Older amplifiers, receivers, and cassette decks, etc. often develop problems with their front panel controls (switches and potentiometers).  As an example, turning a volume or balance control may cause static or the loss of a channel.  Putting a cassette deck into record can cause some nasty sounds and/or may not record properly.

Your grandmother's fine silver tea set sure looks nice, however if you let it set and not polish the silver, it will tarnish.  The same thing happens to the contacts of various electronic controls.  If they are not used for some time they may tarnish and will not perform correctly when you try to use them.  Using the switch or volume control etc. causes movement inside that control and will help to "polish" the contacts, keeping them nice and shinny, and clean.

You may be able to clean up these faulty contacts yourself, saving a trip to the repair shop.  Turn off the power to the receiver, the cassette deck etc.. Rapidly turn the control or flip the switch, back and forth several times (10 - 15).  Hopefully the movement of the control will help to clean the contacts.  You may have to do this more than one time, and on a regular basis.  Be very careful not to force any control beyond its limit.  If the problem persists you may have to take it in to have the controls cleaned and/or replaced.

Often replacement of these controls on older units is not possible.  The parts may no longer be available.  However, most times the repair shop may be able to clean up the offending control.  This requires dis-assembly of the part and cleaning. This would be considered a repair and not a cleaning as a lot of labor is involved.  Normally these parts would be replaced.