There are only a few reasons that AC relays in a pingame will buzz loudly, but it can be somewhat tedious to get them to quiet down.

Above all else, make sure that the coil in the relay is the one that is specified by the schematic. If it isn't, you'll want to get the correct coil and install it before trying anything else. If the relay has the correct coil and it's still buzzing loudly, there are some things you can do to try to reduce the buzzing sound.

First, if there is wear on the face of the coil core and/or the inside face of the armature, where these two surfaces come into contact while the relay is energized, that will cause a loud buzz. You can clean and dress down these surfaces to try to make the relay more quiet. Replacing the existing coil with a new one may also improve things.

Second, if the switch blades of the relay and/or the return spring of the relay are providing too much back-tension on the armature, it will cause a loud buzz. You may be able to adjust the blade tension and/or the spring tension to reduce the amount of pressure that is trying to pull the armature away from the coil. However, you need to be careful when you do this, as you don't want to lower the tension so much that the relay fails to actuate all of the switch contacts properly. A switch blade that is binding in the actuator will also cause problems, so make sure all the switch blades are aligned nicely in the actuator slots, and that there is no binding.

Third, if the frame of the relay is slightly out of alignment, and/or the coil is in the wrong position in the frame, it can cause a loud buzz. You can usually bend the frame slightly at the armature pivot to try to straighten things up and reduce the buzz. If the coil seems too far back, you can try using a thin brass shim washer between the back of the coil and the relay frame to move the coil forward. However, these adjustments are fairly tricky to do, and it's possible to make things worse or even make the relay stop working if you deform the relay frame too much. So, be aware of the level of risk if you attempt this type of adjustment.

Doing one (or some combination) of the above three things, I have had very good success quieting down the lock relay on Williams EM games. Sometimes, it requires a lot of futzing around (and much patience), but it almost always works.

If you want to bypass the lock relay, you can usually put a small wedge made of cardboard, plastic, or wood in the relay (between the armature stop and the armature) to keep the armature pressed up against the coil (that is, in the actuated position) all the time. Then you can disconnect one of the lock coil wires. Be sure to insulate and tie off the disconnected coil wire so that it doesn't short out against anything.

Be aware that if you bypass the lock relay, the game will no longer go into game over mode when you power it up. So, if you turn off the machine in the middle of a game, and come back a week later and turn it back on, the game state will be exactly where you left it.

- TimMe

Yep. Over time, the inside face of the armature plate develops a depression in the metal that exactly matches the outline of the protruding coil core post. When you change to a new coil, that depression doesn't line up exactly with the outline of the new coil post, causing a buzz. You need to smooth down the inside face of the armature plate so that it is as flat as possible. Sometimes the frame alignment also needs to be adjusted, but as I said in post #3, that's a bit tricky to do, so I think it's better to avoid messing with that if possible.

The buzzing noise is being generated by the armature plate vibrating at 60 Hz against the coil core post. If the vibration is severe, you'll hear a buzz. Basically, there are only a few alignment positions of the armature face while it is pressed up against the energized coil core post that are quiet. And unfortunately, there are many more positions that are noisy. So to stop an AC relay from buzzing, you're essentially trying the change the armature plate tension, and the alignment and positioning of that contact point, to quiet things down. The reason this is often a tedious exercise is that it can be difficult to find a tension level and position that are both operational and not buzzing.

- TimMe