How to Fix Up Your Old Radio – Part 2
When fixing up your old radio bear in mind that there are two basic types of valve-radio, superhet (superheterodyne), and TRF (tuned radio frequency). Most of the parts for both radios will be the same, the biggest difference is the way they process the radio signals.
The main parts for a valve-radio are:
- Power Supply – which normally will have two inductors.
- The Antenna – including coils which act as transformers.
- RF Coils – differ depending on the radio’s set up and type.
- Oscillator Coils – if the transformer is an intermediate frequency type.
- Output Transformer – to drive the speaker.
All of the above have to be working correctly for the radio to operate, if not then your radio will not receive any signals and will not work.
Before you get started with your restoration work it is a good idea to first check the status of the transformers and coils. Firstly, pull the speaker and chassis from the radio cabinet, and then carefully remove the tubes. Before taking them out, label the tubes and which corresponding socket they belong to. Most tubes are not interchangeable and will not work in the wrong socket. Some tubes have a wire on the top cap, be careful taking this off. If you damage the wire or the top of the tube, then this could ruin the tube. Now the tubes are removed you have complete access to the rest of the radio, and most importantly the connections. All these need to be checked thoroughly, and the best way to do this is systematically from the front to the back.
Most of the connections can be checked with a multimeter, with this kit you can check continuity and measure the resistance. An infinitive reading will generally mean there is a broken wire somewhere, but good readings can be anywhere from a couple of ohms to a two dozen.
Measuring the Antenna Coils
Measuring the primary of the antenna coils can be a little bit tricky, so a good trick is to take advantage of the built-in wiring of the radio. The radio’s antenna connects to all of these coils, and in turn they all connect to the ground strip after the band switch. So just connect the multimeter onto the ground strip and the antenna terminal, and then flip through the ranges to get your readings.
Checking the Oscillator Coils
Depending on the radio it will have a number of oscillator coils, and they can be mounted in different parts of the radio. Normally these parts will have four terminals, and they are pretty easy to check. There will also be two sets of lugs that have continuity to each other.
Finally, you will have to check the transformers, these will be mounted on the chassis and each will have a primary and secondary winding, which both need checking. There may be a filter choke as well to check, but not every radio has these. Now you have checked, you know where to repair any damage and you are well on your way to have a fully functioning valve-radio that will be the envy of all your friends.