An amp circuit can malfunction in a number of places. If individual components -- such as resistors, transformers or the amp's speakers -- are broken, the circuit may not be complete, and an ohmmeter can help you identify the location of the problem. An ohmmeter measures the resistance between two points in a circuit. If two points are electrically common, they will ideally have no resistance. If the circuit is broken, the circuit will have infinite resistance. If you know the expected resistance between two points, you will be able to locate the problematic parts of your amp's circuit.
Turn off the amp. The ohmmeter supplies its own test current.
Turn your ohmmeter to the range of resistance you expect to find between two points. For example, if you expect to find a resistance in the thousands of ohms, set the ohmmeter to the kilo-ohms scale.
Touch the leads of your ohmmeter together. The ohmmeter should read zero. If it doesn't, turn the zero dial so that the ohmmeter reads zero when the leads are together.
Connect the two leads across the component you wish to measure. The polarity of the leads does not matter, as resistance does not depend on direction.
Observe the reading on your ohmmeter. If you see a reading of "OL" or a "1" in the far left on a digital multimeter, your scale is too low for the present reading. This indication can vary depending on your ohmmeter model. In this case, turn the dial on your ohmmeter to a larger scale.
Connect the leads of your ohmmeter between two points that are electrically common. The resistance between these points should be extremely low. If the reading on your ohmmeter is infinite, your circuit is broken between these two points.
Tips & Warnings
- Some components, such as transistors, only conduct under certain conditions. Make sure you understand how each component functions so that you know what to expect when using your ohmmeter.
- Capacitors can store large voltages even when your amplifier is turned off. If you need to test a capacitor, discharge it first by shorting it with an insulated screwdriver.
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