Quoted from aobrien5:
On a System 1, could the interconnect harness in any way be responsible for blowing a driver board transistor and locking on a coil?
Yes, when the ground potential between the two boards gets too far out of alignment.
Bad connection from MPU to Driver board transistor causes one of the drive transistors to turn on a little, but not enough to actually move coil plunger. Therefore, player/owner may not notice anything is happening,..... yet.....
Coil begins to heat up.
Windings begin to burn enamel off wires and windings begin to short out.
Shorted windings on coil cause transistor to overheat and short out.
Transistor, since it is not fully turned on, creates a ton of internal heat.
Transistor can't take the heat, and shorts out.
Shorted transistor burns up coil.
Shorted coil and transistor blow fuse.
This is why the ground mods are so important on any Gottlieb System 1/80 game. All devices need to be at the same ground potential to insure off means OFF. Bad connections either through Interconnect harness, MPU power input connector, Power Regulator board connectors, or even the individual grounds to the various clusters of drive transistors can mean sure disaster. By common-izing all these grounds together, it minimizes the potential for this kind of failure.