Multi Meter is your friend

I've found the multi-meter to be more and more useful in my guitar repairs.

The continuity setting is probably the most used. It's invaluable in tracing wiring paths. Yesterday, I installed three mini switches in a guitar with three pickups. After I completed the job, I used the tap test; hooking up a line cord from guitar to amp, I selected each pickup, one at a time and gently tapped the pickup coils for each pickup. At first, only one out of three worked. Each pickup is hooked to it's own switch, the positive and negative outputs on the switch is hooked to the other two switches in a series, and then terminates out to the volume pot, the positive going to the leg 1 input of the pot, and the negative grounded to the back of the pot. With only the bridge pickup working, I decided to take an ohm's resistance reading on each pickup to make sure the pickups were functional. Each pickup read 5.84 ohms, telling me that they all were working properly. I began with switch 3, controlling the neck pickup. Following the path of the wiring on that switch, I got positive continuity on the switch in all terminals. I took continuity checks starting with the positive output on switch 3 which travels over to switch 2, then over to switch 1. Touching the probe at the output terminal on switch 3, and the other probe at the output terminal on switch 2, I got positive continuity, so I kept my probe on switch 3 and moved the other probe to positive output terminal of switch 1. Again, I got positive continuity, so, keeping the probe on switch 3 and moving the other probe to the final destination of the positive wire output path, the volume pot, I got positive continuity again. The open circuit was not in the positive wire circuit. I repeated the test, probing the negative wire circuit, probing the negative output from switch 3 to switch 2. Ahh; no continuity! I looked with a magnifying lens and found the negative wiring at switch 2 had an open circuit. I carefully re-soldered the open connection and bingo; my pickups worked. I always use the DC voltage setting to test the 9V battery common to active pickups and pre-amps. This is one of first things I check when diagnosing a problem with no output or distorted output. In guitars or other audio systems, effect pedals, etc. the 9V battery is technically dead below 7V. I also use the voltage setting to trace the path of the pre-amp output to all points leading out to the output jack.