I first saw this old receiver at an auction house in Christchurch about six years ago. It was painted in a sort of cream coloured emulsion over its original finish with plenty of rust showing through. The chassis, visible under its lift-up lid was red with rust and altogether it was in a very sorry state. I can't exactly remember what I bid for it but it probably wasn't more than a couple of pounds as old radios, only a few years ago, rarely fetched more than £15. After getting it home I decided to restore it to what I imagined was its original condition, being considerably helped by finding a picture of it in my 1938 copy of The ARRL Radio Amateur's Handbook, where it was listed at $49.50.
I dismantled the metalwork, removed the old paint and rust from the panels, and repainted them with a dark grey crackle finish Hammerite. After derusting the chassis with a proprietary gel, which I think, turns the rust into harmless phosphorus compound, I reassembled everything. The dial mechanism and gears needed cleaning and oiling and worked OK although the dial surface shows a lot of wear. Under the chassis the majority of the components were serviceable but to my dismay I found that someone had added a long wave band at the top short wave position, discarding the original coils, padders and trimming condensers in the process. This must have been when this particular band had been dead due to the vagaries of sunspot activity. Being keen on restoring the receiver to its original spec I decided to remove the long wave band and refit the original band which covered the unusual range of 17 to 44Mc/s. After some experimentation this turned out to be no mean feat! To carry out the reconversion I had to turn to my computer and Lotus 123, which, apart from its usual role of producing spreadsheets, allows complex equations to be processed. I managed by experiment, using a test coil and a grid-dip meter, to measure the minimum and maximum values of each section of the tuning condenser gangs. The position of the wavechange switch in relation to the tuning condenser and the grids of the valves wasn't too bad at low frequencies but very awkward at low VHF. I had to measure stray capacitances of leads and valve grids and the wavechange switch wafers etc. By developing an algorithm based on all the factors for each of the three coils (2xRF and oscillator) I was able to determine the required inductance of each coil. This included calculating values for trimmer condensers for tuning and a padder condenser for the oscillator coil. The latter is necessary because the oscillator tracks the RF coils by the IF frequency which I also fed into the equations. I also had to determine the proper side of the RF to run the oscillator although I recall, that given the known parameters, my equations told me this was only possible one way.
Eventually after much trial and error and by careful selection of components of the right vintage at the bottom of my junk box I wound the coils, got the oscillator to oscillate and the RF stages to amplify and NOT oscillate, everything turned out OK and the desired short-wave coverage was achieved. I have a little suspicion, although I am writing this after some years have passed, and looking back without too much clarity, I might have cheated when it came to overall performance and added a little extra amplification in front of the audio output valve. Who's going to look for a transistor under the chassis of a pre-war radio!
The cone of the old loudspeaker had perished and I remember fitting a new unit designed for a car radio. Apart from this and a new mains lead and 13amp plug (and I suppose a transistor) the restoration is fairly original although I never got round to finding a couple of aluminium extrusions missing from the sides.
For the reader's information the receiver has the following valve line-up: -
6K7 RF stage; 6L7 frequency changer; 6J5 RF oscillator; 6K7 IF amplifier; 6Q7 detector, AVC and AF amplifier; 6F6 power output; 6J5 BFO and a type 80 rectifier.
And the frequency coverage is again: -
Band 1 545Kc/s to 1800Kc/s
Band 2 1.7Mc/s to 5.8Mc/s
Band 3 5.8Mc/s to 18.8Mc/s
Band 4 17Mc/s to 44Mc/s