Servicing old radios

Posted on Feb 6, 2014

A radio that was stored for years on a dusty shelf must be awakened gently to return to its reliable operation and provide us with warm and pleasant sounds for years to come. Here are few generic notes on how a radio can be easily returned to life. Of course, more details can be found looking at each specific model. In addition to common tools, as screwdrivers,

Servicing old radios
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pliers, soldering iron and some brushes, we need few basic instruments. Some of the types, commonly in use in the past, are no longer manufactured. Some of the instruments still in production today can be more or less unsuitable for vacuum tube sets, where voltages could well be in the order of several hundreds volts. Anyway in many cases, old instruments still operating can be easily find at reasonable prices. Here an overview of instruments usually found in old service shops and their usefulness. A step-down transformer, a Variac or even an incandescent lamp, to reduce the mains voltage during the early power-on, is the most useful tool. The step-down transformer is to be preferred when servicing radio sets with chassis directly connected to the AC mains. When a transformer is used, a 2 to 1 voltage ratio can be selected. We will start at half voltage and switch to the full voltage after the reforming of filter capacitors. A soft start-up procedure is advisable even for radios already serviced in the past, if left inoperative for several months or years. Most of the digital multimeters, today available, have high input impedance, in the order of 10 Megaohms. and are suitable to measure voltage and resistance inside our radio. Unfortunately the voltage range of modern multimeters is limited to few hundreds volts, unsuitable for some old electronic sets. I burned some digital multimeters, attempting to calibrate the HV...

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