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Vintage Radio
The amazing NZ-made ZC1 MkII
military transceiver
In the early
phases of
WWII, the
New Zealand
Government
decided that
their troops
required a better
standard of field
communications
radio than what
they had. They wanted a transceiver that suited the conditions in New
Zealand (bushland) and the tropics (jungles).
By Dr Hugo Holden
T
he task was given to Collier and
Beale of Wellington, NZ. They
designed the first model, the ZC1 MkI
and, by April 1942, they had amassed
enough resources to build 750 units.
By December 1942, the first production batch was shipped.
There were a few minor variations of
the MkI model that are not discussed
here, as this article is primarily about
the MkII. The subsequent re-design
was handled by R. J. Orbell of Radio
Limited (Radio Corporation of NZ).
At least 5000 MkI units were manufactured, and around 10,000 units
of the MkII, although estimates vary.
I have seen one estimate that 30,000
total units may have been made, but
that figure could have been a target. The exact numbers may never
be known. The serial numbers were
somewhat non-specific and not helpful due to secrecy.
The ZC1 radio project was not a
cheap undertaking for the NZ Government. Accounting for a total number
of around 14,000 to 15,000 units, the
cost was $3,000,000 NZ Pounds in the
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1940s, equivalent to about $2,660,000
AU Pounds at the time. Translated on
the RBA’s pre-decimal inflation calculator, that is equivalent to AU$234
million today.
If the estimates of the ZC1 units
made are correct, the cost per set was
around $15,600 in today’s currency.
56 factories and 900 workers produced parts and sub-assemblies for the
radios. It took about 60 man-hours to
build one set; about 20 sets per week
could be made initially. Production
must have sped up as time passed
to at least 100 sets per week to complete around 15,000 units by the end
of WWII.
I have been unable to determine if
many sets were made after 1945. It is
possible that some new ZC1s were
manufactured to support the NZ and
British occupation forces in Japan
(J-Force) during 1945-1948.
The ZC1s saw service in the Pacific
war campaign, and many were sold to
the Middle East; however, it was too
late for them to see any significant use.
After the war, ZC1s were deployed
Australia's electronics magazine
by NZ Government agencies for various mobile and fixed applications
until the 1960s. They then started
turning up in Army Surplus stores in
good numbers, many being cannibalised for components. They were typically used by radio hams on the 40m
and 80m bands (7.5MHz and 3.75MHz,
respectively).
A ZC1 radio was installed in the
Radio Room of the Grammar School
that I attended in Auckland in the
1970s; I cannot recall if it was the MkI
or MkII model. By then, I had already
seen ZC1 radios and many components that had been removed from
them in Army Surplus stores.
In the early 1970s, my brother used
an open-frame relay taken from a ZC1,
in conjunction with a capacitor, to
build a mains light bulb flasher.
Marine conversions
ZC1 radios also found their way
into fishing boats and other marine
applications. Many were modified
to be marine band radios; one of my
MkII radios had its transmit VFO
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Photo 1: this crystal module allowed
a ZC1 radio to be easily converted to
operate on marine frequencies.
Photo 2: the red and blue screws
on the tuning dial, plus the two
small windows at the top, allow the
operator to set it up to flick between
two specific frequencies instantly. The
radio’s front panel has a space for a
pocket watch.
replaced by a Pierce crystal oscillator
circuit running at 2128kHz, a marine
frequency. I converted it back to the
original spec.
Collier and Beale supplied a conversion kit for marine use in the post-war
era. Photo 1 shows the modification I
found in one radio; it may well be by
Collier & Beale.
Many ZC1 radios acquired all kinds
of modifications; unmodified ones
became very hard to find. These days,
due to the historical significance of
these radios, most owners want them
restored to their original condition.
Unusual features
As seen in the photos, one of the
attractive features of the radio’s front
panel is a pocket watch holder. Finding a period-correct military-grade
pocket watch to fit in that holder is a
challenge, but I did.
Also note the red and blue rods on
the main receive and transmit tuning knobs, called “Flick Set Screws”,
shown in Photo 2. These allow
mechanical storage, if you like, of two
frequencies; the tuning knob returns
(flicks) to the position and frequency
where the screws were tightened when
the Flick knob is deployed.
One thing that characterised both
models of the ZC1 was the ability to
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transmit and receive on two different
frequencies.
Design and specifications
The radio is a very solid affair, built
into a steel enclosure, the inside of
which is heavily copper plated. The
front cover (Photo 3) fits tightly with
a rubber seal. No harm would occur if
the unit were dropped in water with
the front cover on.
The main assembly is ejected from
the housing by two large front panel
screws and slides out for easy servicing.
The vibrator transformer (at lower
right on a sub-chassis, see Photo 6) is
encased in a shielded container; all
measures were taken to prevent RFI
from leaking out of the vibrator power
unit and creating radio interference.
There is minimal background interference with the original V6295 mechanical synchronous vibrator.
When using an electronic vibrator replacement (as I described in the
June-August 2023 issues; siliconchip.
au/Series/400), no interference of any
significance occurs. Those articles
described several different suitable
designs. Besides no contact wear, some
of those designs have the additional
advantages of higher efficiency and a
higher HT output.
The ZC1 was specifically designed
for easy servicing (unlike much modern equipment). It was very well documented, not just with a comprehensive
working instruction manual for the
operator but also circuit diagrams and
Photo 3: the front cover is a tight fit to protect the radio from mud, water etc
during transportation.
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October 2024 97
Photos 4 & 5: a photo of a suggested ground station setup from the radio’s manual, and how the radio could be mounted in
a truck.
a parts list with extraordinary detail.
The two manuals were labelled with
“New Zealand Wireless Sets & Stations
No. ZC.1, MK.II.”.
Photo 4, taken from the working
instructions manual, shows a typical
setup of a ZC1 MkII radio in the field
with a vertical whip antenna. Photo
5 depicts a mobile application in the
back of a truck.
As well as parts lists, the manufacturers supplied the Army’s Signal
Engineers with comprehensive details
about the radio that were never generally supplied for domestic radios.
For example, they include detailed
descriptions of each of the coils and
transformers, including things like
the exact number of turns used, the
size of the former, the type of wire,
the SWG wire size, the inductance
value with the % tolerance, whether
the coil was wound bifilar and the
coil base diagrams. The DC resistances of the inductors were also
documented.
This is by far the most detailed information available for any radio I own.
If any of these parts fail in the future,
it would be an easy task to replicate
them. The voltage on every valve electrode is also well documented in the
manuals.
Power supply
The radio is powered by a 12V storage battery, typically two 6V units
in series for the ground stations, or
the 12V battery in a jeep or truck for
Differences between the ZC1 MkI and MkII
The MkI model was a single-band 2-6.5MHz transmitter and receiver (transceiver). The MkII version was split into two
bands: 2-4MHz and 4-8MHz. Other differences include that the MkI model did not have an MCW (Morse code) transmit
mode.
The other major difference between
the MkI and MkII units is that the MkI
used a non-synchronous vibrator supply and two 6X5 valves as HT rectifiers, as shown in Fig.a. Also, in the
MkI unit, there was a switch to select
between a higher or lower HT voltage.
In the MkII, however, the switch
was dispensed with, and a synchroFigs.a & b: the ZC1
nous vibrator, the model V6295, was
MkI power supply
deployed. The 6X5 valves were dis(above) differs
pensed with too – see Fig.b.
significantly from the
The negative output of the MkII
MkII (left) as it uses
a non-synchronous
supply is connected via resistors to
vibrator and HT
ground and a voltage of around -50V
rectifier valves
to -60V is developed across them.
(6X5). The ZC1 MkII
This is used to cut off the valves in the
power supply used a
transmitter section when the radio is
synchronous vibrator,
in Receive mode. In Transmit mode,
dispensing with the
the resistors are shorted out, boosting
two 6X5s.
the HT voltage by an additional 50V.
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Australia's electronics magazine
siliconchip.com.au
Photo 6: a
top view
of the ZC1
MkII chassis.
Note the large
brown tapped
antenna
tuning
coil at top
middle.
mobile use. Although the unit was said
to be “portable”, it weighed 27kg, and
somebody had to carry the batteries
too. For this reason, many units were
fitted into jeeps and trucks.
Two people could carry the ZC1
easily as it had handles on each side
of the cabinet. For one person to carry
the unit long distances by themselves,
they would have to be fit and quite
strong.
The MkII radio’s current consumption is quoted at 2.8A in Receive mode
with Sender off and 3.8A with Sender
on. In send RT mode, it is 4.9A; close
to 2A of that is for the valve’s heaters. The 6.3V heater valves are strung
in series pairs across the 12V power
supply; since there are 11 valves in
the radio, one valve requires a series
heater ballast resistor.
With an 80Ah battery, the usable
life is in the vicinity of 20 hours,
with the transmitter used 25% of the
total time.
Transmission power & modes
The ZC1 MkII RF output power
is in the order of 2W. A near-perfect
impedance match into a 50W load can
be made with an impedance-matching
transformer and slightly modified coupling, giving 3W output on 80m and
easily 2W on 40m.
The transmission modes are CW
(carrier wave), RT (carrier wave amplitude modulated by the microphone)
and MCW (Morse code telegraphy,
where an audio tone modulates the
carrier wave).
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The 800Hz tone oscillator was
enabled in both CW and MCW mode
(even though the modulator is not
used in CW mode). The oscillator output was cleverly coupled to the audio
stage and headphones so the operator
could hear a ‘sidetone’ or beep when
the Morse key was pressed.
In RT mode, the sidetone was
instead the sound picked up by the
microphone, helping the operator to
‘hear himself talking’ in the headphones (similar to analog telephones).
Antenna
The ZC1 was generally used with a
vertical 34-foot (10.4m) rod antenna,
supplied in several sections. The
transmission range was 25-35 miles
(40-55km) in CW mode and around
10-20 miles (15-30km) in vehicles
with 8-to-12-foot (2.5-3.5m) whip
antennas.
Wire antennas were also an option,
such as an inverted-L or T-shaped
wire. The ZC1 has a large two-inch
(51mm) diameter antenna tuning coil
with many taps, allowing a significant range of antennas to be used.
This large coil with the brown former
can be seen in Photo 6, sitting above
the chassis and behind the front panel
and switches that select the coil taps.
Component selection
The components in the ZC1, like
knobs, potentiometers, switches, dials,
valves, sockets, coils, shielding cans,
variable capacitors, resistors and fixed
capacitors were all of outstanding
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quality. These radios made for an
extremely attractive and economical source of parts for many projects.
These were especially good for young
people interested in learning radio and
electronics but short on cash.
The solid black phenolic knobs and
other parts, even today (80 years later),
look good as new.
There was a shortage of components
in the early 1940s, especially capacitors. Many of the capacitors, including the mica types used in this radio,
were made in New Zealand. The mica
came from local mines. Many of the
wax-paper capacitors were also made
in NZ, although some were imported
(see notes on the “Dwarf Tiger” capacitor found inside the metal housing of
one capacitor below).
The electronic components in the
ZC1 were heavily ‘tropicalised’ with
wax impregnation. Even the usual
wax-paper capacitors in the unit were
double-sealed inside additional metal
housings with a waxy oil to prevent
moisture ingress. All the other transformers were impregnated and sealed
in metal containers as well.
Even the hook-up wire was said
to have been treated with a “non-
vegetable lacquer”. This was all in aid
of reliability in moist bush or jungle
environments.
Transmitter circuitry
The circuit diagram is shown in
Fig.1. The modulation source for the
MCW mode is acquired by creating
a positive feedback pathway so the
October 2024 99
Fig.1: the ZC1 MkII transceiver circuit. The signal inputs (mic, line & key) are towards lower right, while the earphone outputs are just above those. The top half of
the circuit forms the transmitter, while the lower half is the receiver. They share the antenna at lower left.
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Australia's electronics magazine
microphone amplifier stage based
around valve V1G (6U7G) oscillates.
This is easily achieved because the
microphone, being a dynamic type,
requires a microphone-matching
transformer to drive the grid of valve
V1G. A feedback capacitor is switched
in to make the preamp stage oscillate
at 800Hz.
The 6U7G valve was used extensively in both the transmitter and
receiver sections. It made sense to use
the same valve type for as many applications as possible in the one radio to
save on carrying different spare parts.
Valve V1G drives the 6V6GT Class-A
modulator valve, V4B.
The transmit VFO (V1F) is another
6U7G, followed by a 6U7G buffer stage,
V1E, and a 6V6GT RF output stage,
V4A. Generally, a 6V6 can generate
around 2-4W of RF (or audio) output
power in a single-ended application.
These valves were also popular in
domestic radio audio output stages
and as guitar amplifiers.
6U7s are a very capable RF pentode, described by RCA as a “Triple
Grid Super Control Amplifier”. This
means they are suited to applications
involving AGC circuits and gain control. They were also a common valve
type in the 1940s era. It was said that
the 6U7 was the most common valve
to find in junk sales in NZ. The 6U7
is very similar to the 6K7 found in
domestic radios of the time.
The 6U7 was abundant in Australasia and had many manufacturers
besides the usual RCA, Kenrad and
National Union brands. Australian
Philips made them, too, for the Department of Defence, and supplied them
in very attractive boxes with Art Deco
artwork (see Photos 7 & 8). The logo
engraved on the 6U7G valve base in
my set indicates it was made for the
Australian Department of Defence.
Receiver section
The receiver in the ZC1 is a single
conversion AM superhet radio with a
BFO (beat frequency oscillator) added,
based on a 6U7G pentode, V1D. The
valve lineup is a 6U7G RF stage (V1A),
a 6K8G triode-hexode converter (V2A),
a 6U7G 465kHz IF stage (V1B); a 6Q7
detector, and first audio preamp stage
V3A.
The receiver’s sensitivity was quoted
as 1.5μV at 8MHz, varying above and
below that over the bands a little, being
3μV at 2MHz. However, the output
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Photo 7: the Philips valves for this set
came in decorative cardboard boxes.
Photo 8: the original 6U7 variable-mu
pentode.
level was not stated; it probably was
around 50mW into the headphones or
a 100W dummy load.
The audio output stage is only
designed to drive headphones, so
the designers deployed yet another
6U7G RF pentode, V1C, in a triode-
connected configuration to act as the
audio output valve.
The audio output power of a ZC1
is a mere 50mW with low distortion,
although it will deliver 150mW with
significant distortion, pushing the
6U7G RF valve to its limits in this
application. This result is satisfactory
for the 100W headphones used and for
speech but is not good enough to drive
an extension speaker or music.
Some historical articles mentioned
distortion in the audio. The main cause
for it, aside from the non-linearity of
the grid voltage versus anode current
transfer function, is that even by 100150mW, the 6U7G’s G1 grid is drawing
current due to the high drive level
exceeding its bias voltage.
Restoration
I had replaced the electrolytic
capacitors in my ZC1 radios over
30 years ago. The other capacitors,
which included wax-paper types and
moulded mica types, were still in good
condition when the radio was 50 years
old, but that was 30 years ago. Now
those capacitors are about 80 years old.
On re-testing them, I found that
all the capacitors had deteriorated,
including the mica types; nearly all
had developed measurable leakage.
While the wax-paper types fared better than most due to being immersed
in oil inside steel canisters, over time,
the rubber seals failed where the canister and the phenolic end disc mated
together, and the lower molecular
weight part of the oil or wax started
to leak out.
Many of the mica caps in the ZC1
were custom-made by Radio Corporation, while others were American
types made by El-Menco. These were
also amazingly good for their age. The
ZC1 MkII also used three 1in (25.4mm)
diameter twist-lock electrolytic capacitors.
In vintage radio restorations, people often replace the original chassis-
mounted capacitors with radial or
axial types under the chassis. I don’t
subscribe to that, as it looks non-
original and messy.
New twist-lock capacitors are sometimes available in that size. Of late,
though, they have been more difficult to acquire, so now I re-build them
instead. I start by machining out the
base of the capacitor using a lathe. If I
find any latex rubber, I discard it and
clean the inside of the canister, as latex
can contain halides, which attack aluminium.
I machine a 10mm-thick plug from
phenolic material to fit the hole I created in the capacitor’s base. I then cut
two M2 threads in it for screws and
lugs. I also drilled 1mm holes beside
those screw holes to pass the wires
through from the replacement electrolytic capacitors – see Photos 9 & 10.
I glue the plug in place with 24-hour
epoxy resin. Don’t forget to label the
polarity of the pins before gluing! To
do that, I drill a small countersink and
fill it with a dot of red paint. When a
multi-section part is required, I stack
the capacitors on top of each other in
the canister and add more terminals.
Replacing the wax-paper
capacitors
There are many wax-paper and mica
capacitors in the ZC1. I replaced the
mica capacitors with new resin-dipped
18.7mm diameter hole
10mm thick Phenolic
plate (18.6mm diam.)
Panasonic 47μF 450V (18.1mm diam.)
Photos 9 & 10: after replacing the electrolytic capacitor within the can, I glued
the end back on. The new eyelet tags are soldered to the capacitor leads.
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Australia's electronics magazine
Photo 11: soldering the end onto one
of the wax-paper capacitor cans.
October 2024 101
◀ Photos 12 & 13: end
caps for the waxpaper capacitors
made from PCB
material and the
finished capacitors.
Fig.2: an easy way
to add an extension
speaker to the ZC1
MkII.
500V silver mica types and the wax-
paper types with polypropylene film
capacitors, fitted inside the original
metal canisters.
When replacing the wax-paper
capacitors, I found the best method
was to first desolder the internal
capacitor wire from the eyelet/tag at
the end with the phenolic insulator.
Then, holding the capacitor (with
protective tape around its body) in
the lathe chuck, I carefully go around
the circumference near the far end
with a junior saw to create an initial groove.
After that, I cut the end off with the
saw and slide the capacitor contents
out of the canister. Next, I drill out the
rivet and tag in the phenolic insulator
and discard them. These tags were in
poor condition; the brass was quite
brittle where it was sharply folded,
and prone to cracking.
After that, I smooth the end with a
file while rotating in the chuck, then
smooth it further with 400-grade sandpaper. Once ready, I fit 1/8in (3.175mm)
diameter silver-plated brass eyelets to
the phenolic end.
I use fibreglass PCB material to
replace the end that was cut off. It is
easily cut into discs using a 22mm
diameter hole saw in a drill press.
I then make a 1/8in central hole and
attach a screw and nut to secure it.
I then used the lathe to machine the
perimeter down to 16.8mm, to be a
close fit inside the end of the metal
canister. I fit the same eyelet type to
this end cap, visible in Photo 12.
The replacement capacitor is prepared with a phenolic spacer and
some Scotch 27 fibreglass tape, so it is
a firm fit in the original canister. I then
recess the discs about 0.5-0.8mm into
the end of the metal canister before
soldering it. This way, a small well
for the solder is created between the
canister’s edge and the eyelet projecting from the copper side of the PCB
material.
Polyimide tape must be wrapped
around the capacitor body, right up
to the edge being soldered, or the solder will track down the outside of the
canister, spoiling the appearance of
the capacitor body. I use a soldering
iron set at 400°C to heat the edge of the
canister all the way around initially to
create a strong bond, then fill the well
with more solder.
The same principles apply to
re-building the 200nF capacitors,
except I initially used a 25mm hole
saw to make a larger disc.
I decided that having flying leads
on the capacitors was a better way to
mount them than the tags they once
had.
An interesting finding while restoring these capacitors: the 20nF types
were custom-made by Radio Corporation with a brown paper valve over
them inside the canister, also filled
with wax.
They must have been running low
on their own production because
one of these four capacitors had an
American-
made 20nF 600V “Dwarf
Tiger” capacitor hiding inside.
Replacing the mica capacitors
Most of the mica capacitors that had
become leaky were American-made
El-Menco parts. One was made by
Radio Corporation in NZ.
Photo 14 shows the underside of the
Photo 15: the
custom 12V
DC power
connector
used by the
ZC1 radios is
now hard to
obtain.
Photo 14: the underside of the chassis is pretty neat; it was made to be easily
serviced. Most resistors have already been replaced, as the old ones were way
out of spec.
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Australia's electronics magazine
Photo 16:
my newly
manufactured
replacement
12V DC
power cord
for the radio.
siliconchip.com.au
ZC1 after re-capping it. In the past, I
had replaced nearly every carbon resistor, except just a few, as they measured
way out of spec.
As well as many resistors having
gone high in value, one 50kW power
resistor was open-circuit. I carefully
removed the paint to inspect it to find
out why it happened. It turned out
that there was a discontinuity in the
carbon film.
Optimising transmission on the 40m and 80m bands
The RF output impedance of the ZC1 best suits long wire antennas. I found that by using
an impedance-matching transformer (an ‘unun’) with modified coupling to the output
coil, the output could be optimised for a 50W load. This also makes measuring the output power with standard equipment very easy. It requires the addition of two capacitors
inside the unit and the unun outside.
The capacitors are selected with positions 10 & 9 on the switch, as shown in Fig.c.
The unun matches the resulting ~12.5W output impedance to 50W (Fig.d). The Amidon
core and wire (see photo at the bottom of the panel) come as a kit (AB200-10). With
this arrangement, 2W is easily delivered to a 50W load on 40m and around 3W on 80m.
The 12V power cord
One of the tricky parts to get for the
ZC1 these days is its polarised 12V DC
power cord and plug. The original type
was a substantial black phenolic connector with two large-diameter rubber-
covered wires – see Photo 15.
I used my lathe to hand-make a compatible 12V plug from some phenolic
plate, machined brass inserts, electrical insulating valves and brass rod
– see Photo 16. A friend in the USA
also made a CAD file to 3D print this
connector.
Making an extension speaker
As noted, the ZC1 uses a 6U7G radio
frequency valve (triode connected) as
the audio output amplifier. The designers pushed this valve to near its maximum ratings: a plate dissipation of
up to 2.25W and a screen dissipation
of 0.25W.
The 2kW cathode resistor for the 6U7
can be reduced to 1.8kW to gain a little
more power, which is in the range for
the specification of the original carbon resistor. If the valve is exchanged
for a 6K7G, which has higher plate
dissipation but is otherwise similar
to a 6U7, the cathode resistor can be
lowered to 1.2kW, which gives a good
improvement.
I wanted to keep the set original
but add an extension speaker. It is
best to match the speaker with a small
autotransformer, the design of which
is shown in Fig.2. The taps can be
selected to suit any speaker impedance (the impedance ratio is the square
of the turns ratio). At this low power
level, the laminated iron core transformer I used has a flat, undistorted
response from 50Hz to 20kHz.
I mounted the matching transformer
inside a speaker box with a spare 32W
speaker – see Photo 18 (shown overleaf).
Other options to increase the audio
output power include moving to a
higher power rated valve such as a 6V6
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Fig.c: this simple modification to the coil switching arrangement can be used with an
external impedance-matching transformer to obtain good performance into a 50Ω load.
Fig.d: this ‘unun’ matches the 12.5Ω output
impedance of the modified radio to a
standard 50W antenna.
Right: the autotransformer that adapts the modified set’s 12.5Ω antenna impedance to 50Ω
is housed in a small diecast box.
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October 2024 103
or 6K6. However, that requires modifying the radio’s circuitry, and the small
output transformer’s primary current
can only be pushed so far.
According to the data sheets, transformer T1A’s primary has 3000 turns
of 43 SWG wire, which has a current
rating of only 18mA. Another option
is an active external speaker.
Adding a frequency counter
Photo 17: the frequency counter connects via the lamp socket on the front,
modified to pass enough of an RF signal for this to work.
Accessories
My ZC1 headset
and microphone.
The headphones’
cord is a little
frayed but both
still work fine.
The ZC1 came with several accessories, many of which supported its
use as a ground station – see Fig.e.
The minimum requirements, aside
from the batteries and the antenna,
were the headphones, microphone
and Morse key.
The headphones and microphone (see Fig.f and photos) are
both dynamic types. They use the
same dynamic inserts with a DC
resistance of around 40-45W. The
ones in the headphones are wired
in series and have a total resistance
of around 95W and an impedance
close to 100W at 1kHz.
Other items included a remote
control box for the radio (Fig.g), the
whip antenna kit, the battery pack
and a spare valve kit containing every
valve plus a spare V6295 vibrator.
There were also two 6V lead-acid
80Ah batteries in wooden boxes.
The remote control allows the ZC1
to be operated 100m away via a connecting cable. Two remote control
units could be used, and the operators could talk to each other like a
telephone link. The remote control
units came with a satchel to carry
the microphone and headphones.
An add-on power amplifier, type
ZA-1, was an option. It incorporated
type 807 power valves to boost
the RF power. Not nearly as many
booster amplifier units were made
as the ZC1 radios.
104
Silicon Chip
There is a connector on the front
panel of the ZC1 to power a reading
light. One of its connections is via a
resistor.
Adding some coaxial cable and
small coupling capacitors into the
radio allows the signal from the Transmit and Receive VFOs to be exported
via that connector – see Fig.3 & Photo
17. This modification does not alter
the original function of the front panel
lamp socket.
The dynamic
microphone
insert (at upper
left) is easily
removed from the
handpiece. Two of
the same inserts
are used in the
headphones.
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Fig.3: adding a couple of small capacitors and some coax allows the front
panel light socket to be used for monitoring the LO or transmitter frequency
with an external frequency counter.
Due to the low values required for
the coupling capacitors (1.1-2.2pF),
the set barely requires retuning after
adding them. The C7G and C7H trimmers can be adjusted on the transmit
side and C7C and C7B on the receive
side (L/O) to fractionally reduce their
capacity if required, but I found it
unnecessary.
The capacitance of the coax forms an
Fig.e (above): some of the available
ZC1 accessories.
Fig.f (right): the microphones,
headphones and Morse code
key available with the set. The
microphones and headphones used
the same type of dynamic insert.
AC voltage divider and transforms the
impedance. The presence or absence of
the external frequency counter results
in a negligible effect on receive or
transmit frequencies.
Since one of the connections on
the lamp circuit is to positive and not
ground, it is a good idea to put two
DC isolating capacitors in the banana
plugs in case the chassis of the frequency counter and the ZC1 chassis
come in contact.
In receive mode, the peak voltage
is only 30mV; not all counters could
work with that low a level and might
need a buffer amplifier. My counter
has an internal buffer/amp. In transmit mode, the output level is higher
at just over 200mV peak.
The frequency counter can be modified to switch out its 465kHz offset in
transmit mode to automatically show
the correct receive and transmit frequencies without manually switching
the offset on the counter.
Conclusion
Fig.g (left): up to two remote
control units could be used with a
ZC1 radio. They could be located
100m or more away from the
radio, connected by wires.
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Photo 18: the completed extension
speaker. The impedance-matching
transformer is also inside the box.
Australia's electronics magazine
The ZC1 MkII radio is a masterpiece
of high-quality radio engineering and
a very impressive feat for New Zealand’s wartime radio engineers. It is
so well built that many are still functional 80 years on.
As expected, the capacitors and
resistors deteriorated over that time
frame. In my ZC1 radios, all the coils,
transformers and original valves
remain in good order.
The radio is an excellent, sensitive receiver for shortwave listening. It remains one of my favourite
radios. Unfortunately, many that were
deployed for Marine use rusted significantly, but with enough work, that can
also be remedied.
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October 2024 105
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