William’s Terminator 2 Sound Board Hum

Here’s a sound issue I found on a William’s Terminator 2 pinball last week. I get a call because the game has a few issues but oddly enough the customer didn’t mention the really annoying sound hum.

The main concern was the display wasn’t working and he couldn’t start a game. When I got there i found just a bunch of garbage and dots on the display. Reseating all of the ribbon cables fixed the display issues. The game wouldn’t start because the battery was dead.

Fortunately the game had a TNT Amusements lithium battery installed about 15 years ago so corrosion damage wasn’t a concern. Of course I installed a WPC battery board so the customer can easily change the battery in the future.

The sound hum reminded me of a similar sound hum I’ve seen in several Gottlieb System 3 pinballs. That was a problem I chased around the first time for more hours than I’d like to admit. I’ll delve into more detail on that in another post.

William’s Terminator 2 Pinball Sound Board

Suspecting a blown fuse, I pulled the sound board out only to find one of the traces burned up on the back coming off one of the fuses. On the front I saw a cap burned up. C28 to be exact. When capacitors fail, they usually go short. In this case the trace burned up.

Detail of C28

I replaced C28 and I also replaced U3 which is a 7912, the -12v regulator, for good measure. Audio amplifiers *usually* need a positive voltage as well as a negative voltage and and that’s why you’ll see some games use negative voltages.

Many sound boards have their own power supplies onboard for the power amplifier. Early sound boards will regulate their own +5v as well. The sound board is fed a center-tapped A/C voltage meaning there’s power sent on two legs and a return leg. That’s why there’s 3 wires on the power connector. Losing one of the legs can cause this hum.

The final step was to repair the trace on the back of the board. I try to avoid running a jumper wire and use copper foil tape instead but in this case there are some tight clearances and it can end up being more of a pain in the ass than it’s worth to try to make it look pretty.

Solder side of sound board
Detail of burned out trace repair

A final test verified the repairs are good and the board can go back in the customers game.

If you encounter a hum in your game first always check that the boards are secure and all grounds are good. Next check your fuses. In a lot of cases one of the fuses for the amplifier are blown causing the hum.

Until next time, Frank

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