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Need to get MG Electronics Power Supply repaired...or NOT!?

Former Member

Hi.

I just bought a lot of Slot Car stuff, yes those cars that go around a track. The guy was serious about his hobby he didn't have the usual wall plug in power pack, he used a MG Electronics PS-10AD, 0-20 VDC at 10 Amps. Well this power supply doesn't work anymore. It does turn on and the red overload light stays on. It doesn't put out anything. I used my multimeter etc., nothing. I know that these are made overseas . They did retail for about $200. So the questions is were can I get it repaired OR should I ? I'm in Western MAss.

I did check the fuse , it's fine. I've got a heavy door stop on my hands, it does have a nice handle and cord on it !

Many thanks for info, Ted

 

http://www.mgelectronics.com/shopexd.asp?id=179

Parents
  • 0

    7812 voltage regulator test results:

    w/ Power off and writing facing me

    Black lead on the center pin, read lead on the left pin = 1.2mV and it trickled down to 0

    Black lead on the center pin, read lead on the right pin = .5mV and it trickled down to a negative number

     

    w/ Power On and writing facing me

    Black lead on the center pin, read lead on the left pin = 30V

    Black lead on the center pin, read lead on the right pin = 12.04v

     

    I re-tested the D313 for continuity (after the unit has been turned on and off)

    There is NO continuity between the base and the emitter or collector

    There IS continuity between the emitter and the collector

     

    I retested the corroded diode and it is ok.  The corrosion was giving me a bad reading previously but I scraped it off and the diode shows a voltage reading of .519v in one direction and nothing in the other.

  • 0 in reply to mpulliam

    All sounds good and normal except the D313. We need to test that one out of circuit and verify if it is good or bad.

     

    At some point I mistyped Read when I meant to type Red. I mention it so that we don't start a new convention for what is the correct nomenclature.  Sorry about that. I always try to proofread but at 1:30 AM I may miss things.

     

    John

  • 0 in reply to jw0752

    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

  • 0 in reply to mpulliam

    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

  • 0 in reply to jw0752

    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

  • 0 in reply to mpulliam

    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

  • 0 in reply to jw0752

    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

  • 0 in reply to mpulliam

    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

  • 0 in reply to jw0752

    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

  • 0 in reply to 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 $$.

  • 0 in reply to mpulliam

    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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