Master Oscillator Tuning Circuit
I tend to be very careful when taking on something new, that I know nothing about and gaining access to the metal box hiding this circuit is a prime example of that action. I had no idea at all what was inside, or where and how it was mounted. I could see an access hole in the lower rear chassis wall for purposes of tuning the coil, the V5A Holder (socket) on top of the lower chassis and that was about it There also had to be a switch lurking inside somewhere for purposes of selecting Crystals.
The first discovery was this piece of metal did not exist in the Illustrated Parts List at all, first or second Issue. It was not a cover, a plate or a shield. Not an encouraging start. I then thought about the existence of the Modification Card Holder fitted to the bottom and wondered if the installation instruction for the holder might shed some light on things. It did. It was called a ‘Plate’, four screws held it in place (2 top and bottom in the photo in the previous Post), and the Plate was to be removed carefully.
After looking closely at how the Plate was installed in the Sender chassis, I could see the outside end was slipped in behind the lower left rear side of the Sender chassis rail, in behind the heavy reinforced rail the Sender slides in and out of the Carriers No. 4 on. The inside end of the Plate just slipped in beside an adjoining chassis compartment wall. So, with the Sender resting on its right side, putting the heavy rail at the top, I removed the four retaining screws and lock washers. The plate was a snug fit. I had to very carefully insert a slot head screw driver blade just into the open slot either side of the Plate at each lower side and ease the Plate out enough to be able to grab the lower edge with my fingers and pull it free, but not out. While holding it in place, I then slid the screwdriver blade carefully up the slot to the top of the Plate to keep it flush with the chassis rail. If this end of the Plate slipped inwards, it hung up on something unknown. Once you can clear the inside end of the Plate from the partition wall it attaches to, you can move the lip of the Plate inwards enough to allow the upper end tucked behind the chassis rail to drop free, and remove the Plate.
The four photos here illustrate the location of the four screws the Plate is mounted to the Sender Chassis with, what the bottom of the Plate looks like once removed and what the inner (top) of the Plate looks like, full of dirt and dust bunnies.
The real surprise in this exercise was the discovery of the hole, roughly centered in the Plate, which is effectively blocked by the Modification Card Holder. My initial though was it being another access hole for tuning something. NOPE! Turns out it is a ‘finger hole’ thoughtfully designed into this Plate by Canadian Marconi to make removing and installing this Plate an easy task to accomplish. My guess is that any surviving Senders in the UK will still have an easy time working with this Plate. Canadian owners, not so much.
David
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