Thursday, November 28, 2019

Schematic for the "Novice" Transmitter

Schematic for the "Novice" Transmitter
I guess it has taken a while. I wrote about my Novice transmitter over ten years ago in January of 2019 as one of my first articles in this blog. But I never published a schematic.

Well, the wait is over. This summer I took some time to hand-draw a schematic reasonably neatly. At least neatly enough for someone else to make heads or tails out of it.

The result is in the image on the right. The circuit is pretty simple.

A couple of notes on the tank circuit. The coil is 1 3/4" diameter, 12 turns, about 7/8" long, which computes to be about 6.4 uH. A typical pi network for a 6146 at 7 MHz should be L = 6.2 uH, C1 = 90 pF, C2 = 500 pF. So it seems my design is pretty close.

The keying arrangement is unique. The oscillator is cathode keyed, but the PA is biased beyond cutoff for Class C operation. When properly the grid tuned circuit is properly adjusted, there is little to no chirp.

This radio works pretty well on 30m, too. The crystals I have for 30m are HC-49U crystals soldered in FT-243 housings. These drift a little bit key down, likely due to the high crystal current. I added a 100 ohm resistor in series with the crystal to limit the current.