Technical Short – Cap Lore sample

Technical Shorts’ is a series of (fairly) short articles written by Gerry O'Hara, a CVRS Director and vintage radio enthusiast, each focussing on a technical issue of relevance in repairing, restoring or using Eddystone valve radios. However, much of the content is also applicable to non-Eddystone valve receivers. The articles are the author’s personal opinion, based on his experience and are meant to be of interest or help to the novice or hobbyist – they are not meant to be a definitive or exhaustive treatise on the topic under discussion….

References are provided for those wishing to explore the subjects discussed in more depth.

Eddystone Cap Lore: Failure, Testing, Replacing and Fixing

Valved Eddystone radios contain several types of fixed capacitors (caps). After several decades of continuous or intermittent use or non-use/poor storage conditions, many of them may have become faulty; commonly losing capacity, or becoming ‘leaky’ (measurable resistance much lower than expected for a particular dielectric type), open or short circuit. This is not surprising – who would have thought, for example, in the late1940’s through mid-1950’s that the S640’s, S740’s and S770R’s then in operation in commercial or amateur use would still be in service in 2006? I would suspect that even high quality radios such as the various Eddystone models, would have anticipated working lives of little more than a decade at best (not six decades or more!) – in my view it’s quite remarkable that some of the components in these radios, manufactured using primitive materials such as oil impregnated paper, cardboard, wax and rubber, often subject to high temperatures and voltages are still functional at all.

Capacitor Failure Modes

The problems that develop in caps are usually caused by the encapsulation and/or sealing materials having become unreliable over prolonged periods of time due to effects of heating/cooling, moisture or direct mechanical damage, resulting in changes to the electrical properties of the caps dielectric material. Alternatively, the dielectric may have
A selection of ‘failed’ caps removed from valve radios (alas, not all Eddystones…)

We hope you have enjoyed reading this article sample. Members of the Canadian Vintage Radio Society (CVRS) can download the entire article for free from the CVRS website here (You must be logged in first).