Gillies6
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Replica rotary knife switch. detectors that could easily be switched into the circuit to.see which type gave the best signal in the headphones. I was lucky enough to see a later model Telefunken crystal set being restored in the conservator’s workshop of the German Museum of Technology in Berlin, Germany, when I visited in 2008. I was able to examine it closely to see how it operated and take many photo- graphs. When I returned home and examined the circuits I had photocopied from the museum archives, I was able to determine eventually just what every- thing did. The rotary knife switch, at the rear of the receiver unit is not visible in any previous photographs and caused me some trepidation as to whether I could replicate it. On the same operating arm was a lever that operated the aerial changeover switch. It had a heavy arm which was used to make sure there was a good con- tact between the aerial and the trans- mitter in the transmit mode. It took some figuring out, using the circuit drawings and the photographs I had taken, how the chamgeover switch worked. My replica arm is a mixture of the earlier 1911 model and the 1917 model that was being restored in the Berlin Museum. The rotary knife switch has a num- ber of banks: three banks switch the two crystals out of the circuit completely in the transmit mode, probably to prevent any damage to the crystals; two switch Rotary knife switch components. Page 16 - Aurora - June 2012 Insulator and aerial changeover switch on the Berlin Museum unit. the 220-volt power to the morse key and transformer, in the transmit mode, pre- venting power being applied to the transmitter in the receive mode; and one bank removes the earth from the crystal set in transmit mode, an extra safety feature. The arm that operates the aeri- al changeover switch, also carries the aerial to the crystal set in the receive mode. I cut the rotary switch blades from the copper sheet that was used for the heatsinks. They had a square section hole cut in the centre, which fitted along a square section of paper bakelite, so that they all rotated together. They were separated by short sections of Delrin tube. The pivot went though the centre of the square section piece of paper bakelite, which insulated them all. The angle of the operating handle took a bit of trial and error until I was happy. The handle is attached by some small grub- screws. The fixed contacts of the switch were fabricated, again with a lot of trials, using bent pieces of brass strip, until I had the dimensions right, to make a jig to bend all the pieces of brass strip to the same shape. I used brass shim, nickel-plated before assembly, but the original was probably silver-plated phosphor bronze to maintain the con- tact pressure. The aerial changeover switch on the model being restored in Berlin, was mounted on a large porcelain insulator, with special fittings to hold it in posi- tion, and hold the switch clear of the Delrin insulator and aerial changeover switch on replica wireless. Coil coupling rotator and tilting axle. rest of the unit. I could not find anything like this, so I turned a large diameter section of natural (white) Delrin to a similar shape. The aerial switch weighted arm was milled from a square section of brass, after numerous trials were done with wooden arms to get the shape correct to do the job. The pivot that is fixed to the insula- tor, was milled from another piece of brass and later nickel-plated. I was fortunate that one of my sons worked in a factory where I was able to use their drill/mill machine and a small lathe for those parts. The receiver coils The Telefunken crystal set was made to cover a number of wavelength bands with different coils that could be inter- changed. The secondary coil could also be swung upwards to vary the coupling between the coils. To change this coil, it had to be swung up to clear the primary coil in the centre, then rotated through 90 degrees to enable it to be unplugged. This mechanism took some figuring out from the photographs I had taken in Berlin. It was eventually made with some plumbing pieces welded together. A retaining ring to prevent over-rotation, which would break the wires to the set was made from a section of brass tub- ing. Mini Leyden jars for aerial matching circuit.