Audio equipment

This page covers audio equipment, in particular radios, Hi-Fi systems and portable CD and cassette players.

Summary

Audio equipment not infrequently fails. The problem may be a simple mechanical fault or a failure in those parts handling the most power, or it may be something more subtle. This page covers the commonest faults and what to do about them.

Safety

Audio amplifiers of even modest power output ratings can operate with quite high voltages internally, often in the range of 50 - 100V. Though not as dangerous as the mains, such voltages need nevertheless to be treated with respect.

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 even after the mains has been disconnected.

A separate page covers CD and DVD players.

Amplifiers

Power output transistors in an audio amplifier.

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, especially 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 or two pairs of transistors connected in series between the positive and negative supplies. There will be one set for the left and one for the right in a stereo amplifier. These may be either bipolar or field-effect types. Bipolar transistors 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, 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.

Check whether there is any possibility that the batteries (if used) have at any time been installed the wrong way round. If so, there is a strong possibility that this would have done irreparable damage.

Corrosion of the contacts in the battery compartment is common, caused by leaking chemicals from dead batteries which have been left in the device. Clean the contacts with switch cleaning fluid or isopropyl alcohol on a tissue or cloth.

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.

DAB radios ought to be very reliable as they have few mechanical parts, but nevertheless failures are not uncommon. Firstly, look for broken wires or connections and signs of poor workmanship such as bad solder joints. A faulty power supply is another possibility. Check whether the device works on batteries but not on mains. If it uses a separate power adapter, test its output voltage with a multimeter. If it runs directly off the mains then it will have an internal power supply. In this case, extreme caution must be observed if any tests are carried out with the device open. Only do so if you are completely confident that you can do so safely. Whether the unit runs on mains or batteries, it may well have internal circuitry to provide different voltages for the analogue and digital sections. You may be able to identify this and test it with a multimeter.