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CHASSIS RESTORATION  

WOOD  BAKELITE  CHASSIS  ETHICS  DIODE, TRIODE  TETRODE, PENTODE  STANDARDS PHOTOGUIDE  VIDEOS

PILOT  BUSH  EKCO  COSSOR  PHILCO  BAKELITE HISTORY VALVES DIODE, TRIODE  TETRODE, PENTODE  STANDARDS

 

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 cabinet.

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, electric shock.

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. NEVER HIGHER.

*Don't worry 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.

 

From top:

Scraping rust with craft knife blade

Preparing for respraying

Components removed

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