1 5 6 7 8 9 120 Replies Latest reply on Sep 14, 2020 1:11 AM by Jan Cumps Go to original post
      • 90. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
        shabaz

        Hi Matt,

         

        I tried following the thread and it's hard to follow, but it got me wondering, does slot car racing need such a supply?

        It seems overkill, and I can see that it could fail through accidental misuse, or even damage the slot-cars if set accidentally to too high a voltage.

        Probably a few amps is enough, if it is a 'scalextric' style slot car racing system.

        • 91. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
          mpulliam

          HI Shabaz,

           

          My problem isn't my purpose for having the supply.  My problem is that it doesn't work and I'd like to fix it.  I do not do slot car racing.

           

          Thank you for the thought, though.

          Matt

          • 92. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
            mpulliam

            Hi Gene,

            I'm just trying to fix this one for fun, I realize I can buy other power supplies.  I also wouldn't purchase another MG Electronics power supply out of principle now, they have neglected to return emails, neglected to provide contact information on their website, and flat out refused to honor warranties to their distributers.

             

            Matt

            • 93. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
              fmilburn

              John,

              As you well know this isn't my area of expertise.  Too bad there is no schematic.  Some thoughts on things I didn't see but may have missed:

              • have all of the electrolytic caps been checked
              • can the transformer be isolated and checked
              • if the caps, diodes, transistors, assorted ICs, and transformer are good then is it worthwhile to go through the circuit with an iron and touch up all the joints to make sure all the connections are good

              Frank

              • 94. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                jw0752

                Thanks for the input Frank, Shabaz, and Gene. I will keep your suggestions in mind.

                 

                John

                • 95. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                  jw0752

                  Hi Matt,

                  Let's Continue:

                   

                  Test the output transistors with the Analog meter by choosing one. Since I believe they are all in parallel we will look at all of them at the same time. With the unit unplugged and turned off. Put a meter probe on base of transistor I believe it is the yellow wire and test to the other two. Note meter readings. Switch probes and test in the other direction. Now leave out the base (Yellow) lead that was common in the first test and test both polarities between the other two ( Emitter and Collector).

                  Next test. Still using the Analog meter with the unit off. Test to see if there is connection between the negative of the big capacitors and the black (negative) banana plug jack on the front of the unit. Test to see if there is a connection between the negative of the large capacitor and the #4 pin of each of the LM741 ICs.

                   

                  Now with the unit still off test to see if there is a connection between the Blue wires of the power transistors and the RED positive Banana JAck on the front of the unit. If not is there a connection between the yellow wires and the Red Banana Jack?

                   

                  With the unit turned on and using your digital meter set on DC volts measure the voltage between the Black binding post on the front of the unit and the Collectors of the output transistors (Orange? RED? Wire I can tell the color clearly) Try turning the range selector on the front. We should see the same voltages that we saw when we tested the large capacitors.

                   

                  With the unit turned on and the black lead of your digital meter still on the black banana jack check the voltages on the yellow and blue wires to the power transistors.

                   

                  Let me know what you find.

                   

                  John

                   

                  You are getting some really good practice in using your meters.

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                  • 96. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                    jw0752

                    Hi Matt,

                     

                    I just wanted to let you know that I feel that we are making real progress. I have a preliminary concept of how they are making this circuit work. We hopefully are only a couple steps away from zeroing in on the source of the problem. You are doing an excellent job of interpreting my measurement requests. I understand the #3 pin reading on the lower Right LM741 and I believe we are good on the circuit through this 741.

                     

                    John

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                    • 97. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                      phoenixcomm

                      mpulliam Mat you have three (3) bridges on the diode board, remove the screws holding it in place. Then turn it over and draw or take photos of the bottom. You should be able to tell the outputs of the transformer to feed the bridges. On a bridge 2 there are two connections for the AC, one for positive and one for negative;  here is a little video that explains them. with bridge in place check input voltages (AC) if that's ok check the(+/-) side. If no joy on the inputs that might mean you big transformer is open, only what to check is to desolder the leads to the transformer and Lable them!!! then check the secondarys. tells you real quick!

                      • 98. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                        mpulliam

                        If I'm measuring Resistance, then it's 15 Ohms between Yellow (base) wire on different transistors in both polarities (at X1)

                        Between Red to Reds it's also 15 Ohms

                        Between Blue to Blues it's also 15 Ohms

                        Between Red to Blues there is no reading

                         

                        Affirmative continuity between negative on both big capacitors and negative banana plug

                         

                        The continuity between the big capacitors and the LM741 is screwy.  It seems to change...

                        Negative continuity between negative on both big capacitors and #4 pin on each of the LM741 ICs, except that there was continuity for a second

                         

                        Positive continuity between negative on both big capacitors and #2 pin on each of the LM741 ICs, and then there was no continuity

                        Intermittent continuity between the positive on both capacitors and some of the pins of each of the LM741 ICs

                         

                        Positive continuity between the top row of rectifier diodes on the non-line side and the #2 pin on each of the LM741 ICs, then negative continuity

                        Negative continuity between the bottom row of rectifier diodes on either side of diode and the #2 pin on each of the LM741 ICs

                         

                         

                        Positive continuity between each of the blue wires on the power transistors and the Red Banana Jack

                        Negative continuity between each of the yellow wires on the power transistors and the Red Banana Jack

                         

                        16.5V between Red wire of power transistor and black banana plug, then turning the front range selector

                        26.14V

                        36.9V

                         

                        Black banana plug to yellow transistor wire (all were the same)

                        36.7V

                         

                        Black banana plug to blue transistor wire (all were the same)

                        .6V

                        • 99. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                          jw0752

                          Hi Cris,

                           

                          We have shanghaied the original posting bt Ted Bier as Matt has the same problem as the original poster. Several posts ago we established that we have 3 levels of Positive DC voltage available depending on the setting of the front function switch. We have voltage to the LM 741 ICs that appears to be proper. At this point it seems that the output of the Op Amp is not being applied to the bases of the output transistors. Matt is running tests right now to get us closer to the point of failure. If you have time, go back to the point in this thread where Matt first weighs in and check out our progress to this point. If you have some insights it will be appreciated.

                           

                          John

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                          • 100. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                            mpulliam

                            I wanted to thank you for your patience and thoughtful explanations!   I really appreciate it!! 

                            • 101. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                              jw0752

                              Hi Matt,

                              The voltages that you provided tell me that the Output transistors are likely failed open. I just need to confirm that the voltage on the blue wire was 0.6 Volts?

                               

                              I recommend that you bite the bullet and remove one of the output transistors so we can test it and verify that it is bad. Don't order replacements without talking to me first. Once it is out use the analog meter to run the same test we just ran on them when they were in the circuit.

                               

                              With 36 volts on the collector and 36 volts on the base the emitter should also be at approx 36 volts not 0.6 volts as this is an NPN transistor.

                               

                              John

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                              • 102. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                                mpulliam

                                Confirm, I'm reading .6V between the blue wires and the negative banana plug

                                36V between Red and the negative banana plug

                                36V between Yellow and the negative banana plug

                                • 103. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                                  mpulliam

                                  Actually, it's -.6V

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                                  • 104. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                                    jw0752

                                    Hi Matt,

                                     

                                    Here are some thoughts on the Output Transistors. If the wires are long enough you might try pulling the wires through the holes in the heat sink and then desoldering them or just clip them off and make new connections to the new transistors. I believe that the coils of wire on the circuit board are emitter resistors. One blue wire should run to one end of each coil and the other ends of the coils should be connected to the red banana jack on the front panel. Confirm to me that this is the case. Otherwise we may need to install our own emitter resistors to help the outputs avoid thermal runaway. When you remount the new output transistors you should either use a teflon insulator or if you are going to use the original mica insulators silicone heat sink compound needs to be used. We want the heat from the 2N3055s to easily transfer to the heat sinks. I saw that Newark has some reasonably priced 2N3055s

                                     

                                    https://www.newark.com/multicomp/2n3055h/bipolar-transistor-npn-60v/dp/78M7081?st=2n3055

                                     

                                    Get yourself at least one extra just in case. Also you should be able to find the teflon heat sink insulators on Newark. I still want to see the results of your out of circuit test of the 2N3055 before we go ahead but I had some time so I jumped ahead to these ideas on replacing the 2N3055s.

                                     

                                    John

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