Trampmitter

A past Winter project

       

          I collect old transistor radios. Most of them are the long distance kind. They have extra parts in them and a longer ferrite antenna. I use them late at night to hear Zoomer Radio out of Canada and other stations from parts of this country. There is nothing on AM radio that I enjoy during the day so I made this transmitter. It is the combination of several circuits. It puts out a full 100mw. I jointed a Yahoo group and a guy there kept redrawing my circuits as I posted them so I left. I don't like redrawers!

Here is the circuit board.

         The circuit uses a LM386, 1MHZ crystal oscillator and a 2N3904 as an RF output amp. I used the parts that I had. You will notice I used three RF chokes in series in order to have a 330uh choke. Also note the four .01uf caps around the diode bridge. See below No Hum Supply. I use a Proto Board and then build the project on Perf Board. It's faster! From Idea to working circuit in minutes.

Here is the schematic I used.

         The circuit is very simple and it is made up from several circuits I found on the Internet.

A No Hum 9 volt supply.

          Transmitters have a nasty problem. If the supply voltage is not clean you will get Hum on your signal. Hum comes from a few places. Lets kill the first ones. Not enough capacitance in the supply. The diodes in the full wave bridge put Hum into the circuit. So you put .01uf caps across each diode in the bridge. See Circuit. Then the sneakiest HUM off all! The transmitting signal goes into the wall of the house. It then rides the AC back into your transmitter supply. See .01uf caps in the 120 volt AC section of the supply. My supply also lets you adjust the 9 volts for best results.

My Perf Board Layout

          50 years later the circuit was working!

Under the hood!

          Notice again the .01uf caps around the full wave bridge and the ones at the input of the transformer's AC input. This circuit is for the advanced hobbiest. The information here is more than enough!


Email: John a quick question


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