Published in the HRSA magazine "Radio Waves" ( April 1998 No. 40) was an article I wrote describing the construction of a miniscule "B/C Transmitter" followed by a test report by Ray Kelly. The following has been taken from those articles.
| I have been a collector of old wireless sets for some years and my collection consists of some
good old timers mainly 20s and 30s. All of which I have been able to get working quite well. However here's the rub
- what do you listen to on the broadcast band? Being keen on light classical music, FM is for me. I decided therefore to design a small "B/C station" with the audio modulation coming from my FM tuner. Three requirements: 1) since I am rather busy on various projects it had to be quick and easy to make. 2) It should have only sufficient power to be heard within the room that confines my collection. 3)That no discernable distortion should appear on the output. |
The result was a 2 fet oscillator and amplifier device with a bipolar transistor for the modulator. It provides a very clean distortion
free modulated envelope even up to and in excess of 90%. The aerial is a piece of hookup wire draped around the skirting board behind the wirelesses
This provides plenty of RF for any autodyne/anode bend detector or neutrodyne set etc. The oscillator should be tuned to a quiet place on the broadcast band and the PA tuned to the same frequency ie. tune to give max audio output on a wireless preferably without AGC. Construction should conform to the usual requirements of separation of inputs from outputs, however since there is no tuned circuit between the two stages, stability can be assured even though both coils are tuned to the same frequency |
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| I used pot cores to make my own coils, but old b/cast RF or Aerial coils in cans can be used providing the windings are isolated from one another, the number of tickler turns is relatively unimportant however the polarity is all important on the oscillator coil. |
| A test report. By Ray Kelly. GrahameJeffery's submission stimulated me into action. For a year or so I had planned to try Peter Lankshears's "Mate" described in Electronics Australia , May 1989 but I never seemed to get there despite the need. Once Grahame sent in his design I thought I would like to test it for myself and I am very pleased with the results. To be fair I have I have never had the chance to compare it with Peter's design though I am confident either would satisfy anybody who wants to hear the type of programming that suit the elderly machines that they love still. Your choice could be decided by the technology that you feel most at home with, and the number of components you can find in the junk box. | I built mine in a salvaged metal cabinet, available from Dick Smith cat No-H 2744. The smaller components were assembled on a piece of veroboard that had been around since I bought it for some long forgotten project. the transformer also came from stock, it was once part of some Japanese equipment. I used a small two gang tuning capacitor from a 50s radio, and a pair of oscillator coils that came from Antique Electronic Supply in the USA. I used them because of the ease of mounting as they were one hole fixing but I screened them with a couple of cans salvaged from old TV ratio detector coils. These coils are slug tuned so I first tuned a radio to find a clear spot on the dial. This is best done at night as interstate stations can create interference on what seems to be a clear channel during daylight hours then I peaked the slugs for greatest output on that channel. However as the aerial lead is a parallel capacity across the output tuned circuit if you move the aerial lead you may need to adjust the slug in the output coil. The results seem to be very good the signal seems to be as free of distortion as you could wish and as Grahame says the output range is limited to well within legal limits. Try it, I am using a small transistor cassette radio to provide either FM music or a selection of suitable material from my cassettes I certainly prefer it to the talk-back programs which seem to be the current trend on most AM stations. |