or the UTC 7500 controller will help but from previous experience I guess that replacing the 7500 controller probably will not make any difference here!
The 8pin dil SD6863 primary controller is according to its datasheet : “datasheetspdf.com/datasheet/SD6863” , a max 11 Amp Drain Current Pulse current mode PWM + PFM controller with built in HV mosfet driver with a max constant current of 3A at max 28V. (10 of them cost about 1,82 euro incl. Shipping). I ordered those – just in case I may need them in future -but I doubt it will help to salvage this unit in the present condition it is in.
Conclusion : lesson learned here is… better use more different Power Supply Testers to rule out unexpected major shorting circuit currents! Especially use a tester with decent load resistors on the voltage outputs. It will certainly save you time not having to remove the afterwards still failing power supply from your computer again.
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article in the below link: