Wireless ZC8: A number eight set for New Zealand – Part 2

By Kelvin Barnsdale ZL3KB
Originally published in Radio Bygones June 2015 Adapted for NZART Break-In. Thanks very much for allowing us to republish this article.

In part one of this article, we looked at the origins of this unique, New Zealand made back-pack radio from the second world war. In this second part, we will see how one example, now over seventy-six years old, was resuscitated to warm the airwaves again. Breathing life back into its circuits without damaging its authenticity is a delicate path to tread.

A surviving ZC8
I received this ZC8 example, serial number 54, from a long-deceased estate of a Christchurch ham radio operator, and on initial inspection, it appeared in clean condition, but with few accessories. It had no head-phones or aerial, except for the base. To help me through the restoration, I, fortunately, had the instruction manual and circuit diagram. However, a nasty surprise was waiting for me when the battery compartment was opened, it became evident the internal foam padding had decomposed into a molten “goo”, covering the internal headphone/ battery connection cable, and the microphone that had been stored in here too. Resembling a sticky, black pitch like substance, it was obvious I had to clear this mess as the battery cable was trapped, which prevented the removal of the radio chassis. Some thought was given to my approach to this restoration, and the only way forward was to try the removal of this mountain of goo, with microphone, in one lump.

Using a trick I use when removing large globules of hot melt glue, I heated the bottom of the case with a hot air gun to melt the interface between the “goo” and the metal, and theory says the bulk would just lift away. It partly worked, allowing the cables to be pulled clear, and the microphone and plug be removed, albeit in a gummy black mess. Many hours of scraping later, with several applications of petrol, methylated and white spirits, and with hands ingrained with sticky bitumen, the battery compartment was looking better, the microphone and plug were rescued (hooray), but unfortunately the cloth-covered cables were “a goner”.

The radio chassis is freed
Once the battery/headphone cable was released, the radio was removed from the case, this time revealing a pleasant surprise. The chassis and components were in excellent condition and looked like it had not been touched since it was made. However, the rubber insulation of the wires had suffered from old age; when the battery and headphone wires were moved the rubber insulation cracked and disintegrated like an Egyptian mummy. Therefore the first job was to replace these in a way sympathetic with the original construction. I looked for rubber\ covered wire with no luck, so I used a silicone insulated wire which had the same “feel”. Strangely, the old battery wires had no evidence of\ plugs, indicating the batteries used (Eveready 762 HT and C116 LT) had screw terminals or clips. As the mechanical remote control cable was missing, I fabricated a shaft and knob to fit into the side of the set chassis, so it could be activated
manually. With the power cable repaired, I made some careful measurements of the supply resistance. All looked OK but an intermittent connection was found in the power contacts of the main wafer switch, and that was buried deep behind the front panel. The only thing that could reach it was to switch cleaner spray. Luckily, after a tsunami of “no more dodgy contacts”, the connection eventually came right, but the low-quality wafers will forever be saturated. When trying to gain access to the wafer switch, I removed the meter plate from the front panel. To my surprise, this revealed the meter had never had any nuts fitted to its connections, with the meter shunt just pushed onto the threads; this must have been a rush job! Risking detraction from originality, I fitted some 4BA nuts, tarnished brass of course.

Now for the receiver
A jammed volume control was a fault evident from the moment I received the set, so this was the first job to be addressed. The knob was removed, thankfully the grub screw did not fight back, but no amount of twisting would rotate the shaft. Despite the connecting wires being very brittle I carefully removed the pot and removed its backplate. It was
a design I had never seen before. The wiper was not a contact, but a fibre button pressing on the inner surface of a steel ring, which in turn pressed onto the carbon track. This seems a great idea as there is no wiping action to wear out the track. Much disassembly was needed to eventually find that the shaft was seized into the bush, but when it was extracted no sign of seizure was seen, just shiny metal like it was born yesterday! It was reassembled with a dob of grease and it worked like new.

I have some self-imposed rules when restoring old radios; in the interest of maintaining historical integrity, I always try to minimise the disturbance to the set when “getting it going”. For instance, I never change components unless
they are proven bad, I never disturb solder joints or their inspection paint unless there is no alternative, and when changing components I always leave the original connected by one leg. I know this sounds strange, but I feel we have a responsibility to the future owners of the set to keep it as genuine as possible, and at least keep the original components. That being said, I was delighted when the receiver worked the first time which no tweaking at all, and I
have to attribute this to the use of American components everywhere, as these seem to survive much better than British components of the same era. This was also experienced when restoring the British WS18 and its American brother the WS48, and is plainly indicative of the production hardships experienced in Britain at that time. I measured <2uV PD sensitivity for 10dB SNR using the aerial coupling of 50pF as per the user manual. The auto-bias resistors R17/R18 were slightly high so I shunted them with new resistors to the correct values.

Transmitter
This section caused me the most head-scratching. The output power was initially only 50mW, whereas I was expecting 200mW based on the WS108 spec. I had no spare valves, but swapping the oscillator and PA valves showed no improvement, so I assumed the valves were OK. A few capacitors were replaced with new types (leaving the old ones hanging by one leg, for the next generation), and I heated the PA tank to get rid of most of the moisture laden wax, but still no improvement in output. Eventually, by tweaking the antenna coupling capacitance to the external power meter I managed to see 100mW, so that would have to do for now. At some stage, I will need to look at this again, maybe replace valves, check for RF loss in the insulators, or look at the drive level from the oscillator.

String problems
Unusually for a military set, the transmit and receive tuning uses dial cord for its slow-motion drives; with the receiver having a locally made die cast wheel. Maybe it is inaccurate or the shaft groove is the wrong shape, but on this set the dial cord does not run easily, sometimes overlapping itself or jamming at either end. I noticed the wheel was not aligned with the shaft groove, so I filed and refitted the drive wheel, but it showed little improvement. This will have to wait for further inspiration.

Headphones and Microphone
The headphones were missing, but my colleague Ross Jowitt notified me of a suitable pair on an online auction. These were Brandes high impedance domestic phones which were used in this set, and I was lucky to win the auction. As described above, the microphone was rescued from the goo but the two core cloth covered cable needed to be replaced. After a long search, and requests to all my colleagues, I found the ideal cable in my junk box! As in many army sets the carbon microphone capsule was red, but I have been told by GPO engineers that red capsules were for high voltage systems, so that remains a mystery to me. The jack sockets had been locally made, hewn from some fibre rod, with sprung buttons for contacts, and up to now, they have proved reliable. One sad discrepancy was the hole size for the boss of its No 9 headphone plug was not drilled large enough, so I had to rectify that. This set was starting to get
better than new!

Batteries
The operator’s manual specifies the batteries as two 45V Eveready type 762 in series and one 1.5V type C116. Particulars for the former are readily available, but the C116 has defied all my attempts to trace its details, and I wonder if it is a misprint. By scaling a photo of the WS108 batteries, I concluded the LT battery is very similar to Eveready type 742, or maybe AllDry4. The other strange evidence is the lack of plugs on the ends of the original battery leads, making me wonder if the original batteries were fitted with Fahnestock clips. Just imagine having to change batteries in a field of mud, with it raining, blowing a gale, and someone shooting at you! It must be even worse than the WS18 set battery change. The manual mentions a battery connection diagram on the battery door, but this set had no sign of the diagram remaining. Compared to the normal army sets, the big difference with this is the type of supply used, being domestic Eveready batteries. I normally fit inverter/switch mode supplies in my sets, but as this radio used three separate batteries I chose the good old Zinc Carbon solution for once. Never having made “faux” batteries before, I thought this would be an interesting exercise to try. The box structures were made of 3mm thick MDF, glued together and were fitted with five 9V batteries to give 45V for HT, and the LT had a D cell installed. The only image I could find of Eveready 762 and 742 batteries were very poor photographs (1), so I took these into Corel Draw and redrew the patterns, matching the fonts as best I could. Despite the final result being quite acceptable, I realised eventually it was a little futile as the batteries are tucked away in the bottom of the cabinet, never to be seen. As the photos of these batteries showed socket connections (the 742 has a Fahnestock adapter) I chose to fit plugs to the ZC8 and will puzzle over the “missing battery plugs” ambiguity in the future.

Final Testing
Once the bench tests were complete, the next step naturally is a field test, and what better set could I chose for the other end of the link than a ZC1 MK2 (3). As the aerial for this ZC8 was missing, I made up a substitute using four “B” section rods and a couple of 13” brass tubes to bring the size up to the 8.5mm diameter of the base spigot. We set up a short-range test, but this showed up another fault in the ZC8; when the signal was very strong the receiver became unstable. This also explained another fault I had chosen to ignore – the netting function caused all kinds of whistles
and squeaks in the receiver. Back on the workbench, I discovered the decoupling cap on the auto-bias was low capacitance, but changing that only partly cured the problem. It was also improved by adding 10k across the AF transformer primary. Running off a bench supply was now OK, but from my batteries, the instability was still apparent. I eventually cured it by adding an extra 50uF electrolytic across the HT rail, a requirement probably caused by the high internal resistance of my fake batteries. I did think about fitting the capacitors inside the battery cases, but could not bring myself to add decoupling so far from the load! Now I could inject signals up to 500mV with no instability. Back out
in the test range (garden), I had a fine business QSO with my son on his ZC1, and I wondered how long had it been since a ZC8 had worked with a ZC1?

Conclusions (so far)
I still feel there is more work to do on this set, the receiver dial cord issues are not sorted, and the transmitter output remains poor. Despite this, the set is now operating reliably, and will be taken further afield for some “DX” working, maybe up to a few hundred metres! I may also have a go at making a harness as per the original leather straps and belt.
In New Zealand we characterise our practical abilities and local fabrication with the term “number eight wire”, meaning we can make or fix anything with fencing wire; by chance, the ZC8 wire-less has the perfect number! B.

References
[1] <http://commons.wiki-media.org/wiki/File:B_battery_%28Eveready_-762-S%29.jpg>
[2] <http://commons.wiki-media.org/wiki/File:Abattery_%28Eveready_-742%29.jpg>
[3] Battery pictures <http://www.zc1-radioclub.org.nz/what-is-a-zc1.html>