Antique radio FAQ
Collecting what ?
Radio is a very wide field and radio collecting can become addictive and invasive. New collectors tend to take everything they find. If you don't own a castle or if you are not planning to create a museum, you must specialise in things that you like best. If you live in a small flat, you can collect crystal sets, Philips sets made before 1935, portable valve sets, or pocket-shirt blue transistors ! Or collect old components : tubes of course, but why not capacitors or resistors. A small but precisely defined collection seems to me more interesting than heaps of miscellaneous stuff.
How to get a crystal set to work again ?
Crystal sets are easy to repair as they have a very small number of components arranged according to very simple schematic diagrams ! The main thing is to eliminate any trace of oxide, which causes faulty contacts. The cat's whisker must touch the crystal very lightly and it must be very sharp. It is easy to make one with thin copper wire coiled around an 8 mm drill. Flatten the extremity with a hammer and bevel it with scissors. Natural galena is generally not sensitive enough. Synthetic crystals are much better and their sensitivity is equivalent, sometimes better than a germanium diode. The impedance of the headphones should be at least 500 ohms, but better results are achieved with 2000 ohms. Provided that the coils, made of wire thinner than hair, are not cut and that the magnets have not lost their magnetism.
An outdoor aerial is a must (although an outdoor antenna will work just as well !) It must be as long as possible (30 meters is usually fine) and the extremities must be well insulated. A good ground is also necessary (water pipe, central heating...but never use the "ground" from an electric socket !) Your reception depends on your location, but in general you can only receive the nearest and strongest station. A crystal set is not sensitive nor selective, but the audio quality is very pure.
How to clean Bakelite and plastic cabinets ?
Bakelite is in fact the ancestor of what we now call plastics. But unlike modern plastics, Bakelite does not melt but gets carbonised. If in doubt, try with a small soldering iron somewhere invisible inside the cabinet, but with caution ! If the radio is very dirty, first wash it in soapy lukewarm water. Then polish with some Miror (normally made for shining brass but also very efficient on plastics) and cotton wool. That way, you will restore the original shining aspect. It is important to eliminate all white traces left by the Miror (or equivalent) in the hollows of the cabinet. The sometimes very dirty knobs are easily cleaned with a tooth brush or nail brush and detergent.
How to tell if a valve is good?
A tube tester is necessary for that, but few collectors possess such a piece of equipment. If you have a multimeter (you should have one !) you can only test if the filament is not burnt. You will have to find which two pins correspond to the filament... If a valve is black it does not necessarily mean that it is burnt. Many valves were black inside when they came out from the factory. And a valve which does not become hot can be well alive : all triodes like A409, A410 etc do not glow and do not produce any heat, same with miniature tubes used in portable radios of the fifties.
The radio is working but with a very strong hum. What to do about it ? If your radio produces a strong 100 or 120 Hz hum, it means that the filtering capacitors are dry. That's what generally happens when you plug in a radio that has not worked for decades. Those capacitors are usually aluminium tubes screwed to the chassis. You must disconnect them under the chassis and solder new capacitors (they are much smaller) of the same value and voltage. Respect the polarity ! Leave the old ones for decoration. The mains hum has now completely disappeared.
The set works but the sound is weak and distorted. What to do about it ? Also something very common. It generally means that the grid capacitor of the final AF valve is "leaking", which means that it lets some direct current through. It is usually a small glass or cardboard tube of 0.1 MF. Just replace it with a modern Mylar one, preferably rated 400 volts, which will last for ever and perhaps longer !
In fact, purists will also replace all other old capacitors which tend to leak more or less, so the radio will work perfectly. But, as it can be rather costly, you should do that only for exceptional sets that deserve such a treatment.
AC/DC sets : Danger !
AC/DC sets are very common in all countries. They can be powered from Direct current or alternating current. They were cheap because they didn't have a transformer. The mains voltage was used as the anodic current and also as the filament current through a large heating resistor coiled on ceramic. This allowed for more compact chassis which were designed to be housed into miniature cabinets (bedside radios). But the problem was with the metallic chassis connected directly to the mains, which meant a danger of electrical shock when operating the radio in a humid environment. The most horrible of those sets used a "heating cord" in which the mains cord includes a third wire surrounded with asbestos to drop the voltage ! According to newspapers of the thirties, lots of fires were caused by those devilish things. The moral of that is : Keep those sets for their (sometimes very remarkable) design, but be extremely cautious if you insist on repairing them. Use an isolation transformer if you can find one. Anyway, those small sets with small loudspeakers sound like saucepans !
Are there any books on repairing antique radios ?
Yes, check with Antique electronic Supply. They have a lot of literature on antique radios.
Last updated february 24th 2009
Copyright Jean-Luc Fradet 1997 - 2009