Place all of your unwanted electrical devices on your work bench. It’s preferable to place them in order of relevance to your project. For example, if you want to build a distortion pedal, an old distortion pedal is most likely to have the parts you need. A reverb pedal will have some of the parts you require, such as circuit board, 1/4-inch jacks and potentiometers, so other effects pedals are also relevant. Old radios may contain usable parts such as jacks, wire, transistors and potentiometers.
Dismantle each device partially. Don’t disconnect the power supply. Unscrew the chassis using a small Phillips screwdriver. Each device is different, so the dismantling process will vary. To dismantle an effects pedal, you typically unscrew the base of the chassis to separate it from the top, then unscrew the top to expose the circuit board.
Remove all 1/4-inch jacks. These are the small plugs where you connect your guitar. Melt the solder joint fusing the wire to the input terminal on the inside of the jack, then remove the washer holding the jack to the outside of the chassis.
Inspect the circuit board. Examine each transistor, resistor, capacitor, diode and potentiometer for signs of leakage or blow-out. When a circuit-mounted component blows, it typically produces a small amount of soot which creates a brown discoloration. If you spot any discolored components, mark them with a piece of electrical tape.
Connect your multimeter cables to the circuit board, to the right of the part you’re testing. The power supply for a circuit board is typically connected to the right, so the power flows right to left. Set the meter to “resistance.”
Turn on the power or connect a 9-volt battery if testing an effects pedal with no DC adapter. If the meter gives a zero reading, it’s probable that the part you are testing is bad. If the meter gives a reading within a 5 percent tolerance range of the value printed on the component, it’s probable that the part is good. For example, if testing a resistor with a resistance of 120 K, any reading between 115 K and 125 K is fine.
Unscrew the circuit board, flip it over and use a clean soldering iron tip to melt the solder joint connecting the connector pins of the transistor, resistor, diode or potentiometer.
Pull the component out from the top, so the connector pins slide out of the board turret.
Remove and discard all bad components so you can reuse the circuit board.