[R-390] 200KC calibration crystal source

[Richard]

My humble opinion for what it's worth - Since the little crystals were
becoming very scarce even back when I got my radio I disconnected the heater
in the oven.  Seems plenty stable enough compared to the other radios on
hand.    I also have IERC tube shields to improve heat flow from the tubes,
a fan in the cabinet (home-made oak) , a variac for power-up, and an
isolation transformer.   A little paranoia can't hurt when dealing with a
classic.

73 de Richard W5SRB


> I have heard of one or two failures of the little plug-in oven: the 
> thermostat stuck ON, and overheated the thing. I n at least one case it 
> damaged at least one crystal.  Maybe a switch in that line would be a good

> idea - it runs on 6.3 volts filament power. The other ovens run on the
25.2 
> volt line.