An oscilloscope is a device for testing electrical currents, specifically to display for observation the shape of the signal wave. In audio amplifiers, you can use a scope to check for incorrect biasing, blown fuses, signal distortion and any other potential source of poor sound quality. The signal wave is displayed on either a cathode ray screen or a digital screen. The shape of the wave holds clues to the performance of the amplifier. Typically, the smoother curve the better the sound.
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Things you need
- Dummy load
- Sine wave generator
Unscrew the back and top panels of your amplifier. Use a small Phillips screwdriver. Stick the screws to a piece of electrical tape in the order that you removed them. This makes it easier and quicker to fit the screws back in the right holes. This exposes the circuit board and chassis ground, which is required later.
Connect the sine wave generator to the input of the amplifier. You don't always need the sine wave generator; it depends on which specific test you are carrying out. For example, if you performing a basic voltage test on the circuit board, you don't need a sine wave. But it's easier to have it connected and unused rather than constantly connecting and disconnecting it.
Connect the red cable on the dummy load to the output socket on the amplifier. The dummy load receives power in an attenuated form to simulate normal operation without forcing the amplifier to process the signal. During testing, the amplifier must function in the normal way, but to test with speakers connected is potentially damaging to the speakers and your hearing. However, the current needs to go somewhere. The dummy load absorbs the current. This protects the output stage of the amplifier during the testing process.
Clip the scope's ground cable onto the chassis ground on your amplifier. The chassis ground is typically a bolt mounted to the side or back of interior of the amplifier chassis. Turn on the sine wave generator.
Turn all controls on the scope to zero. Set the scope to "DC Coupling."
Turn the amplifier on. From now on, take great care not to accidentally disconnect the ground cable. This cable is for safety; if it is disconnected while the amplifier is on, you risk electrocution.
Press the oscilloscope probe against the part of the amplifier you want to test, such as the output transformer. Hold it in your hand so it doesn't slip away from the amp.
Adjust the "Time" and "Volts" dials. This adjusts the view on the oscilloscope grid. The vertical axis of the grid represents voltage and the horizontal axis represents time. The resultant curve represents the dissipation of power as it flows across the amplifier.
Move the probe around the various parts of the amplifier and observe the grid. If the part of the amp your are testing gives an uneven waveform with irregular peaks, this suggests a problem in that particular part of the amp. Components that display a regular, "ripple" like waveform are typically fine.
Turn off the sine wave generator. Switch the oscilloscope to "AC Coupling" to test the power supply. Press the probe against the power transformer. If the waveform doesn't ripple, there may be a shorted or about-to-short primary winding.
Tips and warnings
- Take great care at all times during the testing process. If you are unsure of any step, consult a professional.
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