pair of 807 transmitter valves in parallel forms the transmitter, with
both the screen and control grids separated by resistors to reduce the
risk of oscillation. Pairs of heaters are in series to allow
operation from 12v DC. The RF input is
connected directly to the ZC1 aerial terminal and feeds the RF through a
send/receive relay into the grid circuits. The input impedance would be
essentially capacitive and high resistance, although it is not known how
well the tuning unit in the ZC1 copes with this impedance over a wide
The anode circuits are combined into a tapped Tank
tuned circuit, with the aerial connection taking power from the tap via
an RF current meter. The unit above has had its Tank circuit removed at
sometime, and a substitute fitted in its place.
The built in
power supply of the ZA1 Mk-1 provides approximately 500 volts using a
vibrator and transformer, with the HT output rectified by a thermionic
diode. This prevents fast changeover between transmit and receive, as
the diode heaters are supplied from the HT transformer and will take
some time to heat up. However, having operated many Army sets, a long
wait between receive and transmit seems to have been a standard
procedure! Note the RF amplifier valve heaters are powered all the time.
The vibrator is a synchronous type, with the unused contacts paralleled
to provide extra grunt for the primary drive.
A bank of buffer
capacitors has been provided on the transformer HT secondary, with the
required capacitance being selected in production test. Hence one
capacitor is left unconnected in the sample model."