The 73 kHz transceiver is based on a cave radio (1) which in turn was based on boards designed for an HF band phasing transceiver (2).
The on-board cascode RF stage is not used: instead a small piggy-back board RF amplifier using an OP-37 low noise HF op-amp and a single tuned circuit is fitted. With the series tuned loop aerial, a bottom-capacitor coupled circuit results in a Q of 14. An alternative aerial with a 50 ohm feeder would produce a Q of about 22, plenty wide enough.
In the working model, the pre-set gain control on the piggy-back board is replaced by a front panel 1k lin pot as RF gain.
The whole receiver now forms an image-cancelling DC receiver. The CW filter and S meter omitted from the cave radio are reintroduced in this.
The exciter is a phasing method SSB generator, which has two 1496 balanced modulators terminated in a single tuned circuit at 73 kHz.
A piggy-back board carries a crystal divider, and in fact off-the-shelf crystals at 4.608 MHz giving an output at 72 kHz are available, but a VFO at about this frequency is used instead. The VFO again is taken from the HF phasing rig, is very stable, and when divided by 64 results in the most stable signal source. After the 4060 divider, a 4013 produces the I and Q quadrature signals for both transmitter and receiver. A two pole changeover switch enables simple sideband selection.
The power output stage is a TDA2003 audio amp capable of 10W into 1.6 ohms.
For CW transmission, a simple circuit which unbalances one of the modulators on key-down can be used; alternatively there is a suitable audio tone from pin 2 of the 4060 which can be keyed into the microphone circuit.
With a 100 pF single gang tuner, the transceiver range is 71 - 75 kHz.
(2) J R Hey, 1993 July and August Rad Comm "An HF Phasing Transceiver"
(3) Main amp printed board £4.85; SSB Exciter board £3.60
PCBs available by mail order from:
John R Hey (G3TDZ)
8 Armley Grange Crescent
LEEDS
LS12 3QL
Tel: 0113 263 7885