Is it starting to burn, or could this have happened a while ago?

Scorching like this could be caused by:
- dirty contacts that increase resistance and generate heat
- a high current circuit (e.g. to a pop bumper or other solenoid) that cuts off power without a bleeding resistor or some other place for the residual current to go
- having a contact land between two pads (as your BS relay) and barely making contact with either one of them which could increase resistance and generate heat.

If your contacts are landing in the center of the pads the scorching may no longer be an issue. Without a schematic it's hard to be more specific.

/Mark

Steve Young of The Pinball Resource contacted me offline about this problem and offered me some clarity:

"....the phenomenon is called carbon tracking and is caused by repeated tracing and particulates building up between lands...eventually will conduct and burn....must be removed and can be filled with epoxy or the disc replaced. Disc is available."

"Seen alot on this disc because wiper can stop between patterns and create a bit of an arc and deposit debris. Seen also on AS discs as patters are close together and maintenance difficult and ignored. Common on Williams ball count discs that are foil (not rivet) type. Common on Chicago Coin everywhere where they used fiberglass board material....believe a bit more abrasive and more prone to leaving debris."

"Keep disc clean and properly lubricated. Where adjustments are available, make them to keep stopping position centered on the pattern center."

Thanks Steve. I learned something new.

Did you just clean the board, or the whole assembly? I'm curious where the carbon is coming from if the problem returned.

I haven't repaired a board myself that way nor do I know how super clue holds up compared to epoxy. Give it a try and maybe order a spare just in case.