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diagnosing an old ATX PSU help identifying resistors (PowerComputing TCX-20D)

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    Re: diagnosing an old ATX PSU help identifying resistors

    remove the switch and it's wires,
    and check the bridge rectifier for any shorts


      Re: diagnosing an old ATX PSU help identifying resistors

      no, the black wires link one of the incoming AC lines to the common point of the 200v electrolytics.
      One of the black wires connects the common of the capacitors with the AUX PSU.

      The other wire, which has the 240V/115V selector switch on it, links one side of the AC mains to the very same AUX PSU point where the other wire goes.

      The bridge tests ok, no shorts.


        Re: diagnosing an old ATX PSU help identifying resistors

        My apologies you are right @stj.

        What I said is also true but since the wires end up in the same place on the AUX PSU, by selecting 115V I end up connecting one of the mains lines to the common leg of the capacitors.

        Forgive me for my ignorance, how is that even possible? Connect mains to an electrolytic capacitor?


          Re: diagnosing an old ATX PSU help identifying resistors

          Here are many atx smps schematics, I'm sure one will be close, and many are 110/240v switchable
          Any chance of getting the make and model of this power supply?
          Last edited by R_J; 08-07-2023, 07:03 PM.


            Re: diagnosing an old ATX PSU help identifying resistors

            you have AC on one side of the cap, the other side still gets dc from the rectifier.
            when the polarity changes the cap boosts up the voltage.


              Re: diagnosing an old ATX PSU help identifying resistors

              Try a different DMM. I mean 480vdc from 240vac, even with a PFC you wouldn’t get there! It’s gotta be the DMM.


                Re: diagnosing an old ATX PSU help identifying resistors

                The PSU is from a Power Computing old Macintosh clone system, model TCX-20D. The model on the PCB says CY-03V0 and the little vertical board which has the 5VSB regulation and ON/OFF logic says CY-03 so I suppose it's CY-03 Rev 0.

                Oscilloscope also reads 240V RMS at the caps so I don't think it's the DMM.

                Thanks, I'll take a look

                That makese sense thanks. I am not familiar with voltage boosting circuits so this helps.

                Still, the caps are rated 200V and I've got 240V across them. Is that expected? Not to mention the burnt bleeding resistors. Is that just bad design?
                Attached Files


                  Re: diagnosing an old ATX PSU help identifying resistors

                  Doesn’t make sense either. If you rectify 240VAC x 1.414 = ~340VDC. All your numbers are way off. So these 200V caps have to be in series on the 240V setting.
                  Last edited by CapLeaker; 08-08-2023, 04:40 AM.


                    Re: diagnosing an old ATX PSU help identifying resistors

                    Yes but as @stj pointed out, the thing is has a voltage doubler in it. Something like the below - which should only be on when 115V is selected though?

                    Hence I guess the recommendation to try without the 220/115 switch I think.
                    Attached Files


                      Re: diagnosing an old ATX PSU help identifying resistors

                      That is correct. We thought maybe that switch was sitting on the 120V position and then you feeding it 240V. Your measurements are all over the place, so I am not sure if it is your testing equipment or its the ground reference point you are using, or both?

                      The voltage doubler part on these old PSU are all the same. 2 caps in series for 240VAC operation. You can see, there is no chance in hell for these voltages you are getting. Unless you have not the right point for your ground reference, or faulty equipment.


                        Re: diagnosing an old ATX PSU help identifying resistors

                        With the Fluke DMM I am measuring across the caps and across the (-) and (+) of the bridge.

                        With the scope I am using a differential probe, still across the caps and the bridge. So the reference should be ok - happy to be proven wrong of course.

                        My DMM is reading the correct AC input and other lower DC voltages. The Scope is backing up the DMM.

                        I agree those voltages are weird but I would be surprised if both my DMM and my scope broke the same way. Happy to test another DMM but I doubt it would make a difference!


                          Re: diagnosing an old ATX PSU help identifying resistors

                          i always remove the wires to the switch,
                          it stops other people making expensive mistakes.

                          last time a saw someone get the switch wrong the caps exploded and it killed the motherboard!


                            Re: diagnosing an old ATX PSU help identifying resistors

                            well clearly the switch was not switching properly. I removed the 115/220V switch and now I have 390V at the bridge. 194V per cap. Much better! Yes I tested it but in continuity mode, maybe I missed something? Maybe with high voltage it would leak something? I've now tested it in resistance mode and it switches perfectly so.... no idea.

                            Thanks for that! I suppose that's what killed things in the first place. I'm actually surprised the caps haven't exploded.

                            The PSU still won't start - but now I see why. The CS pin is getting to 8V and I understand that that is the overvoltage optocoupler stopping the IC. The CS pin would be normally be clamped to 3.6V (it turns on the IC at 0.24V). 8V means "stop".

                            So I have a lead to follow. Any ideas appreciated.

                            Example design from the controller IC datasheet
                            Attached Files
                            Last edited by tony359; 08-08-2023, 09:34 AM.


                              Re: diagnosing an old ATX PSU help identifying resistors

                              390VDC is closer to to what it should be, but it still not correct. You should be having roughly 340VDC not 390VDC. You should have 390VDC IF there is a PFC circuit in there.


                                Re: diagnosing an old ATX PSU help identifying resistors

                                ah you're right. I remember 390V from the PFC boost.

                                I did leave the other black cable connected, that is connecting the common of the capacitors to one leg of the AUX Transformer - but I assume it's powering the AUX PSU itself. I am going to lose 5VSB if I remove that.

                                I looked expensively and did not see a PFC in there. Could that AUX PSU wiring designed to boost the voltage a bit?