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

        Hi John,

         

        The wires are not very long and there is no slack. It would be a delicate operation without much room for error to desolder and solder.  I would probably need surgical tools.

        The blue wires go to the bottom of the circuit board then up to I believe the curly q resistance coils (I don't what they are called) then on to the resistor next to it.  From there it's a maze.

         

        Working from the red banana jack back, it's tied to the front Amp Meter and then back to the circuit board (it's the thick red cable coming out of the middle of the board)

         

        In my research, bad power supplies seem to have usual suspects:

         

        If not blown fuses, loose wires, corrosion, broken traces, or cold solder joints then -

         

        Typically goes out:

        Transistors

        Electrolytic Capacitors

        Integrated Circuits

        Diodes

         

        Sometimes goes out:

        Mechanical switches and potentiometers

        Resistors

        Non-polarized capacitors

         

        Usually never goes out:

        Transformers

        Variable capacitors

        Inductors

         

        Also testing components in circuit could yield bad readings.

         

        So I'm willing to order replacements for the ICs, transistors not yet replaced, the power transistors and whatever else you'd recommend.  The parts are cheap but shipping adds up.  I'll give replacing the components a go and if it works great, if not, I'm not sure the unit is worth any more trouble. 

        Thanks,

        Matt

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

          Hi Matt,

          At this point based on your voltage readings the Output transistors are definitely a problem. I would start by replacing them. If that doesn't solve the problem we can go back and look at secondary causes. There are two wires on each output transistor that need to be replaced. The collector is connected through the screws that mount the transistor. You could even cut the pins off the old transistors and then solder on a short piece of wire and seal the connection with heat shrink. This would give extra wire to come through the heat sink and solder on to the new transistor. This extra wire could then be pushed back through the heat sink when the transistor is remounted to the sink. Hold off on replacing anything else until we have done the outputs. If you want to take more detailed pictures of the difficulty replacing the outputs I will see if I can come up with more ideas to make it easier.

           

          John

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

            Hi John,

             

            I have a thought.  If the output transistors (or is it pass or power transistors?) NTE130 or 2N3055H are giving a bad voltage reading in the voltage test, could it be that any component between the blue terminal and the red banana jack could be bad, (and not the output transistor) thus creating the low voltage?

             

            For example, the blue terminal on the output transistors tested positive for continuity with pin 2 on one of the LM741 IC (the lower right one) but gave a weird electronic sound when testing for continuity with pin 2 on the top left LM741 IC.  If the LM741's function is the same on this linear power supply as the ICs on switching power supplies, as detailed HERE in the 6th paragraph down, (which regulates the output voltage of the transistors) then a bad IC could result in low voltage.

             

            Thoughts?

            Matt

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

              Hi Matt,

               

              Sorry but due to the physics of a functional transistor the voltages that you have measured are not possible. Therefore we must conclude that these are not functional transistors. there is an energy barrier of about 0.7 Volts between the Base and the Emitter. Any time that energy barrier is exceeded current flow in the BE circuit. This also causes the CE current to flow. I think the BE junction can handle a few volts for a short period of time but anything more than that would cause it to burn out. Your measurements indicate that there is over 36 volts across this junction and therefore it is apparently already burned out. Until these transistor are replaced and functional we can't further trouble shoot the circuit easily.

               

              John

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

                Hi John

                So what do I need to order?

                NTE130 or 2N3055H

                Do I need thermal paste?

                 

                What type of LM741CN IC?  This one?  If so, it's backordered, is there a suitable replacement you'd recommend?

                Ceramic capacitors? Diodes?  Resistors?

                 

                It cost $9.99 to ship a .10 cent component.   I'm already in for about $35 with the analog multimeter and the transistor.  I could buy a new unit with free shipping for $83.

                It doesn't make sense to buy one component at time to see if it's a fix.

                 

                Thanks,
                Matt

                • 110. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                  Jan Cumps

                  mpulliam  wrote:

                  ...

                  It cost $9.99 to ship a .10 cent component.

                  ...

                  For companies that have free delivery above an amount, I add consumables like solder, flux, wick, spare cutter blades ...

                  Your repair will never be economical. If you do it for the fun of learning, you'll have a good time. Even if it fails.

                  But it 'll never be worth it in $$.

                  1 of 1 people found this helpful
                  • 111. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                    jw0752

                    Hi Matt,

                     

                    I would get the 2N3055 as opposed to the NTE 130s if I had the choice.

                     

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

                     

                    You will also need to get some of these:

                     

                    https://www.newark.com/nte-electronics/tp0001/thermal-insulators-thermo-pad/dp/79M5626?st=to-3%20insulator

                     

                    Which can be used without thermal paste. If you want to reuse the current mica insulators, if they are in good condition, you will need to put a thin layer of thermal paste on the contacting surfaces. Be sure to install the insulators before soldering the wires to the pins..

                     

                    If you want to save on shipping you might check to see what your electronic store resources are in your city. I would be surprised if you couldn't find a local store that would have these two items though what you save in shipping might be offset by higher component prices.

                     

                    John

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                    • 112. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                      Jan Cumps

                      jw0752  wrote:

                       

                      H...

                       

                      If you want to save on shipping you might check to see what your electronic store resources are in your city. I would be surprised if you couldn't find a local store that would have these two items though what you save in shipping might be offset by higher component prices.

                       

                      John

                      I do that here. In Brussels there's a shop called Elak. They have loads in stock. And I can order anything from element14 / Farnell.

                      Because they order from there, my components are a tiny part of their order.

                      They have a markup. For most orders lower than what I'd pay for shipping.

                      And I prefer to spend the extra money there rather than on FedEx/UPS - who try to make my life miserable whenever they spot the opportunity.

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

                        Well, I spent all day replacing four 2N3055 transistors....it wasn't easy

                         

                        and...

                         

                         

                        Success!!!

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

                          BTW

                          I tested the old 2N3055 transistors, the continuity is the same as a new 2N3055.  There is no continuity between any of the terminals

                          But when I run a diode test, a new 2N3055 tests at .486 volts with negative on the emitter, and the positive on the base.  .OL in any other test configuration.

                          The old 2N3055's test at .OL in every test configuration.

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

                            Hi Matt,

                            Hurray!  Well done.  I am curious what the theory is on why these failed.  If I remember correctly it was working one day for you and the next time you turned it on it wasn't working.  Maybe John has a thought.

                            Frank

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

                              Congratulations Matt,

                               

                              You stuck with it and you won the battle. Your diligence in taking good measurements and notes as well as useful pictures made all the difference. I hope you stick around the forum and continue your adventure in electronics. Don't hesitate to share your projects. The next time someone needs one of these fixed you are the go to expert.

                               

                              John

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

                                John, thank you very much for your knowledge and ability to communicate it!

                                 

                                Matt

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

                                  Hi Frank,

                                   

                                  I think that the problem comes from the paralleling of the (4) 2N3055 output transistors. When the unit is used at High current and low voltage the 4 transistors have the challenge of getting rid of the extra power that is not being used. In a perfectly balanced system they would each carry 1/4th of the load and all might be well. Unfortunately they are not perfectly balanced and the one transistor that insists on taking more of the load gets hotter and hotter until it fails. In this case it seems that they fail open Base Emitter which takes the most ambitious transistor off line and transfers the load to the remaining three. One by One they fail until we are left with the situation that Matt had. The designer realized that this was a potential problem so they put emitter resistors on each of the Outputs as this helps to balance the load. This are the stretched out coils that we see on one circuit board. I am sure that they help but apparently not enough to prevent failure. Since we have seen two of these units with the same problem on the forum I would guess that these failures are a design problem.

                                   

                                  John

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

                                    I thought that might be it.  It is a common problem in parallel systems whether they be splitting fluids or electrons unless there is some sort of active control.

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