Vectrex Shielded Wire Buzz Reduction

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Parts List
About 40cm / 16" of coaxial cable
Small length of 12-14ga wire

Whenever I find TV coax cables with Quick-F connectors, I like to chop 'em up for these mods. Any reasonably flexible coax cable will do. I've never tried 50ohm impedance cable, but I suspect it will work just fine.


Goals
Minimize ground loops
Shield the audio signal from interference


This modification has been floating around the internet for some time. The popular version eliminates ground loops by ensuring a single, stout ground source. I like to change it up a bit by adding and tying grounds together instead of removing them. In all my years of troubleshooting ground issues in automotive 12v systems, never has "remove all grounds except one" been a viable option. Sometimes rust will do that for you and the result is that the one remaining cable goes up in flames, hence my tendency to supplement and tie grounds together.


Remove original audio cable & audio cable connectors from power (J107) & CPU (J203) boards

Oh sure, that sounds easy enough, but you'll have to do quite a bit of disassembling to get to the connectors. This is why I tend to give a Vectrex the business all at once, doing cap kits, cleaning, etc.

Tie J204 ground to CPU ground plane

J204 ground pin jumpered to adjacent ground. Wire positioning shown prevents pinching in the CPU PCB frame.

The ground pin of J204 doesn't have a very solid connection to the perimeter ground of the CPU PCB. A small jumper wire improves this.

By design, the Vectrex has split grounds (digital, analog, bypass) for noise control. In practice it doesn't seem to help much. This modification just adds some redundancy from the power board ground and the perimeter ground of the logic board.


Connect shielded cable to CPU board

Center lead soldered to audio signal, shield soldered to adjacent ground

Once the audio connector is removed, solder the center conductor to the audio signal pad at J203, and connect the shielding to the adjacent grounding trace. Remove the coating on the ground trace with a scratch pen, or even a bit of medium grit sandpaper.

Reinstall CPU PCB

Because coax cable isn't very compliant, I prefer to make the connections to the power board only after the CPU PCB assembly is reinstalled into the front casing. If you are using very thin coax, or just don't care, you can skip this step :)

Connect shielded cable to Power board

Center lead soldered to audio signal, shield soldered to adjacent ground

Solder the center conductor to the original audio lead at J107. Solder the shield to the large nearby ground trace.

Done!