A second version, the ZA1 MkII, shown
above, differed in that the parallel 807's were modulated by a second pair of 807's fed from a 6V6 phase
inverter and a 6U7 microphone preamplifier. The ZA1 Mk-2 unit is a
much larger and heavier unit housed in a modified metal ZC1 Mk-1 case.
For this unit the ZC1 acted only as an RF driver and keying or modulation
are achieved via the ZA1 Mk-2 amplifier.
The Mk-2 was a contemporary rather than replacement for the Mk-1 and
both versions were manufactured around the same time in 1943.
The amplifier is built on a modified
chassis reduced down to half height to accommodate the height of the four 807's
in the ZC1 case.
The case is a modified Mk1 ZC1
one fitted with louvers at
one end and with a slide cover at the opposite end allowing access to the final
coil tapings for matching purposes. A slide cover on the top was
included for ventilation
Opening up the top slide reveals the plate
connections for the modulator, something O.S.H. would go "bananas" over in
this day and age, but of no consequence in 1943!
A dummy microphone lead connects the ZC1/ZA1 to
key on the ZC1 when transmitting.
In summary, the major differences between the two
versions is the addition in the ZA1 Mk-2 of an AM modulator, with associated
microphone amplifiers, MCW oscillator, and a second built in vibrator power
supply. On CW just one power supply is used, but when phone or MCW is needed, the
extra PSU provides HT supply for the power modulator. The inclusion of
this extra hardware considerably added to the
weight and size of the Mk-2 version.
When a short mobile whip aerial was used, a ZA1 loading
coil could be fitted to the case upper flange slide, to date no surviving
example of this component has been found.