short answer
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check the end-of-stroke switch on the line unit step-up plunger arm. The switch should open when the plunger sucks into the step-up coil. The switch should have yellow wire 30 and blue/orange wire 27-1 on the two blades.
tmi
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the line unit step-up is a solenoid coil with a plunger. Those coils should never be held powered unless you have a bag of marshmallows handy and nothing else to roast them with :-)
the line unit wipers actually move when the step-up coil loses power, so if your coil is getting stuck on, it won't look like anything happened until the smoke starts. The obnoxious buzzing is a hint a solenoid is stuck on.
anyway, your problem is likely the line unit relay is stuck on. If you don't have a schematic, look for one in
http://bingo.cdyn.com/slots/ . If your full model number off the plate isn't just 873, look at something like the 873-C schem as it's likely the same for your problem and post what your full model number is on the plate below the handle.
the sequence is:
1] deposit coin
2] coin relay powers
3] handle release relay trips
4] line step-up relay powers
5] a switch on the line step-up relay closes and powers the line step-up solenoid coil
6] the end of stroke switch opens and the line step-up relay loses power
7] the line unit step-up coil loses power, and as the plunger comes out of the coil the wipers are pulled around one rivet on the contact plate.
don't confuse the "line step-up (S.U.) relay" with the "line unit step-up coil". Two different things. The relay controls the solenoid coil.
you probably made it to [5] above, and [6] isn't working.