Hemiroid:
Welcome to the forum and congratulations on your Bally 873. WHEN working they are lots of fun to play. Reportedly they have the most frequent pays of any of the old bally EM games. I have 2 and they are the favorites of my guests in my game room. If you are going to get a few of these machines, and you will, (obviously already bitten by the EM bug)then I would suggest you get a set of cables so that you can operate the hopper, reel mech, and even multiplier unit out of the cabinet. There is a member here, I believe his name is Jim, who makes an EXCELLENT set of cables.
I have a folding work horse that I set the components on when I have problems. The cables are about 6 feet long so you can see the whole component operate in real time.
I have never experienced a runaway hopper that was not related to the timer, which it seems that you have already checked so I dont have any specifics on your problem. Please keep up posted.
My opinion is free and worth every penny.
Ra