ANCIENT MAINS RADIO

I first saw this radio in Birketts, which is one of the few remaining government surplus radio shops around (it's in Lincoln at the bottom of the steep hill leading to the castle and the cathedral). I thought it was there for display but on enquiring and after a bit of haggling it was mine for £40. The radio was built in what looks like a dark oak chest of drawers from which the bottom drawer has been removed. The centre drawer has been taken out and its front panel cut and refitted with a centre-mounted loudspeaker, behind which was presumably mounted the power supply, which has long since disappeared. The top drawer has been modified by removing its sides and rear panel and its base then covered with a sheet of copper to act at a baseboard. To this are fitted the components including the valve-holders. Tucked in behind the speaker I found a letter and various documents including a hand-drawn circuit diagram, data sheets for the commercial coils and valves etc. The set was said to have been made by a Mr Robert Laurence of St.Catherines Grove, Lincoln and was apparently given by a Mrs Doris Mackenzie of York to Lincoln College? Presumably from there it eventually found its way to Birketts. The set is a TRF type and uses a collection of early "mains" valves with indirectly heated filaments and a number of heavy, metal-cased, Wearite coils. The tuning condenser is a three-gang type with a very smooth slow motion tuning dial calibrated 200 to 550 metes and 700 to 1900 metres together with a logging scale. The set hadn't been touched for donkey's years and was thick with dust. After hoovering (I always use blow rather than suck because this works better- out of doors of course!) and once the bits were visible, I discovered the original construction was more hurried than reliable and I found numerous minor reasons why it would not have performed well. Also, one of the interstage transformers was open-circuit which is often a problem in sets of this age, probably because the very thin copper wire of the high impedance windings, in contact with the iron terminals and a touch of damp results in electrolytic action, corroding the copper wire. The valve line-up is as follows:- RF amplifier: Standard Super Valve, VM/SG AC Detector: Standard Super Valve, Detector AC LF amplifier: Mazda AC/PEN The first two are thoughtfully marked 4v 200v/100v and 4v 200v respectively Other components used in construction are:- Three Wearite "Universal Coils" Large air-spaced 3-gang tuning condenser Type X-673 dial and slow motion drive assembly Dubilier Type 9200 1uF 500v condenser Two C29 234 No 102 2uF condensers Dubilier Type 40 HF choke 2uF 500v bakelite condenser Sovereign rheostat Volume control with switch Telsen 0.00015uF reaction control Telsen 0.0003uF aerial coupling variable condenser "Utility" wavechange switch Two small Dubilier Type 655 condensers Graham Farish Ohmite resistor Three paper tubular condensers Small RF choke Six various types of resistors Interstage coupling transformer, Gecophone BC725 4:1 Overall dimensions of the case are 27" wide 15" deep and 31" high. The baseboard is 24" x 10.5" and the front panel 6.5". The loudspeaker is about 7" in diameter. The next step is to build a suitable power supply and then to get it going again after 70 years. I measured the filament current currents, using a 4volt DC supply, of the three valves as 1.07, 1.18 and 0.88 amps respectively. The circuit diagram indicates the HT requirement as 300volts (I wonder how many times the constructor inadvertently grounded the HT with his fingers?) Unfortunately I binned all my old "4volt" mains transformers years ago before valve radios became desirable objects so it may be a long wait before I get round to making a power supply.

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