Sometimes software bugs can appear to be a hardware failure.
I was fixing a solid state 1978 Bally Black Jack and when it was
finally all back together I started to test it. The upper left ball gate
was broken such that the ball came from the shooter, around the top, through
the spinner and straight down to the flippers. As soon as the ball hit the
spinner the game tilted. This happened every time. I was convinced this was
a switch matrix or wiring problem. But in checking all the diodes and
wiring everything was normal. It turns out that this is a bug in the
software. Normally the ball bounces back and forth between the ball gates
upon launching from the plunger and then settles down through one of the top
lanes. If any other switch is activated before the spinner then everything
is fine. If the spinner is the first switch hit when the ball enters the
playfield then it produces an immediate tilt!
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