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Williams Display Power Transistor Replacement

Many of the Technical Articles on our site contain information and directions involving electronics and circuit board repair. They are authored with the assumption that the reader has adequate experience and knowledge required to do the work being described.

If you do not feel qualified, or are in any way uncomfortable doing any of the work described in any of the articles, then we strongly recommend enlisting the help of a qualified repair person or shop who can do the work for you. It may save you cost, time, and further repair work.

We (Action Pinball & Amusement, LLC) are not responsible for any damage to you, your game, or your property, from doing any work on your game related to any of the articles listed on this site.
  • Parts & Supplies: Free free to contact us if you have any questions about parts that we sell which are mentioned in any of our Technical Articles.

  • Technical Help & Questions: We do not provide remote assistance with diagnosing and repairing games. We are mainly a parts supplier and do not have the time or resources to provide such services. Here are some other options you can explore for help:

Williams used two different types of transistors, and slightly different circuitry routing, for the display power supply circuitry on their games from 1977 through 1989. Early original power transistors, and several other parts used in the circuitry, are obsolete and no longer available, however some modern equivalent replacements are available.

Looking for replacement parts or a full rebuild kit? We have both available! See our Electronic Parts page for full listing, and online ordering.

Below is a listing of the original part numbers, and their new replacement numbers. Underlined part numbers are available for online ordering on our Electronic Parts page- either individually, or as a complete kit:

Williams High Voltage Display Components- Cross Reference Part #'s:

Old Style (late 70's thru mid 80's):
  • Q1 Transistor: SDS201 = NTE 171, 2N6557 (obsolete), (or MJE15030 with modification- see note below)
  • Q2 Transistor: MPSD52 = NTE 288, 2N5400, or 2N5401
  • Q3 Transistor: SDS202 = NTE 296 (or MJE15031 with modification- see note below)
  • Q4 Transistor: MPSD02 = NTE 287, MPSA06, or MPSA42
  • ZR1/ZR3 Zener Diode: 1N5990 = 1N5228B
  • ZR2/ZR4 Zener Diode: 1N4764 = 1N4764A

    New Style (mid/late 80's):
  • Q1 Transistor: MJE15030
  • Q2 Transistor: MPSD52 = NTE 288, 2N5400, or 2N5401
  • Q3 Transistor: MJE15031
  • Q4 Transistor: MPSD02 = NTE 287, MPSA06, or MPSA42
  • ZR1/ZR3 Zener Diode: 1N5990 = 1N5228B
  • ZR2/ZR4 Zener Diode: 1N4764 = 1N4764A

    Williams recommend upgrading resistors R2 and R5 to 1/2 watt 1.2K ohm resistors for better reliability.

    Looking for these parts? We have them in stock- see our Electronic Parts page for online ordering.

    MODIFICATION NOTE: The original early style Q1 and Q3 transistors are no longer available- in original or aftermarket parts. These parts must be replaced with the newer MJE15030 and MJE15031 parts, respectively. The MJE transistors have a different pinout (standard) than the original SDS type had (non-standard), so they must be installed differently on the board, requiring a slight modification, described below.

  • Old (SDS): E B C
  • New (MJE): B C E

    Note that the new pinout is just shifted one place to the left. To install the new MJE type on a board that originally used SDS, do the following 4 steps:

    1. From the front side of the board, drill a new small hole to the right of the rightmost hole for the old transistor mounting- like this (the "x"):

      o   o x
      E B C    (Pinout)
    2. Insert the left two legs of the MJE transistor into the RIGHT two holes of the old mounting. The right leg of the MJE will go through the new hole you drilled. So the MJE transistor will basically be mounted one position to the right on the board. Like this:
        | | |
      o   o x
      E B C E  (Pinout)
    3. Solder the two left legs of the MJE transistor to the board.

    4. On the back of the board, install a small jumper wire between the right leg of the MJE and the first hole of the old mounting. (ie: connect the leg that goes through the hole you drilled to the leftmost hole (empty hole) of the old mounting.)
    Alternate Method:
    Note that the MJE can also be installed without drilling a hole, but this can be a bit tricky (not advised). To do it this way, the left two legs must be installed in the 2nd and 3rd holes, respectively (as described above), and the rightmost leg will then criss-cross under the first two legs, and install in the 1st hole. (Click here for a picture) This is actually the way Williams recommended doing it in a service bulletin, but the above method with drilling a hole is much safer, cleaner, and you can still mount the transistor without any risky leg criss-crossing nonsense.

    If you mount the transistor via the criss-crossing method, we strongly recommend installing some insulation on the rightmost leg (such as heat-shrink tubing, etc) to prevent any arcing or shorts to the other legs. And as always, these modifications are done at your own risk- we are not responsible for any damage to you or your game. Use caution, and good luck!

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