MFJ 4245mv power supply help

Discussion in 'Radio Circuits, Repair & Performance' started by DV2NAH, Nov 13, 2017.

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  1. DV2NAH

    DV2NAH Ham Member QRZ Page

    I am trying to repair my friend's MFJ 4245 power supply

    Initially it does not power up, so I replaced the PWM ic TL494 and open R9 on the primary side
    and now powers on ok.

    The problem is everytime a load above 2amps is added it shuts down and has to be reset(turned off)

    I cannot find any other defective part, all the switching transistors on the primary are ok, so with the secondary diodes

    Can you please help with ideas what to do next?
    thanks and 73!

    Jonathan
    DV2NAH
     

    Attached Files:

  2. NN4RH

    NN4RH Premium Subscriber QRZ Page

    Just a guess ...

    The power supply is designed to shut itself off if it detects currents in excess of 45 amps.

    Maybe it THINKS the current is over 45 amps when it's only 2 amps?

    So ... figure out what part of the circuit detects the output current, and check all the components there.

    ??
     

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