Bally Beat the Clock Feature Lamp Circuit Explained

So here’s part 3 of the Beat the Clock saga. You can find part one here and part two here.

The 6803 feature lamps are driven in a way that’s different than probably any other pinball machine ever, and it hasn’t been used since.

One obstacle that designers face is there’s only so much hardware to work with. An 8×8 lamp or switch matrix will only give you 64 lamps or switches. Most early games only had 16 drive transistors for coils or other things such as motors.

Adding more would be easy but you have to remember the bean counters have to keep within a budget to keep the games within a particular price point. So what do they do? They come up with clever ways to work around these limitations such as relays used on Data East and later William’s System 11 games to switch between two possible coils or even between coils and flash lamps. Even early Bally games used solenoid expander relays.

The 6803 feature lamps are done in a similar way. But instead of controlling multiple lamps by a relay to alternate between which bulbs to turn on, which by the way wouldn’t work because of the high-speed switching needed and a mechanical relay just couldn’t keep up. It’s done by controlling which lamp to turn on depending on which lamp has power at that particular moment.

This allows the controlling of two individual lamps with one transistor. The lamps are split into two phases, an “A” phase, and a “B” phase to power each side individually. It is this turning on and off one side at a time that allows a single transistor to drive two different lamps without both being on at the same time. The Bally 6803 system, and two earlier games, Grand Slam and Gold Ball, are the only games I know of that drove the feature lamps this way.

In this system, most bulbs have a “companion” bulb, there are of course some unused ones. For example, if you look at the chart below, you can see the “T” in “STOP” is driven by Q70 on phase A. If you look a few lines above you can see Q70 referenced again but the B phase is unused.

Transistor and Phase assignments for feature lamps in Bally Beat the Clock pinball machine.

The game turns on the lamp it wants by activating the drive transistor when the particular lamp is in its power cycle, then it turns the transistor off if it doesn’t want the companion bulb turned on when the phases switch. Of course, this means the lamps are being turned on and off very quickly. The same thing is done in a matrix system, we just can’t see it because of persistence of vision, the switching happens faster than our eyes can detect.

If a lamp isn’t to be turned on, then it is bypassed so only the lamps that we are meant to see are visible. The lamps are turned off and on so fast that the lamps the game wants us to see are on long enough for our eyes to process it. If it wasn’t done that way it would take a LOT more hardware to drive the lamps. See the video below for more details and to see how the feature lamp circuit alternates between the “A” and “B” phases.

Hopefully, you found this educational. If you liked it or learned something, please share it!

‘Till next time, Frank

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