Master Oscillator Tuning Circuit
Finally, my first look inside this section of the Sender chassis. What struck me was how clean everything was, with little or no dirt and most important, no sign if heat discolouration or burnt smells lingering about.
The most notable thing was the pristine looking L31A COILS, RF sitting front and centre, complete with its CMC Part Number stamped on the side of it – CMC 119-209. I am not sure what prompted me to do it, but I decided to run that number against the Parts List at the back of the Operators Manual. Did that ever turn up the excitement!
According to said Parts List, L31A had Part Number CMC 119-107. The number stamped on the assembly in my Sender was nowhere to be found. So next stop was the 1945 Illustrated Parts List. It showed L31A as being, COILS, RF, 1.75-4-mc, MO, No. C1. It had a 1-1/32 inch diameter with two riveted 6-32 mounting screws at one end and no sign of a central, adjustable core. The ZA-Number reference given was ZA/CAN 4355.
A pail of tea later, I decided to check the 1948 Issue of the Illustrated Parts List for a possible explanation. Bingo! The same reference for ZA/CAN 4355 was found, but this time, directly below it was a second item, ZA/CAN 4221. It was a stink smaller diameter and was fitted to its mounting Bracket, that also held the central, adjustable iron dust core. So this complete assembly bears the CMC 119-209 Part Number.
My thought is the original design, in theory, allowed for replacement of either the coil, or the core, but the amount of work required to do either, was more than making the Coil and Bracket a single item replacement. Interestingly, the Bracket itself does not show up at all in either the 1945 or 1948 Illustrated Parts Lists. Having confirmed at this point that all the components in the Master Oscillator Tuning Circuit were replaced during the Senders last overhaul when in service, I then traced the connections to Pin 1 on the V5A Holder (socket) and found nothing worth a red flag. To be on the safe side, I retested the resistance of this Pin circuit to ground after getting a good clean contact point on the pin and now found the reading to be a steady 1.25 meg. I liked that reading and took the time to recheck Pin 1 on V5B and V5C in the same way. They both now showed 1.25 meg as well. So I was glad in the end to have opened up the shielding plate for this circuit and had a closer look.
The surprises were not quite over, however.
David
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