(Topic ID: 197793)

Black Knight LE Display Woes

By Talonslair

4 years ago


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  • 16 posts
  • 2 Pinsiders participating
  • Latest reply 4 years ago by Talonslair
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    PSU-Display-PWR.pdf (PDF preview)
    F1 Display Fuse (resized).jpg
    20170912_171936 (resized).jpg

    #1 4 years ago

    I have a system 7 Black Knight LE. The machine boots into attract mode and will start a game but the displays don't work. The F1 1/4 amp display fuse on the power supply blows when the machine is turned on. I have unplugged all of the displays but the fuse still blows. The displays used to work before I bullet proofed the power supply per Vid's guide. I've replaced the capacitors, diodes, resisters, and all of the male and female .156 connectors. I'm not sure what to look for next. Can someone please help? Thanks.

    #2 4 years ago

    check you power at the supply board, may have a bad bridge rectifier

    #3 4 years ago
    Quoted from wdennie:

    check you power at the supply board, may have a bad bridge rectifier

    I'm a little ignorant of what to check. For the bridge rectifier, it is BR1 on the schematic(35 amp, 100 volts). I check this like I would be checking a diode. Correct, like below?

    Testing a Bridge Rectifier.
    The following test will check if a bridge has an open circuit or a short.
    1. Turn the game off.
    2. Put the DMM on diode setting.
    3. Put the black lead of the DMM on the "+" (positive) terminal of the bridge.
    4. Put the red lead of the DMM on either AC bridge terminal. Between .4 and .6
    volts should be seen. Switch the red DMM lead to the other AC bridge
    terminal, and again .4 to .6 volts should be seen.
    5. Put the red lead of the DMM on the "-" (negative terminal of the bridge.
    6. Put the black lead of the DMM on either AC bridge terminal. Between .4 and .6
    volts should be seen. Switch the black DMM lead to the other AC bridge
    terminal, and again .4 to .6 volts should be seen.

    Edit: If I did this correctly, here is what I found:
    Black lead on "+" red lead on AC - 494 on one leg and 506 on the other.
    Red lead on "-" black lead on AC - 484 on one leg and 495 on the other.

    #4 4 years ago

    I did some reading about blown fuses and I copied some text from a Pinball Rehab article:

    "The first thing to know about checking for a short is that 9 times out of 10 the problem will be caused by a diode, bridge rectifier or transistor".

    I looked on my power supply board and saw this: The little pad looks like it is half lifted(might be hard to see in the picture). Could this be a problem?

    How do I check the 2 transistors(Q3 PNP and Q1 NPN)?

    20170912_171936 (resized).jpg

    #5 4 years ago

    looks like the actual trace is on the back side.
    With the fuse that blows out, what voltages do the fuse clips read?

    #6 4 years ago
    Quoted from wdennie:

    looks like the actual trace is on the back side.
    With the fuse that blows out, what voltages do the fuse clips read?

    94.2v AC to ground and 0 on the other

    #7 4 years ago

    Sound like a bridge rectifier went bad, that's the fuse that blows right?

    #8 4 years ago
    Quoted from wdennie:

    Sound like a bridge rectifier went bad, that's the fuse that blows right?

    The one that blows is the F1 display fuse.

    F1 Display Fuse (resized).jpg

    #9 4 years ago

    Ok, there are 2 heatsinked transistors that provide 100+ and 100- voltage, located in the upper left side, one went bad.

    #10 4 years ago

    should be the transistors in you pic. good luck, I've tried several times to rebuild this part with no luck, and just would buy a aftermarket replacement.

    #11 4 years ago

    Ok. I really appreciate all of your help with this. I will order and replace both transistors(Q1 and Q3). I've read about those two transistors and how the replacement legs should be crossed when installing.

    I'm just a little confused with this section of the power supply. I could be wrong but isn't the current going into the F1 fuse AC current? And does it get turned into DC current after the diodes which are after the F1 Fuse? I clipped part of a diagram from Great Plains Electronics about the high voltage section.

    PSU-Display-PWR.pdf

    #12 4 years ago

    Yes, one transistor will put out a +100 volt DC current, and the other transistor puts out negative -100 volts DC.
    Where as Bally's power supply would just put out just a +216 Volt dc. and use the common ground.

    Is it possible that when you did your grounding mod that you may have grounded out the display's as well?
    That would create a short.

    #13 4 years ago

    My thoughts are that if I am getting good voltage from the transformer to the fuse then something is shorting out after the fuse and could sending current back to the fuse and blowing it. So I might as well replace all of the components of the high voltage section just to be on the safe side.

    Again, thank you for your help

    #14 4 years ago

    One quick though, if you unplug the connector to the displays does it still blow fuse?

    #15 4 years ago
    Quoted from wdennie:

    One quick though, if you unplug the connector to the displays does it still blow fuse?

    Yes. Still blows. I only have the 2 power connectors from the transformer hooked up, (3J1 & 3J2).

    1 week later
    #16 4 years ago

    After rebuilding the high voltage section and plugging it in, the fuse doesn't blow. Hooked everything up and I now have displays! I found a shorted small transistor and a shorted diode.

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