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trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

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    #31
    Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

    Can you check the resistor R0 2.7Ω current-sense is not open-circuit. It looks a bit dodgy.
    I don't think I've ever encountered a small TO-92 HV mosfet before. What is it's part number?

    Also, note this PSU does not have an opto-coupler so it must be using the aux winding for voltage feedback.
    Last edited by SMDFlea; 09-18-2023, 02:57 PM. Reason: 0.5A

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      #32
      Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

      Based on post #26 I suspect the R0 resistor is ok. The mosfet is a 1N65T

      C1 and C2 must be the two 4.7µf/400v capacitors, What is the voltage across each of them? (it should be around 300vdc). What is the voltage across C3 (it should be around 20vdc) and the voltage across C3 is important as it will tell us the state of the ic.
      Best I can do with Paint
      Attached Files
      Last edited by SMDFlea; 09-18-2023, 02:57 PM. Reason: 0.5A

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        #33
        Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

        The fusible resistor blew due to a short or overload on the primary side.
        That would be a bad rectifier diode, or shorted mosfet and that fault current ends up going through the sense resistor who usually loses.
        So far I don't see the cause of the overload, although OP slipped with probes and shorted something out.

        Rectifiers test OK, mosfet tests OK- so something isn't making sense here.
        Last edited by SMDFlea; 09-18-2023, 02:57 PM. Reason: 0.5A

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          #34
          Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

          Thanks.
          R0 measures 3 ohms in circuit.
          The mosfet is not small. It's the size of a normal through hole transistor.
          Last edited by SMDFlea; 09-18-2023, 02:58 PM. Reason: 0.5A

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            #35
            Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

            So far only the fusible resistor was bad, everything else you have checked is OK. Seems a bit odd. To me that happens when I've glossed over and missed testing a part, or I tested it wrong etc. so I will back up and recheck the top suspects.

            Can you measure (carefully without slipping with the probes and shorting things) the voltage across C3 (same as empty "C1" on the board's backside). That is power for the IC. It might be pulsing or hiccuping. RJ also asked for this.
            The SF5920S IC needs over ~15.5VDC to startup, ~9.5V it goes to UVLO shutdown, and ~33V is OVP shutdown.
            Last edited by SMDFlea; 09-18-2023, 02:58 PM. Reason: 0.5A

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              #36
              Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

              i would start with q1 voltages and see what's not right there . then go backwards towards the beginning .
              Last edited by SMDFlea; 09-18-2023, 02:58 PM. Reason: 0.5A

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                #37
                Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                if you cant see anything wrong, check R1 and replace c3 with something decent
                Last edited by SMDFlea; 09-18-2023, 02:59 PM. Reason: 0.5A

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                  #38
                  Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                  Thank you.
                  I am focussing on assembling the current limiter first. So, any more measurements will have to wait until I that is finished.

                  Also, I have another 6V adaptor but the wrong size jack and I might just swap the jack lead and rewire the faulty adaptor mains input lead with longer wires so that it is easier to work on. Atm those wires are falling off due to strain from the mains plug hanging onto it. I had to re-solder them twice already.

                  I was also thinking of wiring up my oscilloscope to the ouput of U1 and to the it's ground to see if there is a signal coming out.

                  No-one said anything when I mentioned that the fast acting resistor temporarily measures a voltage in the reverse bias in circuit. Should I assume that's coming from the circuit?

                  An advice is appreciated.
                  Last edited by SMDFlea; 09-18-2023, 02:59 PM. Reason: 0.5A

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                    #39
                    Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                    If this is a mains powered (not battery powered) oscilloscope DO NOT connect it to the primary side of this adaptor without an isolation transformer, you will blow up the bridge diodes at a minimum.
                    Last edited by SMDFlea; 09-18-2023, 03:00 PM. Reason: 0.5A

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                      #40
                      Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                      Thanks. I thought if I test it first using a multi-meter and only connect the scope to ground or remove the ground it would be ok. Or only test things that have a capacitor between them and negative or postive. 400 V does sound too high. I will take your advice and not use the scope.

                      I am starting to think that it might be the IC that is faulty. Since you don't advise using a scope to test it,
                      does anyone have any ideas on how to test the IC to verify if it is shorted?

                      Less than 50% of the wires showed continuity and there are other paths to that, so that test was inconclusive.
                      I know other measurements on capacitors have been suggested, but if they go ok, I think I am likely to end up testing the IC anyway.

                      There seems to be a problem ordering them, since though I have the data sheet, the only company that I have found that sells them is in China and I doubt they accept small orders and they don't even have an ordering system.

                      Since I have started working on this adaptor, I noticed a smell that I think is similar to the smell when an transistor or a diode is burnt. I don't know what a burnt capacitor smells like. Since I don't seem to have found any other shorted semi-conductors by testing in circuit I suspect the smell came from the IC.
                      Though the fast acting diode did seem to conduct for a split second in the reverse direction.

                      If the IC is faulty and I can't find a replacement then I have to end this project as it can't be fixed.
                      Last edited by Tarot Superstars; 09-18-2023, 04:55 PM.

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                        #41
                        Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                        I took some readings with a multimeter with the adaptor plugged in.
                        Voltage and temperature tests. Powered on. Insulated jacket, rubber gloves, protective goggles.

                        Voltage Tests (Powered On)

                        F1 = 1.5mV AC. ~ 0V DC.
                        C1 = 335 V DC. 0 V AC.
                        C3 = 1.3 V DC. 0 V AC.

                        Temperature Tests (Powered On)

                        Equipment = a digital room thermometer with insulated probe on a plastic stick.

                        Everything I tested was at room temperature. I think that was weird. I expected if U1 and Q1 were working they would be above room temperature.
                        I also tested the largish smd diodes (M7...Schottky) on the reverse side and the small smd capacitors and the resistors near U1. They were all room temperature.
                        I tested the transformer, both inductors and the fusable resistor and they were all at room temperature.

                        Any advice is appreciated.
                        Should F1 have more voltage?
                        Does the measurement on C3 show U1 is broken?
                        Attached Files
                        Last edited by Tarot Superstars; 09-21-2023, 05:23 PM.

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                          #42
                          Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                          if f1 was broke then c1 voltage would be zero .f1 is mains fuse
                          oh and why has d2 a hole in it ?
                          Attached Files

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                            #43
                            Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                            D2. That is the light shining off it. I check and there is no hole in D2.
                            Also, the hole in the board near D4 never had a component in it. It was covered with solder and appeared when I soldered near it.

                            Comment


                              #44
                              Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                              Originally posted by Tarot Superstars View Post
                              I took some readings with a multimeter
                              [...] C3 = 1.3 V DC. 0 V AC.
                              Does the measurement on C3 show U1 is broken?
                              Yes, or C3 or the diode D8 has a problem. You said R1 is good. The IC could be hiccuping too, but it wont start unless over 15V is present.

                              When working on the board, always ensure the main filter cap C1 is discharged before going in. You might have to discharge it or wait for the 335V to dissipate.

                              Comment


                                #45
                                Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

                                Originally posted by petehall347 View Post
                                if f1 was broke then c1 voltage would be zero .f1 is mains fuse
                                F1 is 10R
                                I think he is measure voltage across F1...?

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