Glenelg, MD
Quoted from TimMe:The back EMF spike that is generated when a coil turns off and the field collapses around the winding can cause enough magnetic induction in nearby circuits to make LEDs flicker briefly on an EM. You're right that it doesn't happen on every game. In my experience, it most often happens on games that contain at least one 6-volt relay coil, which Gottlieb used on several 70s EMs for bonus detection.
That coil can act like a little transformer secondary winding, transferring EMF spikes directly into one or more lamp circuits. This also happens when incandescent lamps are in the circuit, of course, but the filaments of those lamps respond relatively slowly and require so much current that they don't flicker from the spikes. LEDs respond super fast, and will emit light with very small current, so you can see them flicker in some EMs during game play.
- TimMe
Perfectly stated. I bought a few comet LEDs to use under an Old Chicago CPR playfield that unfortunately sported washed out color inserts (special - red and extra ball - orange). The ghosting on those LEDs was VERY NOTICEABLE. Ordered some non-ghosting and those did the trick.
Added over 3 years ago:Just an update to my Old Chicago feature lamps... what's interesting is that an earlier poster stated he placed a green LED underneath the double bonus insert on his OC... I did the same by putting a green non-ghosting LED in that spot on mine. I also placed non-ghosting colored LEDs underneath the special and extra ball inserts. Although the ghosting was reduced, the very slight flickering as so well explained by TimMe was DEFINITELY distracting during gameplay, so much so that I took them out and replaced with painted 44s. Moral of the story is you gotta try non-ghosting LEDs in your particular game if that's the look you want. It might be fine, might not.
