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ATX PSU Cooler Master RS-700-pcaa-e3 shutting off after 5 seconds

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    #1

    ATX PSU Cooler Master RS-700-pcaa-e3 shutting off after 5 seconds

    Got this from my kid with a fault on primary side. Main power supply here is AC 220V. All caps were Ok, testing with 2 series lamp of 70W each, got a bright lamp . Checked main heatsink one by one with my component tester and found one 3 leg buddy MDP18N50 that passed away. Got same replacement after a week, tested, short was gone, now I got 310V on primary.

    Happy as a dog, added one case fan for some load and the fan of the PSU. Tested purple wire, got 5.1V, great!. Shorted green and black, fan spins and after about 5 seconds shuts down. I was able to measure doing several iterations after to short green and black 12V 5V -12V (thru a LM7912 with heatsink) and 3.3V....and every time the PSU shuts off after 5 seconds....Checked secondary side all caps, no bulged at all, anyway I pulled them, check uf and ESR all are fine. No burnt parts or brown...

    With only a multimeter I tested all discrete components on the PCB. There are no shorts across components and all diodes' forward voltages are within expectations. Haven´t checked the switching components at secondary but as I am getting all main voltages I guess that should be fine. There isn´t a TL494 on this board, It has a vertical board with a TNY278PN on secondary side of the board

    Any ideas on how to keep going ahead to bring back this baby are really appreciated.
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  • Answer selected by dariolana at 02-29-2024, 08:22 PM.
    You can't use the light bulb in series trick while loading the secondary. After you fixed the short, you should test the PSU no or very little load on the secondary, while using the light bulb in series trick. After you confirmed all rails are up and running, plug the PSU straight to power mains and load it up if you want. Since the light bulb in series with power mains from the PSU, the lamps will limit the current to the PSU, making the PFC in the primary working very hard. That's not good.
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      #2
      Connect this psu with test motherboard and see if it will work.
      I have same issue, see on 15min how psu react... all good few sec and shutsdown.
      https://m.youtube.com/watch?v=d7jXHcYf9wk
      You can check and monitor gray PG wire, powergood while do so.

      Comment

        #3
        hello, more info to consider, with current PSU of 700W after to short green and black and the fan starts to spin, noticed that one or two lamps of 70W were glowing!!, indicating a higher consumption. So I´ve added the AC current clamp meter and read 0.6A on the 220V line. Scratching my head about this, went to my storage and grabbed another PSU of smaller power, about 400W, connected it with the same fan, and one or two ligth were almost off, got the clamp meter and read 0.3A, so it seems that besides the short is gone, there is higher consumption so I seems that there is another component that is bad or not balanced on the hot side I guess...

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          #4
          This look like APFC acting.
          Look on 13:30 behaveour, is the same?
          https://m.youtube.com/watch?v=aMMzu436MBA
          Solution is to use toaster with 700w or so, not lights bulbs, it is on this forum thread about this.
          APFC drow more power for compensating voltage drop...

          Comment

            #5
            You can't use the light bulb in series trick while loading the secondary. After you fixed the short, you should test the PSU no or very little load on the secondary, while using the light bulb in series trick. After you confirmed all rails are up and running, plug the PSU straight to power mains and load it up if you want. Since the light bulb in series with power mains from the PSU, the lamps will limit the current to the PSU, making the PFC in the primary working very hard. That's not good.
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              #6
              Dear CapLeaker, followed your advice and....YES, no smoke and the issue disappear. Also the temperature is equal for both mosfets. Really appreciated.

              Comment

                #7
                Originally posted by harp View Post
                This look like APFC acting.
                Look on 13:30 behaveour, is the same?
                https://m.youtube.com/watch?v=aMMzu436MBA
                Solution is to use toaster with 700w or so, not lights bulbs, it is on this forum thread about this.
                APFC drow more power for compensating voltage drop...
                Thanks HARP for the reply. Following your advice and CapLeaker the issue is solved. Today I´ve reassembled the whole thing and the kid is playing again.

                Thanks again for your help with this

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