So recently, sometimes my knocker goes off for a free game but then seems to stay energized, makes a terrible electrical chattering noise and gets really hot. When I turn the game off and on again it starts right back up making the same noise. It stops once I push the start button. Then it plays a few games fine with the knocker firing as it should but then randomly starts doing the same thing again after the knocker fires a few times.

Where do I start with troubleshooting this?

Here’s a link to the noise it’s making.

Thanks.

Thanks to everyone for the input! I'll be looking into it tomorrow and report back.

So after finally finding some time to dig into this problem I've found that the credit reel is starting to bind when it reaches 18 credits. It binds at red circle and doesn't allow the knocker leaf switch (green circle) to separate all the way which causes the knocker to be constantly energized. It does this every time the credit wheel tries to click over to 19. If I try to manually advance the credit reel the metal wire (blue arrow) gets pulled out of its hole. For the time being I have turned off extra credits for high scores and it hasn't happened since.

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Don_C nailed it. Although I haven’t disassembled and cleaned it yet, just unwrapping the return spring 3/4 and hooking it in the correct position fixed the problem. It now goes up to 25 credits without any binding. Thanks everyone for the help!