So I am getting pretty noticeable speaker hum on a Williams Comet. I have recapped the power supply and verified that all screws are tight and present. Neither seemed to effect any change. What should I try next? Might it be one of the MPU caps?

Sorry to step on your thread but am seeing the same thing on mine. Just got it home and set up this weekend. I notice when sitting in attract mode it makes some noise. I could make it louder with the volume knob or quieter by fiddling with it.
I'm going to start poking about this evening and if I get anywhere I'll post back.
If this post is out of line, my apologies I'll delete it.

The more the merrier

So you are saying sound board, but there is only a speech board. Is that what you mean?

This describes my hum exactly. Every screw everywhere on mine is a counted for.

The volume is inside the coin door. The pot on the speech board controls balance between speech and music.

I had considered hitting both these pots with contact cleaner, but haven't yet. I notice there are caps on the speach board...

Haven't had time to apply any of the suggestions, but I will post once I have a chance to focus on this issue more.

I did have a sec to power on mine and check the volume pot. The hum does go up and down with the volume. No static when I pan it though. I know what a bad pot sounds like and I am not hearing that here.

Come on! Hit Me!

I asked BigB about this since he has TWO comets! Quoting from his response:

"
I have not had this particular problem on either of my "Comet's", but I have had it on other pins and the problems don't seem to be in the game itself. They are usually in the power-cord. It will pick up ground noise very easily. I have had a lot of pins that would seem to develop a hum just by moving the pin from location to location. It would drive me nuts. I would buy a pin from someone and get it home, hummmmmm. WTF!!! WTF!!! Didn't do that where I bouht it.
That or I would move a pin in my house and it would start humming.
Usually what works is adding some sort of "ground-loop isolator" to the powercord. Hope that helps..... BigB
"

Not really a useful update, but I just lost all sound on my Comet EXCEPT THE HUM. Gonna go through and try to find what broke. (based on this http://home.scarlet.be/~fb054529/will9/ewill9.htm )

Does yours hum Shovel?

Where do I get a test eprom? I was just pricing burners assuming I had to make my own.

OK now what?

IMG_20141218_145137_1.jpg

I mean, is the answer just replace all the sound circuit caps? Is there a list of that? I guess I can stare at the schematic. I was assuming it could be *any* component.I know you are not a fan of shotguns vid.

I haven't had time to troubleshoot the no sound at all, but twice now the sound has slowly drifted back in after about a half hour of playing. As if something was slowly warming up (and expanding?)

I now own an Oscilloscope. Trying to find a palatable solution for bench power.

IMG_20141218_145137_1.jpg

I didn't realize some light hacking is required to get an ATX pc power supply working, as I had intended to go that route. I just ordered one of these to avoid having to mess with that:

6ce9364c56fc52d0e1a57feefae74d66.image.530x397.jpg

http://dangerousprototypes.com/docs/ATX_Breakout_Board

Inkochnito, I have recapped the power supply previous to posting are you saying the fade in could be originating all the way from the power board? I would think that it would not work at all in that case.

Ok so I finally got back to this. As far as I can tell these are the only caps in the sound circuit:

#24 Cap., Elect. Radial, 470 MF. 16V +50-10%
#25 Capacitor, Axial, .1 MFD. 50V +/-20% x1
#26 Capacitor, Axial, .1 MFD. 50V +/-20% x2
#28 Capacitor, Axial, 1K PFD. 50V +/-20%
#29 Capacitor, Axial, .1 MFD. 50V +/-20% x3
#30 Capacitor, Axial, .01 MFD. 50v +80V -20%
#31 Capacitor, Axial, 47 PFD. 50v +/-20%
#32 Cap., Elect., Axl., Low L. 10 MFD. 20V +/-20%
#53 Cap., Elect., Radial, 100 MFD. 25V +50-10%

I replaced 24, 32, and 53 as they were the only electrolytic ones and that did not solve the problem. There are only 4 other elec. caps on the mpu (2, 5, 7, and 9) that I can spot, and it looks like they are all in the logic circuit.

So to recap: still have no sound at boot. The Sound slowly comes back starting wit the speech and then the music. Still have speaker hum in time with the lights, even at start up.

I guess my next plan is to start replacing the ceramic caps?

How do I use an Oscilloscope to pinpoint the hum?

After all this it seems like the issue was missing -12v coming out of BR1 on the power supply. Replaced it, sound is back, hum is gone. I think recaping sound circuit may have helped also.

When I said BR1, I meant that I changed out the bridge rectifier at position BR1 by desoldering it and replacing it.

When I discussed this issue with The local repair guy (Steve Kuntz, Pinball pirate) He suggested getting an oscilloscope, starting at the speaker, detecting the hum and work back component by component till you figure out where it is coming from. Something is grounding the wrong way.

Standard advice about check if all the screws are present.