It was a pretty noisy guitar.
There was a nasty hum that didn’t quieten down when I touched the strings. This is generally a good indicator that there’s grounding problem.
The quick test: Touching the metal of the jack on the (plugged in) cable quietened things. This means that somewhere inside the guitar, we’ve got a disconnected ground wire—if everything were working properly, there would be a signal path from the strings to the ground of the output jack.
A quick primer/reminder: The ground wires in a guitar help to ‘shield’ it from unwanted interference from the environment. The exposed metal parts of a guitar or bass are generally all wired to a common point. This includes the strings. Inside the guitar, there’s usually a wire from the bridge or tailpiece that runs to ground.
When you touch the strings (which you would normally do when you’re playing), you ground yourself through the guitar’s wiring and so you cease being a big meat-antenna, picking up interference, which is then picked up by your pickups.
Because this guitar quietened when I touched the output jack but not the strings, I knew that the path to ground had been interrupted somewhere.
Fixing the Bridge Ground
A quick look inside revealed what seemed to be the problem (more in a minute). I could see a wire from the back of a pot (common ground point) disappearing into a hole that pointed towards the guitar tailpiece stud. A gentle tug on this gave no resistance, however, and the wire pulled right out.
Problem found. Have to run a new ground wire to that stud.
Removing the stud bushing from the body isn't too much hassle (but be careful if you ever have to do this yourself). I fished in a new wire, stripped it back so it contacted the metal of the stud-bushing and reinserted the bushing. Then, I soldered the other end to the ground point on the back of the pot.
A quick check with my continuity tester showed that all was well—good connection between the bushing and ground—so I strung up and plugged in and…
Exactly the same problem.
What the heck? I was all set to treat myself to a nice cup of tea. What’s going on?
I pulled out my multi meter again and, sure enough there was no ‘continuity’ between the strings and ground (that ‘I’ on the meter indicates there is—for all intents and purposes—Infinite resistance).
NOTE: In case you’d like some basics on conductivity and resistance.
Fixing the Ground ’Continuity’ Problem
I had a suspicion of the issue so I decided to work my way back with the meter.
The internals all looked fine. Perfect continuity between the output jack’s ground and the pot with the bridge ground wire. From outside, I could just probe the bushing and that was fine too.
Then the trail went dead. Suspicions growing.
This particular guitar has a black powder-coating finish on the metal hardware. I have a feeling that this coating is preventing a good ‘connection’ between the metal parts. Check on the conductivity/resistance article for more on this. Without a good connection, there is no ‘signal path’ from the strings to ground.
Remove the strings again. Time for some ‘scraping’.
What I’m trying to do is to get a sound, ‘metal-to-metal’ connection for the bushing, stud, tailpiece and strings. Working in the parts that won’t show, I remove some of the powder coating. I use sandpaper or just scrape if it’s easier.
I want to see some shiny metal on the threads of the stud and the internal ‘post’ that holds the tailpiece. Same on the slot in the tailpiece (you can see the difference in the photos below)
Lastly, and most fiddly, I scraped a little coating off the holes where the string ball-ends contact the tailpiece.
String things up again and check with the meter… 0.00Ω. That means there’s a clear path with no resistance from strings to ground.
Plugging in reveals a much quieter guitar. Splendid.