I uncovered a fix that eliminated the annoying hum or buzz in my Jokerz, and I wanted to share this on Pinside. This solution worked on my machine, which doesn't mean it will on every other one. But it offers clues as to what causes it and how to understand it. I have read everything I could find on this phenomenon, not just for Jokerz but for other Williams System 11 titles and could find no conclusive answer on how to address it. The culprit, EMI, or electromagnetic interference, only affects sound (through speakers), but not any other playing or electronic functions. By definition, EMI is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The challenge is to find the source so you can address it. It requires a disciplined process.
So I created a list of every plausible idea, suggestion, trick, or hint I could uncover on Pinside searches. This gave me a hit list. I found that if one claim worked on one machine, it didn't necessarily work on another. So why not try them all? I enjoy fixing pins about as much as I do playing them. The solution was out there. I was determined to finding it.
I got my Jokerz in the beginning of March 2020, days before the Coronavirus chaos hit the US. The timing couldn't have been better for me. The machine was in average player condition, some minor hacks, new Rottendog power supply, and an NVRAM installed. Most important, it worked. Not 100%, but I was OK with that. It needed me. Before I could power it up, I had to first replace the line cord crushed in shipment. It needed considerable cosmetic care and mechanical repair work, but that would come later. From the first game I played, I noticed the hum. Not too bad. But over time, it got worse and was intermittent. On a scale of 0 to 10 (zero being none, 10 being intolerable) the intensity would vary between 3 and 9. On rare occasions, when I would do work in the back box it would change. Sometimes better, sometimes worse. During play, the flashers and certain light features would amplify the noise. It was near impossible to pinpoint what was causing it. After a few months of playing and restoring, I was determined to kill the hum.
Each time I researched this issue, I created a log. From that, I prioritized a series of steps starting with the easiest first. Plan was, complete each step, play a test game. Here's how it went:
1. Replace the volume potentiometer. One advisor said don't replace it until you try turning it back and forth many, many times and/or clean it with contact cleaner. Over time when not used, carbon crystals form on the wiping surfaces of these devices and can cause static interference. I cleaned and turned it. No effect on the hum. Five minutes. The new pot, which I had already ordered, is a now a spare in my inventory.
2. Remove the four chips on the audio board, clean the pins and reseat them. I used a soft brass bristle brush. Ten minutes. No effect.
3. Rebuild the EMI filter in the power inlet/fuse box on the lower inside cabinet. I learned EMI filters don't last forever. This one was 22 years old. Inside the enclosure is a Corcom 5KV1 5amp EMI filter with a metal oxide varistor (MOV) across its two line terminals. The fuse holder and power line outlet were left in place. I began a new parts list with these two items. Why not? Easy to replace, less than $25. I had the highest hopes for this step.
4. Install a 68 ohm resistor between the two upper speakers. Not sure which post this came from, but I ordered the 70 cent resistor.
5. Install ferrite core snap plugs on cables. This is like the wild west. I had some notions of where to mount these things, but it would be purely experimental based on logic and assumptions relative to the effects of EMI from cables. Think how many linear feet of cable there are in these machines. This would be a needle in a haystack. I added a batch of four different sizes to my parts list based on the various sizes of the machine's bundles. Photo shows what they look like. Actually quite affordable. They're used everywhere: computer power cords and many other electronic equipment. Their function is to limit EMI in sensitive devices. And they work in both directions. Pretty cool. They snap and unsnap easily, which greatly simplifies experimenting on a variety of different cable locations.
6. "WMS Service Bulletin (PDF), "subject: Jokerz Audio Hum". I couldn't find any posts or comments where this factory recommended alteration actually reduced the hum. It had not been performed on my machine, but I added it to the list anyway. I put the parts on the order. This involved some board work, which I can handle.
7. Replace the line of capacitors C60-C67 , as well as the C30 capacitor on the CPU. This involved removing the CPU from the machine. Whoa, I can do that, but that's an involved project. I added the components to the parts list and moved it to the last step on the to-do list.
8. Replace lower bridge rectifier BR1. This was recommended in a few posts, but a trusted advisor said he'd never seen any case where the replacement of a bridge rectifier had any effect on EMI. It was already on order, so I moved this priority to the bottom.
9. Just for good measure, I ordered 2 new ribbon cables in the parts order: the 26-pin from the display board to the CPU, and the 20-pin from the CPU to the audio board. This had no effect on my issue, but it should be done anyway. I installed them. Nice and snug.
The Corcom EMI filter arrived first, so I rebuilt the enclosure with the new parts. Photo shows the original parts removed. Zero effect on the hum. Hopes were dashed.
Next to arrive were the ferrite core snap plugs. It gave me a chance to try them out on a few cables. I wish I had ordered these bad boys first. I figured if the problem was speaker hum, I'd start there. I targeted the largest cable bundle closest to the first speaker in the back box, which I thought was a logical place to start. It always bothered me that the speaker wires were just laying loosely in the area and not properly routed. Photo shows. There appeared to be some hacks for the ground wires, too. So I worked those speaker wires into the large cable bundle in same ferrite core. One technical paper I found recommended looping the wires through the ferrite core more than once for better effect, so I did that to the speaker wires, as well. The picture shows what the assembly looks like, before and after.
Then I powered up the machine and I started a game.
The hum is gone. Zero. But I remain skeptical. Every time I fire up the machine and play a game, I expect the buzz to come back with a vengeance, but so far it hasn't. About 20 games so far. Pure magic. My hope is that this information helps someone else with audio and speaker hum issues. But most of all, I hope it keeps working! (But if it doesn't, I still have 4 more steps to try and the parts to go with it!)