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Old 2nd Jul 2019, 8:41 pm   #1
mictester
Triode
 
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Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 37
Default Another "Pantry" Rig for the IC-allergic

I cheated slightly - there's a three-terminal regulator chip in there, but that could be replaced by a series-pass transistor and a zener diode - at the cost of some stability - but the aim was largely achieved. The 100n ceramic caps either side of the regulator need to be as close to its pins as possible to prevent RF getting in there.

The circuit's nothing particularly special and is made up of a Colpitts oscillator (for stability), a buffer stage (with a bit of gain) to prevent the variable loading of the final "pulling" the oscillator frequency about, and a Class C final, which is operated at half the supply rail voltage under conditions of no modulation. The only slightly critical component is the NPO 220p capacitor in the oscillator. I had an ordinary orange-topped ceramic there at first, and the drift was horrible.

The modulator provides the nominal half rail voltage, and waggles the supply to the final up and down with the applied audio. It also provides a small measure of audio filtering.

It's worth making sure that the emitter of the modulator transistor never quite reaches zero, so that the final is never quite completely cut off - this reduces the (already small) harmonic output considerably.

I used 3N3904 transistors throughout only because I've got a few thousand of them (thanks Ebay!). My brother made one using BC546 transistors throughout, and it worked just as well as mine. Use whatever devices you have - within reason! Most of the component values aren't too critical - if you haven't got the exact values, "close enough" will probably work.

It's worth pre-processing the audio that's modulating the rig - compression and limiting will help this little device be heard above the noise floor! My prototype - thrown together in the traditional 2oz tobacco tin - worked well over nearly a kilometre when given a good earth and a 5m "bit of wire" dangling from a tree, powered from a "12V" gel-cell (actually about 14.4V fully charged) on a well chosen "quiet" frequency.

Have fun!
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