Sustainiac install

[lespaul1964]

I have an electrical eng. helping me install a sustainiac on a guitar. I have a problem, when I engage the sustainiac through the push pull volume kob, sound still comes out when I bring the volume to 0, it wont cut off completely. This happens only with the sustainiac engaged, when it is off it shuts off completely at 0 volume. Any ideas?

[WRGKMC]

Sounds like something is up with the pot. The pot shunts the signal to ground when its turned down. For some reason when the pot is pulled out, it fails to do this.

If you tested the pot with an ohm meter and know its OK, then it may be you have the signal and pickup wire reversed. There are two ways a pot can be wired to attenuate. The pickup wire can go to a center leg and hot wire to an outside leg, or the hot wire to a center leg and the pickup wire to the outside leg (the other outside leg and pot can are grounded)

These two wiring schemes have different tonal effects on how the pot operates. The modern pot wiring with the hot wire going to the center leg is used in most guitars. In two pup guitars like gibsons when both pups are selected and you turn one volume pot down, it will turn off both pickups when you turn all the way down.

The vintage independent pot wiring style has the pickup wire connected to the center leg. This allows you to to turn one of two pickups completely off without affecting the other.

In the first case, you shunt the amps input off, in the second you shunt the pickups coil. In theory the effect should be the same with a single pickup to the amp, but in the real world, the tonal and noise levels are quite different. 

What can happen with the vintage wiring is the signal from the pickup can leak across the pot and produce sound in the amp. When amps were run clean this leakage didn't amount to much. They switched to the modern wiring for two reasons. One allowed a single pot to turn off both pickups so you weren't fumbling around turning two pots off, the second it was quieter. It didn't matter how much noise of bleedover the guitar had, if the amp was shunted, that noise or RF emission from the pickup couldn't be amplified.

you can also check in the diagrams how your tone pot is wired, The cap may be wired, vintage, modern or a bleedover cap and it too may be pickuping up bleedover from the pickup. Be sure your battery isn't being applied to the pickup too. If the hot voltage on the sustainer is contacting the signal wires, you may have signal bleeding through the battery.

Be sure you don't have any cold solder joints too. Pot cans can be oxidized and often require some sanding before applying solder. It may be you didn't even ground the metal pot can seeing its a push pull pot. The grounded pot can shields any RF emissions the signal wires generate to avoid bleedover across the pot 

The other possibility is the pots been damaged by overheating the contacts soldering. Budget pots are notorious for this. The cheap materials melt easily and the slider may not read zero ohms when turned to an extreme allowing some signal to get through. You can try cleaning the pot with some lubricating contact cleaner. If that doesn't work, but a better pot. Most push pull pots sold today are total crap. You'd be better off using a good quality CTS pot and using an independent switch for the sustainer. If you don't want to drill a hole for the switch, sacrifice a tone pot or install a rotary tone pot that has stepped tone settings and use one position for the sustainer. Using an external tone pot or EQ can always be used on a floor board so its loss is not a huge problem to circumvent.  

http://www.blueskillet.com/Gibson_Mods.htm

[lespaul1964]

Wow, thanks for the detailed response.