I've had this pinball machine a couple of weeks. Starting a couple days ago, the 100 (or optionally the 100/1,000 or 1,000) bell ringer solenoid started sticking. It is not activated, if I tap the plunger it will drop back down. I noticed the 100 relay was stuck on, the paper label was black and it was really hot. I turned it off, let it cool down, and cleaned all the switches on the relay. When I started it back up, I fired it manually a dozen times and it didn't stay on, however after firing one or two times, the bell solenoid tends stick on.

1. Can I clean it or does it need to be replaced? I don't see a spring so I'm assuming returns by gravity.

I'm new to pinball machines and still reading through the guides and https://pinside.com/pinball/forum/topic/em-schematic-fully-described-from-beginning-to-end-bally-bon-voyage recommended by rolf martin.
I have a digital multimeter and can solder.

Thanks chas10e and @hd_fatboy,
I cleaned the solenoid plunger with alcohol and sanded with 700 grit paper (that's what I had). As you suspected it had been oiled and was gummy. My only question, I didn't notice the washers placement.There were 2 star washers and 2 flat one larger than the other. I put them back in this order on the stud, star, larger flat, bell, small flat, star, nut. It rings now but still seems like it doesn't have time to recover (plunger dropping) from two very quick rings. It may have always done that, but now I notice it.
Thanks again for the quick responses!
Paul

Thanks! chas10e I'll switch mine.