how to repair guitar wiring:
output jack, volume & tone controls
Repairing a Loose or Damaged Output Jack
Problems often occur with the Output Jack. At the base of the jack plate is a hex nut which can sometimes loosen off. Attempting to tighten the nut while the jack plate is still in place can, with constant turning and tightening, cause the wires underneath the plate to twist around on themselves and break. The best option is to remove the jack plate by undoing the two Phillip's head screws at either end of the plate and lift the jack plate out gently. Hold the wiring and the base of the jack input socket (underneath) in place, while you insert a 12mm or half inch nut wrench / socket over the hex nut and then tighten it down. This way you'll avoid a twist or break in the wires. If there is a break in one of the wires, you'll need to get out your soldering iron. Cut the old exposed ends of the wire/s, and with some wire strippers, strip the wire back about quarter of an inch. Spot some fresh solder on to the bare wire, do the same other corresponding contact point, then re-solder the wire cleanly to the output jack contact lug and reinstall the repaired plate.
Note: It's good to remember not to yank the jack upwards when removing the lead from your instrument, as this can sometimes pop the hex nut over and cross thread it. If that happens, you'll have difficulty removing the nut at all. Don't try to fix this problem while the jack plate is still installed. Again, the jack plate will have to be removed to unscrew the jack nut.
Noisy or Scratchy Volume & Tone Controls
Dirt and dust contaminates can get into your volume and tone pots (potentiometers), and switches, causing them to sound scratchy. To address this problem you will need to access the underside of the control knobs. On some guitars, e.g. Fender Strats, this will require you to remove the strings, and then remove the screws and pick guard. Remove only the screws around the outer edges of the pick guard, leaving intact all other screws holding in place the pickups and toggle switch. Also leave the volume and tone knobs in place. Then gently pull the guard downward toward the bridge just a bit so it clears the neck and it will lift away. Hold the guard and work out which way you're going to turn it and then gently and slowly rotate the guard (the earth wire should still be attached) just enough for you to turn it over. Other guitars will have an access plate on the back side of the guitar. Either way, once you've accessed the underside, you will be able to see the tone pots, the selector switch, the pickups, wiring configuration, etc.
With a can of electronic contact cleaner (available from any good electronics outlet), locate the small holes in the side or back of the potentiometers, and give them a good spray, control and direct the spray through the small tube supplied with the can of spray into one of the holes on the potentiometer. If no top holes or side holes are apparent, place the end of the spray tube right above the leads where there's a large opening, then give it a good spray and place your hand underneath on the control knob giving each one a good turn all the way from left to right and then all the way back the other way, rotating them back and forward. This will allow the contact spray to spread around the inside of the potentiometer and give it a good clean on the contact points. You can also apply some spray to the metal plates on either side of the back of the pickup selector switch, then move the toggle on the front, back and forward to spread the fluid through the selection points on the p/u selector switch. Once done, carefully flip the pick guard back over (if on a strat) and gently re-position it, making sure you don't pinch any of the wiring between the guard and the body. On rear mount installations, you will simply put the back plate and screws back in place.
More to come on guitar wiring repairs, soldering and electronics soon ;)