(Topic ID: 330926)

TECH - Bally Solenoid Driver Board HV

By Dietrich8403

47 days ago

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    #1 47 days ago

    I got a Kings of Steel years ago and finally am getting around to fixing it.
    The driver board HV had visible burned resisters at R51 and R55 so I rebuilt the whole section with components I had from GPE or acquired from NTE.

    Before putting it back in the game, I was checking paths with my meter and have an issue at R56(82K).
    Meter reads 81.7K with red lead at junction of R56 and RT1, and black lead at junction of R55/Q23/C28.
    Meter reads 77K with red lead at junction of R55/Q23/C28, and black lead at junction of R56 and RT1.
    Position of RT1 (which is also new) only changes the value from 76.4K to 77K.

    If I remove either R55 or VR1, R56 will read 81.7K with the leads in either direction.
    R55 and VR1 both meter properly, both in and out of circuit, and I've tried additional replacements with the same results.

    The board has not been powered up yet so I do not know if voltages are proper or not. Thoughts on this discrepancy of ohm readings?


    #2 47 days ago

    My first thought is that if R56 measures close to 82K and you are reading 81.7K in circuit, that is very close and small variances are due to other components in the circuit. I am not at home, and if nobody answers otherwise I can take some readings for you to compare. If you have rebuilt with all new components, when you are confident you should power it up with the F1 fuse removed, and verify voltage at test point TP2 (~190 VDC, adjustable by VR1)

    #3 47 days ago

    The typical cause of burned components in the HV circuit is adjusting the output voltage too low. The factory spec is 190v. If you want to reduce the stress on the displays lower the voltage to around 180v but no lower as that moves the stress to the regulator circuit.

    #4 47 days ago

    The zener diode at VR1 is partially in parallel with R56 so may affect its reading in one direction.

    Checking two boards here, I got the same reading in both directions across R56 with one board and the other board there is about a 1.5k ohm difference in directional readings.

    Lift one leg of VR1 and measure the resistance across it in both directions. See if it's what's causing the resistance variation.

    With R51 and R55 blown, I presume you replaced the three transistors? They usually all die at the same time.

    #5 47 days ago

    You have resistors on each end of the trimmer and an active device on the wiper pin. Its not going to test exactly right in circuit

    If the half watt resistors are burned. 99% chance at least one transistor is bad, 90%+ all three are. If you don't replace the transistors the resistors will burn up again promptly. Replace all three transistors and burnt resistors and you should be good to go.

    #6 46 days ago

    Thanks for all the thoughts.

    All transistors have already been replaced.

    I also tested a couple boards from my shelf and get identical readings in both directions, so that is why I'm a little confused by this one.
    Wasn't sure if the 4K ohm difference is to be worried about.

    When I lift one leg of VR1, I get .594 in one direction and open in the other so it tests fine.

    Unless something else jumps out at anyone, I will put it in the game and see what voltages I am getting.

    #7 44 days ago

    I installed the board and tested with F1 out and had regulated HV.
    I was able to tune it easily with the new pot, so I tried again with the fuse in and have no issues.
    Apparently the ohm discrepancy is not of concern.

    Thanks to everybody for the input!


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