Williams Linear Power Supply

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This repair guide is for the Williams linear power supply present in Defender, Stargate, Bubbles, Robotron, Splat, Sinistar and Joust. There are two versions of this power supply, once complete with the heatsink on the board, and one with a detached heatsink.

Why fix the linear power supply?

It keeps the equipment original, which for a collector is important. It's rewarding. These boards are fairly simple.

Linear power supplies can provide extremely clean DC power when properly tuned. This supply can be replaced with a switcher with some work, but why hack and splice an aftermarket solution when the original equipment can be easily repaired? In addition you lose the coin door lockouts with a switcher.

There are also potential CMOS corruption problems when using a switcher.

Ok, let's fix it

First, get yourself a schematic, and have it handy. Williams schematics and board layouts in TIFF format.


Check the fuses. If there is a blown fuse, replace it and try again. Meter them for resistance, but do it at the points on the reverse of the board. We've seen a few that have lost "spring" and look good, fuse tests good, but the contact with the holder isn't there. If it blows again, time to troubleshoot. The fuse values are as follows:

F1 = 1A slow-blow (-5V DC)
F2 = 5A slow-blow (+6.3V AC)
F3 = 4A slow-blow (+12V DC / +27V DC)
F4 = 7A slow-blow (-12V DC)
F5 = 7A slow-blow (+5V DC / +12V DC)

Always re-flow the solder on the plastic connectors on the power board before doing anything else. These connectors are extremely unreliable after 20 years of high current operation. Also inspect the connectors on the wiring harness, and check the harness with an ohm meter, as sometimes these connectors burn up. Replace any bad connectors.

There are LEDs on the board to indicate working voltage, but it's best to use a voltage meter to verify.

4J3-1 - 4J3-2 = +6.3VAC

then use

4J2-8 = Reference ground

and test

4J2-12 = +5VDC
4J2- 3 = +12VDC regulated
4J2- 5 = +12VDC unregulated
4J2- 1 = -5VDC
4J2- 4 = -12VDC unregulated
4J3- 3 = +27VDC

What to look for

Transistors and electrolytic capacitors will usually be the problem. Transistors usually short or open when they fail. Electrolytic capacitor values drift as they age. Resistors and other parts will rarely go bad without visible damage (scorching, a smell), although anything can happen.

If a resistor is burning up there is a larger problem with your power supply. For some reason too much current must be flowing through the resistor, usually because a nearby transistor is shorted. Resistors don't spontaneously burn up.

The most common failure on Williams power boards is the 2N3055 transistor (mounted on heatsink), taking out the 12V line, and usually the fuse. If the 12V line is dead, this is a good place to start. Usually it can be repaired by simply replacing this transistor. One caveat: when this transister blows a certain way, it will send 24V unregulated to the RAM chips on a line where they're expecting -12V, frying all of them instantly. This seems to happen in about 15% of the cases where the 2N3055 has failed.

Another common problem is the bridge rectifier. Before it dies completely, it causes problems. If you're having intermittent problems with your game, this is usually the first thing to replace. It gets really hot, and sometimes hot enough to soften its own solder points. When replacing, leave space between the rectifier and the PCB.


This is a good low-fuss approach to these power supplies:
  1. Check the fuses for continuity, on the reverse of the board.
  2. Re-flow solder on connectors on the board.
  3. Check the harness for resistance in wires.
  4. Apply rebuild kit.

You can order a Williams Power Supply Rebuild Kit from our sale page. It contains most of the electrolytic capacitors and the 2N3055, as well as the 5V regulators and sockets, and bridge rectifier. This should fix most problems and give you lots more life out of your supply. If this doesn't solve the problem, it's almost always because the RAMs were fried when the 2N3055 blew up, then the next step is to replace the RAMs. These two measures repair 85%+ of Williams boards. Most of the others are fixed by dropping in new RAM. If you want 97% assurance you will fix your machine, order both the RAMs and the rebuild kit.