MLU FORUM - Wireless of the Week

Ever find one of these at a swap meet and say to yourself “what a nicely marked piece of wartime wireless gear, I wonder what it’s for?” as you were going for your wallet?

Most British and Commonwealth wireless sets of the WW2 period were variably tunable which means they could be tuned to any frequency within the range of the set but it was not always easy to tune to an exact frequency. Add to that the crude markings on the dials and the best a signaler could do is get close to his assigned frequency. That is unless he had a crystal calibrator. If he did, he could independently tune to a required frequency without the need for a long netting call (a netting call is a transmission from a single set tuned to a specific frequency and all the other sets on the net tune or ‘net’ to match it), and he could also accurately determine the frequency of any wireless station heard on his set’s receiver.

Most Commonwealth sets bigger than the small manpack sets had unique crystal calibrators and this particular one is designed for use with all marks of the No.19 set, however on Mk.I and Mk.II sets a slow motion frequency dial needed to be installed as the standard one was too coarse. Mk.III sets had the slow motion dial factory installed and one reason for the upgrade was to allow for the calibrator.

The crystal calibrator is a miniature sender which generates radio signals at equally spaced intervals throughout the range of the set. These signals are accurate markers that generate an audible tone or ‘pip’ at increments of even megacycles (now called megahertz), 10ths of a megacycle and again at each 100th of a megacycle. Actually two notes are heard, one on each side of a quiet position. The receiver is tuned to the pip when it is at the quiet, or “zero beat” point between the two tones. It sounds like someone whistling from a high to a low note, going silent for a moment then beginning a low whistle and going back up again. Go ahead, try it (as long as there’s nobody around to hear you).

To use the calibrator to set an exact frequency, let’s say 3.24 MHz, the operator would switch it on and let it warm up while removing the 19 set ‘A’ aerial cable and attaching the calibrator cable to the ‘A’ socket (see picture 4). He would switch the dial to ‘1000’ and, while turning the “A” set frequency dial, count up three pips heard in his earphones equating to 3 MHz. He would then switch the dial to ‘100’ and count up two pips, then repeat with the dial at ‘10’ and count up four pips.

The Crystal Calibrator is 9-3/8” deep, 2” long, 4-3/16” high and weighs 4-1/2 pounds. Two spare valves are stored in the rear of the case. The control dials are on the face along with a socket that receives a cable from a special 12 point supply unit to sender/receiver cable (a ‘dog bone’) and a lead that attaches to the 19 set ‘A’ aerial socket. The crystal calibrator case has a variety of holes allowing it to be mounted in various ways to the 19 set supply unit depending of the configuration of the set.

(Notice how I snuck in another 19 set?)