Midway Cocktail Hum Bars
by Bob Roberts
Here's a Q that I have answered a k-thousand times over the past 12 years here & 27 years overall & it still keeps coming back so often that I am amazed that it is not something known worldwide yet. I have two people awaiting this answer currently, so I'm going to just throw up this quick note even though I am in the middle of another page that is probably more important.
I have a Super Pac CT, Ms Pac CT, Tron CT or other Midway CT that has a terrible hum bar in it... weaves, rolls, is squiggly, insert your opinion of the symptom... and I cannot find the reason for it. I have noticed that when the cab is open the hum bar goes away, but as soon as I close the cab it is there big time!
What all these Midway CTs have in common is the power chassis in the bottom of the cab known as the "suitcase". Here's a rough pic of it below:
The cause of your problem is induced by the close proximity of the monitor to the "suitcase". If you see what people refer to as the "handles"... red dots on them in the pic... then it is definitely your problem. The red dots are located where there are pre-drilled holes in the "handles" and they are used to fasten a sheet metal cover that will shield the induction between the monitor & "suitcase".
You need to add a flat metal shield as outlined in blue above. A good sheet metal shield can usually be found at your local sheet metal shop as scrap most times & if they need to cut it with a brake they might charge you a buck to do it... at least that is what I found locally... and after I bought a brake I use to swing by several local sheet metal shops for what they call scraps free for the taking, but I always found some pretty nice sized pieces. I've been told that roofers are another good source of shielding material.