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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 96 Silicon Chip 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 siliconchip.com.au 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 siliconchip.com.au 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. Australia's electronics magazine 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. 98 Silicon Chip 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). siliconchip.com.au 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 Australia's electronics magazine 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. 100 Silicon Chip 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 siliconchip.com.au 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. siliconchip.com.au 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. 102 Silicon Chip 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 siliconchip.com.au 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. Silicon Chip kcaBBack Issues $10.00 + post January 1995 to October 2021 $11.50 + post November 2021 to September 2023 $12.50 + post October 2023 to September 2024 All back issues after February 2015 are in stock, while most from January 1995 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com.au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 Australia's electronics magazine 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. Australia's electronics magazine siliconchip.com.au 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. siliconchip.com.au 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. SC October 2024  105