Power Supply Problems

Discussion in 'The Projects Forum' started by Dalaran, Feb 6, 2012.

  1. Dalaran

    Thread Starter Active Member

    Dec 3, 2009
    168
    0
    Hello,

    I am working from the attached schematic (this is the power supply unit for a pinball machine).

    All of my low voltage supplies (5V, -12V, 4V, 8V) are working as expected... it is the lower portion, high voltage area, that is not functioning correctly.

    I have tested all transistors (Q2, Q3, Q4), and all are good. All zener's (CR10, CR11, CR12) also tested good. All diodes of the bridge rectifier (CR6, CR7, CR8, CR9) have also all been tested good. All of these components have been removed from the board and tested with the diode setting of the DMM. I have even checked most resistors and they all seem to be the correct resistance.

    I have ~74VAC at the inputs as expected coming from a transformer. However, at the output (pin1) wrt GND (pin5) I have only ~13.2V. Pin3 wrt GND is 0V. The collector of Q2 (marked as 90.4V) I read ~13.8V.

    I'm really stuck as to what could be causing the issue so any and all help is welcome. Could the large electrolytic C6 being bad cause this, or C8? The fuse for the 69VAC is 1/4 amp slow blow and is not blowing.

    Thanks.

    edit: CR10 was replaced with 1N4753A not 1N4753 but I could not find any differences between these. It is also tested good after seeing the problem.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Since it's a simple discrete NPN regulator, the easy way to troubleshoot is turn ON and voltage map it. Should be able to pick it up easily.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    If I read the values correctly, it looks like the lower rail only puts out less than 10 mA. Is it possible something in the unit is overloading it and pulling it down? Assuming R13 is 33 Ohms: If the current draw on the 60V output exceeds about 8mA, the current limiter Q3 will pull it down.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    NOTE: if C10 is actually a 50V cap, it will fail quickly. According to the schematic, it will have about 48V across it all the time so it should be a 100V part.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    U should change all the high voltage caps first.
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Dalaran,

    Check for a broken connection/track/wire somewhere between the transformer and Q2 collector. Check the GND connections in this area as well.

    Good Luck,
    Ifixit
     
  7. Dalaran

    Thread Starter Active Member

    Dec 3, 2009
    168
    0
    Thanks for all the replies. It is now up and working. I replaced all capacitors with fresh ones, but it was likely C9 or C10 as one of these had <1ohm resistance. I pulled them both off at the same time so not sure entirely which one, but would guess C10 because of the previous comment of the voltage tolerance. I replaced these with 100V 1uF caps.

    Thanks again!
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Told ya . :D
     
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