Quoted from Turtle:Also, the game should work without the diode for awhile (this assume the coil driver transistor is still functioning). But I don't leave it off for a long period of time.
It is NOT a good idea to ever operate a DC coil without a diode when it is energized by a solid state driver (Transistor, Darlington Transistor, Mosfet, Etc.) It only takes one activation and then the following Back EMF to destroy a semiconductor. If you get more than a few kicks without failure, go play the lottery!
The above is not so true with earlier Gottlieb SS games as the sling shots, kicking targets, and Pop Bumpers were self controlled. ie, they used a Tungsten Leaf Blade contact to actually feed the power to the coil. No semiconductor was involved for powering the coil. The secondary leaf switch at end of travel would produce the scoring function. But not having a diode on this type of coil drive will still cause all sorts of weird issues from the high voltage spikes it will send down the wire harnesses.
As others (and I) have stated, it is not the 2 lugs of the coil that mater, but rather which way the diode is facing when connecting wires when replacing the coil. Often, people transfer the wires based on position and do not pay attention to the stripe on the diode. Sometimes, they put the coil in upside down (which flips the lugs). Sometimes, a different manufacturer puts the diode in a different direction. Sometimes, someone has made the same mistake, and replaced the diode without moving the wires on the lugs. Each, can result in a diode facing in the opposite direction of adjacent coils which can be misleading. If you don't know which wire goes where, unsolder both, leave them disconnected, and then power up the machine and find out which wire of the two has power (+28VDC, +50VDC, +73VDC etc.) THAT is the wire that goes to the stripe.