Switching power supply repair help

Discussion in 'Technical Repair' started by inwo, Aug 17, 2016.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    Sorry, this gets long winded. 2 day saga..............

    Power supply stopped working after power outage. Old Life Fitness gym machine.
    5v and 9v outputs. Connected to 115v. There is a jumper disconnecting 1/2 bridge for 230v.
    When the older machines needed repair it was a resistor near the switching transistor. That's where I started.
    A 120 ohm 5w measured 140 ohms. Replaced it and ps turned on.

    Did not work in machine so I returned home.
    After 6 trips chasing phantom problems, such as 115v supply to board dropping under load. I determined that the whole machine is powered thru iso transformer. No taps that I can find.

    Voltage is 5v lower than mine at home. Another 5v lower thru trans. leaving 107vac to ps.

    Testing at home found 107 is about the min. for starting supply.
    220uf 50v on input side had high esr and low value. Was replaced first round.
    Other caps are ok. In and out.
    5v supply adj. as it should. Does not regulate from low to high load.

    Small driver npn gave up its life during testing by me not discharging caps properly.
    Crossed to a 1.5a npn audio. I have it replaced with audio npn in larger package. I also replaced it with a 2n222 for a test. 107v min turn on does not change.

    Decreasing value of the previously replaced 120 ohm, does lower starting voltage, but not enough.

    Looking for general assumptions on cause for failure to launch.
    Not looking forward to drawing up schematic. Only a few parts on the board and most have been scrutinized.

    Can the main bipolar transistor get weak? It osc. in my tester when removed from circuit.
  2. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    Any ideas for kick starting?
    It works at low voltage just won't start below 107vac.
    Tried adding to filter caps.
    Maybe gym voltage is up today. I'll retest.
    At this point, I might use a 6vac boost transformer and call it fixed.
  3. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    I'll never know now. :)
    Cut the board in half to use machines original wiring.
    Wired to 9v and 5v supplies. MVC-017F.JPG MVC-018F.JPG
  4. ISB123

    Well-Known Member

    May 21, 2014
    I gave up on fixing those. Without scope its pretty much impossible unless its a simple problem like a diode or cap.
  5. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    I've fixed a lot of them. Problems repeat making the next ones easy.

    Slight possibility it was only a cold solder joint on the ac input.
    Noticed it when cutting next to it with hack saw. :(

    If you tried to make a connection sensitive to a couple volts it would be impossible. Much less one that would survive repeated tests in two locations.

    What's done is done:
    Lady Macbeth
    absf likes this.