[Contributed by Dave (davemc)]
Radio is a 40-216.
http://www.nostalgiaair.org/Resources/346/M0013346.htm
To align the remote control receiver, Service Notes say to pull the 2A4G thyratron and plug in a 45-2769 aligning adaptor. It is used with a VTVM. Looks like from the schematic that there would be DC on the grid of the thyratron – but not sure! So could this device be as simple as an octal plug with leads to the grid and one side of the filament of the 2A4G?
Anyone know for sure what this is?
Well, no one appears to have any information on this item.
I did some investigating and some experiments and came up with something that WORKS! I started with a dud octal G type tube, broke the envelope and removed all glass, cement and lead wires from the base. I connected a 1N34A point-contact diode to pin 5 of the base (thyratron grid). The other end of the diode was connected to one of the output leads. I bypassed the diode output to pin 2 of the octal plug using a 0.001uf capacitor and connected the other output lead there.
I then insulated the parts with tape and spaghetti and inserted a 1″ wooden dowel (with a hole drilled down the center for the output leads) into the socket, retaining it with small nails drilled through the sides of the octal plug and then used epoxy to firmly attach it. The dowel allows you to easily insert and remove the adaptor even with the radio in its cabinet during alignment (as recommended in the service notes).
After the epoxy dried, I snipped off the nails flush with the outside of the socket.
To use the adaptor, remove the 2A4G thyratron tube and its shield and insert the adaptor in its place. Connect the output leads to an analog VTVM. Connect up the signal generator as directed in Riders or the service notes. There is a fixed DC output voltage present with this adaptor. I used the lowest scale on the VTVM that would not peg the meter, then used the VTVM’s zero control to move the pointer as close to zero as possible.
The output level and frequency of the generator must be adjusted to get a reasonable reading on the VTVM. The trimmers can then be peaked.
I found that the trimmers on my 40-216 were DEAD ON as found (the various frequencies used are documented in Riders and in the service notes). However, my “Mystery Remote Control” box was tuned to a different frequency (it was not the original box for this radio). The adaptor allows the remote to be carefully tuned to the radio’s control receiver for best reception.
WARNING!!!! Note that the thyratron and stepper circuits are connected across the AC LINE, and are NOT isolated by the power transformer! This means that one lead of the adaptor will be HOT! I used an isolation transformer for the radio and took care to always keep one hand in my pocket during adjustment.