MICRO 80 CW TRANSCEIVER
The Micro 80 was first designed by a Russian amateur radio operator Oleg Borodin (RV3GM). After this it became open season for amateur stations building small transceivers. It was this design that lead to the pixie transceiver. The Micro 80 was the first transceiver I have built and can be seen in the two photographs.
The circuit is probably the most simplest design for a transceiver possible. The transmitter section is a crystal controlled oscillator, transistor 2 is the power amplifier which delivers the RF power to the tuned circuit and then to the antenna. For the receiver section the oscillator remains running all the time feeding a signal to transistor 3. Transistor 4 functions as a simple mixer, mixing both signals from the antenna with the oscillator. This part of the circuit forms a DC receiver.
CLICK HERE TO SEE CIRCUIT DIAGRAM
It was even available as a kit with a small PCB 35 x 50mm. I decided to build the circuit using my own components. The original uses Russian transistors. KT315s for the oscillator and audio amp, and KT603s for the power amplifier and mixer stage.
In my version I used a 2n3866 for transistor 2 and BC108s for the other three transistors. I decided to build it ugly style on PCB board using copper clad stand offs, no attempt was made to make the PCB small as this was a first attempt at a transceiver, however it went nicely into a standard tobacco tin and makes a nice little portable unit.
Unfortunately I did not have any specific data for the coil L1, but after making a few enquires I believe the coil would be around 3 micro henrys. I wound 25 turns of 26 s.w.g enameled copper wire on a T50-2 toroid, making taps at turn 8, 16 and 18 to experiment for maximum power output. The original set did not have any RIT receiver / transmitter offset so it was difficult to hear a station if they were on zero beat. I later fitted a 65pf trimmer capacitor and switch in series with the crystal and ground to provide some tuning range.
Please note if you are building this set you will need a pair of high impedance headphones of at least 1000 ohms plus, normal walkman phones are no good. Click here to see my pair of old HI-Z phones. These produce plenty of volume and although there is no sidetone when the Morse key is pressed, there is a loud noise in the headphones which is in time with the keying. Unfortunately after a while this becomes very unpleasant to the ears.
If you are building the circuit good luck and enjoy yourself, I would be very pleased to hear the results of anyone who has built the Micro 80 or anyone with the original data on the coil L1.
M0DAD