The basics of chassis restoration
Firstly, a description
of a typical chassis
The chassis is the electronic heart of
the radio, all the 'works' of the set. It consists of a supporting
structure, the chassis proper, which in turn carries all the components
of the circuitry: the mechanical items such as valve-holders, screens
and, often, the dial assembly and its cord drive system, though the
glass or acrylic dial itself may be mounted within the cabinet shell
along with the loudspeaker. Occasionally, with miniature 'mantel'
radios, the loudspeaker is mounted on the chassis too, but for best
results, loudspeakers need to be firmly fixed to a 'baffle' board within
The work involved
In order to restore a chassis to its
former glory, a considerable knowledge of valve function and radio
circuitry is required and is essential for safety, both for the restorer
and for the final user(s). Mains powered radio chassis use high
voltages. Worse still, AC/DC chassis are almost always non-isolated - in
other words, the chassis metal is live to the mains. It follows that
should the mains plug be incorrectly wired - or the wall socket -
that the touching the chassis would result in a severe, perhaps fatal,
Valves are often the first thing
people think of as likely causes should a radio not function
satisfactorily. In truth, however, although valves certainly do lose
emission over their lifetimes, causing a lack of gain, low volume,
insensitivity etc, and can develop other more baffling faults, there is
the strong likelihood that other components may be the culprits: in
particular, the capacitors. These are more often than not mostly of the
waxed paper variety and are notorious for leakage. A good capacitor
should not pass any measurable DC current. Leaky ones certainly do,
resulting in distorted or low volume sound, inability of certain valve
stages to function, risk of overheating/fire/capacitor exploding* and a possible shortening of valve life.
Under-chassis components may be
replaced with modern equivalents, but the purist may wish to go to the
lengths of cutting open the old ones, hollowing them out and hiding a
new, tiny modern component within. This is to maintain the appearance of
originality. My view is that replaced components on the top of the
chassis should where possible be made to appear original. Under the deck
is not so important.
Typical top deck components that fail
include the valves themselves, the smoothing and reservoir capacitors
(often combined into a single metal tubular or rectangular block) and
transformers, both power and audio output.
Combined capacitors of this type may
be cut open and modern smaller replacements slid into the case, with due
regard for insulation. The cut open section can usually be hidden with
the clamp that holds the component in place.
Wiring is commonly found to have
deteriorated with impaired and crumbling insulation and/or corroded
conductor wires. Replacement is the only safe option in such cases.
Mains leads in particular should be scrutinised for this problem.
On-off switches, where single pole
types are used, will commonly be wired into the mains neutral lead. I
always rewire these into the live conductor. Mains fuse - in the plug -
should be no more that 3A. Throw out the 13A fuse that comes as
standard. It may be quite possible to use an even lower rated fuse.
unduly. Capacitors can explode but rarely cause damage other than to
themselves though the altered conditions leading up to the explosion may
cause the problems mentioned.