This spy set is known as Mk.XV and as Mk.16, while the Mk.16 is the later version. It is another English WW2 clandestine radio set for spys, resistance groups and stay-behind organisations. Other then the paraset, this radio was not well-documented. There is a description in the book "Wireless for the Warrior, Vol. 4, clandestine radio" from Louis Meulstee, PA0PCR. This book is highly recommended for everybody intersted in that topic. Here, you can also find the schematics of the set. Louis supplied the pictures in high resolution, so I was able to make simple drawings by defining the individual scales in every "layer" of the photographs. With some known dimensions (mainly the size of the front panels), it is possible with pretty good accuracy. So this was the basic step to get started. Thanks a lot, Louis! The coil datas were gained from the pictures and some infos from an owner of one of the few sets that survived the time. So thanks also, Edmond!
The front panels were made of 4mm phenolic boards. You can still buy them today. Buy phenolic textile, not paper quality. It's much more stable. The surface is very nice, but the known rub-on letters are not durable on this surface, and the same is known for the layers of clear varnish to protect the inscription. So I decided to go another way. I made a CAD drawing, using the software of the Schäffer company in Berlin. I sent them the panels, and they milled all holes, engraved the letters and filled the letters with white paint. Another advantage: They even had the old English letters in the fonts they provide with the software, so the results looks very close to the original.
Here the transmitter panel:
The next step was obtaining the parts. I decided to build the version "wooden box without lid" from the WFTW-book. The transmitter box was perforated with holes, while the receiver box was slotted. I used a mill to do that. To make it look perfect, you need the original knobs and scales. You can get it from Gary, KF9CM. He supplied the original meter with square flange and round scale. The made a new shunt to get it read 100mA, and supplied the meter face. Remark the sign of the air ministry and the year....nice details...
The knobs with scales on the transmitter are socalled "HRO-knobs". The HRO was the receiver used at Station X in England together with a Mk.III "Tinkerbox" transmitter. This parts, and also the scales and knobs for the receiver, can be found at Garys homepage: www.kf9cm.com
The transmitter is a so called MOPA....Master Oszillator Power Amplifier. Other then the paraset, 2 tubes are used. The crystal oszillator is supplied with a 6V6 steel tube, followed by a power amplifier using the 6L6. The big advantage: It is possible to double the frequency of the crystals. Crystals were rare and expensive items in WW2, and old-style power crystals are still hard to get nowadays.
Here the transmitter finished:
The receiver is usind 3x 6SK7 tubes. In German nomenclature it is a "1V1 Audion", what means we make out a preamp stage, followed by the detector stage and one audio stage. Frontend and detector are tuned by one common 2-section variable capacitor. Great care must be taken to make sure that the 2 parallel resonant circuits tune on the same frequency at all settings of the capacitor. You may need to add a trimmer capacitor on one resonant circuit, and fine tuning is done by bending the slotted side rotor-plates of the tuning-C. Start with 40m, and bend only the "used" rotor sections on 40m that match with the stator. On 80m, only bend the sections that come in the game when using more C on 80m. So you can get equal filter curves at the input stage and the detector stage over the whole bands! Doing this properly is the key to get a nice working receiver. The L/C ratio in this set is lower compared to the paraset. For this reason the detuning by warmup effects in the valves are not affecting the receiver very much. It is much more stable compared to the paraset, but I have the feeling that the selectivity is not so good. It's very sensitive, no doubt, but the problems in nowadays crowded bands are obvious.
The top scale is the main tuning, the lower one the fine-tuning knob. Both use 1:6 vernier drives. You can use this receiver on amateur bands without modification! Try this with an original paraset!
Look at the slotted rotor panels! On the right you see the front-stage, while the other tubes on the left are the detector and the audio stage.
Here the complete set with PSU:
I am on the air with all my spy-sets! So when you call me, and I talk about QRM....my receiving window is large like a gate....please be patient! When you detect some chirp, you know I am working with modern crystals. BTW: I always look for wartime crystals in the CW part of the 80m and 40m band.....FT-171, DC-30, DC-35 (all 19mm pin spacing) or FT-243 versions.
See you on band!