Help diagnosing power supply circuit failure

Discussion in 'General Electronics Chat' started by jimkarl, Apr 20, 2015.

  1. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    Having a problem with this power supply circuit (pdf attached). My main starting point issue is it's supposed to be putting out -15v and +15v to pins 1 & 4 respectively on PN201 connector. Pin 1 is showing -33v and pin 4 is showing 19v. I've noted on the schematic some various voltage points. These were measured with powersupply board isolated - no other boards connected. This is what I've checked so far:
    1) ESR metered all caps 10uf and larger. They all register good. Triple checked those around the voltage regulator as well (C905/C907)
    2) Pulled the Bridge rectifier out and diode checked between all 4 pins. Shows .556v in one direction, infinity in the other and from AC ins (2 to 4) .966v. Seems good, right?
    3) Diode checked various other diodes and they seem to check ok as well but to be honest those were all done in circuit so I realize that's not 100% guarantee on the readings.

    Hopefully someone can see something in the circuit that would be causing such a higher than expected rectifier output, especially the lopsided negative side vs the positive side.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Use a scope to check the signals at the 4 terminals of BD902. Don't know why the schematic is showing +/-15V on PN201 pins 4/1. It's going to be higher.
     
  3. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    Alas - I don't have a scope. I did note the voltages at those pins those on the schematic (15vac on 2&4, 19vdc on pin 1, and -33vdc on pin 4 - obviously the same as on the connector since they run straight there.
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    I'm not sure what a voltmeter would read if C905 went bad. Do you have a cap you could tack in parallel?
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Does your meter have a frequency counter in it? I'd meter the output of the bridge, you can try putting your meter on "AC" and see if you get a reading. That would indicate open filter cap or even a shorted diode in bridge (worth checking over)..
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Your readings, is that with anything else connected to power supply? Or was that totally unloaded?
     
  7. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    It doesn't, but again, I esr meter checked the filter caps and they are good. I also diode meter checked the bridge across all pins combinations. That's what I mention above with the results. Those diode readings seem good, do they not?
     
  8. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    Nothing else connected for those measurements. I had the problem before when boards where connected so I figured I would get a pure unloaded reading to note those values first.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No.
    I posted my take on this problem, over at ETO. The 15V bridge rectifier is duff.
     
  10. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    ETO? What/where is that?
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Electro-Tech-Online. You posted there, remember?
     
  12. jimkarl

    Thread Starter New Member

    Oct 10, 2014
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    AH - Had a bad email addy and account never got confirmed, so didn't get any alerts. . Just fixed thx for the heads up.
    So I did see infinity in one direction but not the other (between pins 2 & 4 when out of circuit) You are saying it should be infinity in both directions, right?
     
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