1 4 5 6 7 8 9 120 Replies Latest reply on Sep 14, 2020 1:11 AM by Jan Cumps Go to original post
      • 75. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
        jw0752

        Diode Tests look normal.

        John

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

          The Ceramic disk capacitors all look normal.

          John

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

            The component in the picture is the range switch which chooses 0 to 7 Volts - 7 to 12 volts and 12 to 20 volts. The potentiometer will be the control to the right as you look at the front panel. It should turn smoothly without clicking and it has a black ramp painted on the front bezel to denote a change in the voltage as it is turned.

             

            John

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

              Here is one oddity I noticed.  It looks like the yellow lead to the center terminal of the pot was in contact with the terminal to its right as pictured.  So much so that I thought it was welded to both.  But I pushed it with some tweezer and it separated as pictured in the last two photographs

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

                It's difficult to test because I don't have three hands, but it seems like the R value between the two outside terminals on the pot is 1.6 kOhms.  The value between the red outside lead and the center yellow lead ranges from .9Ohms to 1.6k Ohms.

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

                  Hi Matt,

                   

                  The readings on the potentiometer appear to be normal and acceptable. I would not have been surprised if the yellow was connected to both terminals. This potentiometer is being used as a resistive trimmer in either configuration. I am waiting to see what voltages you measure on the leads of the two LM 741 ICs.

                   

                  John

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

                    Hi Matt,

                     

                    Here is another experiment I would like you to try. There are two large capacitors on the board with the diodes. I would like you to set your digital meter to DC volts. Turn on the power supply and measure the voltage across the capacitor leads in each of the settings of the range control on the front panel. We may get different voltages and we may get only one voltage. This may tell us something about the actual circuit design. Do this test for each of the two filter capacitors. Note the lead that is closest to the line on the side of the capacitor is the negative. Let me know what you find out.

                     

                    John

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

                      tedbier check to see if you have power in the following places:

                      1. Check the fuse with an dvm ohms..
                      2. where the power cord terminated: yes move on; No replace cord
                      3. check the seconary across the bridge. Yes, Regulator is bad
                      • 83. Re: Need to get MG Electronics Power Supply repaired...or NOT!?
                        mpulliam

                        The diode test on the power transistors yields .010 volts between the base and the emitter in both polarities while the transistor is in circuit.  All four power transistor yield the same results

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

                          The diode test isn't going to tell us much on the power transistors. You could try to test them with the analog meter. They could be hooked in parallel and if one was bad but not shorted you would not see it.

                           

                          John

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

                            Voltage Test for large capacitor (both measured exactly the same)

                            at 0-7V = 16.50V

                            at Mid-mark = 24V and falling

                            at 7-14V = 26.09V

                            Mid-mark = 35V and falling

                            at 14-20V = 36.7V

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

                              LM741 IC Tests

                              Top Left IC

                              Pot at Full CCW

                              1 = 5.2mV

                              2 = 6.22V

                              3 = 6.33V

                              5 = 4.8mV

                              6 = 17.71V

                              7 = 18.33V

                              8 = 35mV and falling past 20mV

                               

                              Pot at Full CW

                              1 = 4.9mV

                              2 = 6.22V

                              3 = 6.33V

                              5 = 5mV

                              6 = 17.71V

                              7 = 18.33V

                              8 = 35mV and falling past 20mV

                               

                              Bottom Right IC

                              Pot at Full CCW

                              1 = 9.6mV

                              2 = 6.33V

                              3 = 6.98V

                              5 = .6mV

                              6 = 18V

                              7 = 18.33V

                              8 = 30mV and falling past 20mV

                               

                              Pot at Full CW

                              1 = 10.1mV

                              2 = 6.33V

                              3 = 10.20V

                              5 = 2.5mV

                              6 = 18V

                              7 = 18.33V

                              8 = 96mV and falling

                               

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

                                Hi Matt,

                                 

                                Good Job. The only one that looked suspicious to me was the bottom right with full CW the pin 3 did not look right but we will wait and perhaps check it again later. I think this information will be helpful. It looks like the range selector switch is picking a voltage tap from the transformer to feed to the output transistors so that they do not have to over regulate and get too hot. I believe the intermediate settings on the switch are just off positions that is why the voltage is dropping.

                                 

                                Back to the output transistors. From the drawings I could see that the yellow wire went to two of the output transistors. Does it go to the same spacial pin on each of the 4 output transistors? Do the other wires from each transistor happen to tie back to the same connection point? If possible draw a diagram of the output transistors and show where the wires go.

                                 

                                Let me know what the results of using the Analog meter to test the output transistors is.

                                 

                                I might ask to anyone who has been monitoring this process, particularly, genebren  if they have any insights or if they can propose any other tests we can run to try and figure out why this unit seems to have voltage to all the power and control circuits but doesn't put out any power. I haven't yet seen any indication of an overload circuit that might be clamping off bias to the output transistors. I expected some variance in the  2 and 3 pins of the LM741s with the potentiometer turned from one extreme to the other but except for the anomalous reading on pin 3 in the full CW position nothing seems to be changing.

                                 

                                John

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

                                  Reconfirm Lower RIght LM741 IC  with Pot at full CW between Pin 4 and Pin 3

                                  10.16V

                                   

                                  I believe all the transistors are wired the same, except the ones on the left side have two yellow wires, the transistors on the right have one yellow wire.

                                  The wires get lost in a bundle, I can't follow them.  It seems they end up at the board and potentiometer.

                                   

                                  But each transistor yellow terminal has continuity to all other yellow terminals, each red terminal has continuity to all other reds, each blue terminal has continuity to all other blues.

                                   

                                  How should I test the transistors with the analog multimeter?

                                   

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

                                    John,

                                     

                                    I have been following your steps in helping to solve this issue with great interest.  I wish that I could be of more help, but my days of being a technician and doing any real heavy device debugging are sadly for behind me.  I tend to fix broken equipment by buying a replacement.  To that end, my search skills have have found a couple of replacement power supplies (same brand - one for 99.00 and free shipping).  Let me know if you need any details.

                                     

                                    I am (and always have been) in absolute awe of people who can fix broken gear and save it from the trash.  I seem to have a phobic thing that prevents me from opening up anything.  With a schematic and plenty of time, I have a fairly good chance of figuring it out.  But my running 'blind' skills are severely lacking.

                                     

                                    Best of luck guys!  I will continue to follow along, and who knows, maybe I will come up with some way to help.

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