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Summary

Initially, this page is mainly concerned with amplifiers.

Amplifiers

Problems with amplifiers most commonly appear in those parts handling the most power, that is the mains power supply and the power output stage.

Power supplies are covered in their own page. Look out in particular for bulging electrolytic capacitors, particularly if there is excessive hum from the loudspeakers. Bear in mind though that overheating components or a fuse which repeatedly blows may be a symptom of a problem elsewhere.

The power output stage often consists of two transistors connected in series between the positive and negative supplies. These may be either bipolar or field-effect types. The former can be subject to thermal runaway: overheating causes an increase in the current drawn, which in turn causes the overheating to get worse, and so on until the transistor fails. This often results in an effective short circuit between the positive and negative supplies and causing a fuse to blow, sometimes spectacularly.

You should remove the transistors for testing and replace them as a pair. They will often have different numbers and it's essential to fit the right one in the right place. Take care to ensure good thermal contact with the heat sink, cleaning off and replacing heatsink paste if used, and not loosing an insulating washer if fitted.

Tuners, portable radios and cassette players

Problems with these are often mechanical such as worn drive belts on cassette players, misbehaving cords and pulleys on tuning dials, and jammed or broken plastic parts. A careful visual inspection whilst trying to operate the part in question may show what is wrong.

Switches and push-buttons can also be troublesome. They are covered in their own page.

Modern equipment often uses a digital volume control activated through push-buttons or a rotary encoder, but formerly a potentiometer was universally used. This can cause loud crackles from the loudspeakers when it gets old or worn. Remediation is covered in the section on potentiometers.

Tuners often contain adjustable trimmer capacitors or coils with adjustable ferrite cores. These are set up and sealed in the factory. Never try "tweaking" them as their proper adjustment requires laboratory equipment such as a signal generator and oscilloscope, without which it would be almost impossible to do anything but make things worse.

Safety

Valve amplifiers are not common, but if you have to deal with one remember that it will contain high voltages which may be just as dangerous as the mains, and that smoothing capacitors may retain enough charge to give you a nasty shock after the mains has been disconnected.

External links

• External links as bullet points