Pinball machine power supply problem

Discussion in 'The Projects Forum' started by wraith644, Dec 15, 2010.

  1. wraith644

    Thread Starter New Member

    Dec 15, 2010
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    I am working on a gottlieb sys 1 power board for a friends pinball machine. When the power is turned on two of the diodes (CR3 and CR4)burn up. The input voltages are reading high. pins 1 and 2 are 12.7v instead of 11.5v, pin 3 is ground, pins 4 and 5 are 15.4v instead of 14v, and pins 6 and 7 are 75v instead of 69v. Voltages on pins 1, 2, 4, and 5 were taken from ground and pins 6 and 7 across the pins. I was wandering if these voltage differences would fry the board or are they within tolerance. I know that there is probably something else wrong with the board, but I haven't gotten that far yet. I'm trying to remember everything I learned about 16 years ago. It's amazing what you forget if you don't use it. Any suggestions will be greatly appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
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    You will have to post a schematic before we can tell what you're talking about.
     
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    It would be very helpful to have a schematic to look at.
     
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  4. wraith644

    Thread Starter New Member

    Dec 15, 2010
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    Ok, This is a schematic that i found online. http://tuukan.fliput.net/power.gif It is very close to the board. the only differences is that C9 and C10 are not on the board an C1 is a 4700uf 35vdc cap instead of the 2900uf 30vdc cap. I'm mapping the actual board myself, but that takes alittle time. I have also found out that lic1 is a LM7912 -12v regulator if that helps.
     
  5. #12

    Expert

    Nov 30, 2010
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    Start by suspecting C4 as shorted. Then check the 7912 to see if it is passing excessive current to some shorted load.

    There's a start. Check back if you need to.
     
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  6. wraith644

    Thread Starter New Member

    Dec 15, 2010
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    C4 is not showing as being shorted. However, as I was removeing the mounting bracket, which aslo serves as a heat sink for Q1, Q2, and LIC1, I noticed there was no mica under LIC1 and Q2. It looks as if there was mica there at one time but is gone now. I checked to see if there was a conection between them and found out that pin 3 on LIC1 and pin C on Q2 were conecting throught the mounting bracket. I think this could be at least be a part of the problem.
     
  7. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    Make sure the tab of the 7912 isn't accidentally grounded; the pinouts are different between the 78xx and 79xx family.
    /mike
     
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  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    SCR201 is part of a crowbar circuit. Something may be tripping it. This might be a case where loading the 14 VAC lines down with a couple of automotive lamps to prevent the voltage from coming all the way up could be useful. If the caps check, then something is going on with the crowbar - the SCR could be shorted.
     
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  9. rushfan

    New Member

    Jan 8, 2012
    1
    0
    hi
    new to this posting business... just wondering if you managed to fix the power supply? Im having the same issue with cr3 and 4 burning... thanks
     
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