Hello sir, similar problem here.
I have a slight hum/buzz while sitting idle
it does play the simpsons and most sound effects - I do not get the homer ice cream sound effects

doing a sound test - I get sound (static anyway not sure if this is supposed to be for test) from L, Center, R, but when it goes to music/level 1-4 nothing plays

any advice?

check the eproms and their sockets and your yamaha chips ym2151 and ym 3012. they produce the music. Could be as simple as chip creep

speaker2.PNGspeaker.PNGactually I'm a dum dum - what made me think the sounds were busted is I have a hum.
During the testing I didn't know I had to hit the start button, sounds test good and all music plays sorry

When the mystery bonuses flash, I get a hum, constant hum as they tick through (during the attract phase)
pulling up the play field, the speaker on the bottom is also making a constant loud/deep hum

I notice on my speaker, I have 4 wires connecting to it, and one goes to a plug (not connected) and another is electrical taped. lol

There looks to be a large transistor or solenoid? what is this? I do not have this and the manual doesn't link it anywhere

that is your knocker. It hits when you get a free game etc. If you have bad hum make sure all the screws are secured nice and tight on all the circuit boards. Very common to have them back out or go missing. Assuming all are good and tight...check wiring and connectors followed by a recap of the sound board .

Oh yea if all the screws are tight on the boards and you have hum. With the power off ........ back the screws off on the sound board by 1 or 2 turns and retighten them all back down . I do not know why...but sometimes it works. Barring that caps are common to fail as they are all getting quite old and way past their useful life.

myspeaker.JPGand here is mine what the heck

i'll try that screw trick - so I don't need a knocker. I guess what is the point of it - free game? I have got free games w/o it.

i am missing a few screws

it is quiet during attract mode, which is strange, so probably something bad on the board

you only need the wires soldered to the speaker. That extra plug goes no where and should be cut off. The other wires laying there go to the knocker coil. It knocks when you win a free game or win a special etc... Not critical to the game.
If you are missing screws than there is your problem. Replace them and play away. They are #8 by 1/4 screws. Make sure you check them all there is a lot of them.

wow. that is crazy. thank you so much sir - i'll try that, I can't belive just a missing scrw would make the top speakers and bottom speaker hum
you have been so helpful, im' new to the forums but can I vote you up or add rank or anything?

#8 any type of screw? don't they have to be a certain material?

No worries...glad to help anytime. Have a game on your machine for me! #8 x 32 x1/4 inch long machine screws. Regular steel is fine.
All your boards are grounded through the common ground plane which is the backbox metal plate....Missing or loose screws...problems arise or at the least ground hum.
Get your screws and install and