I've read that these old ovenized elements "cook out" and become unstable
after many years of reliable service. The rig starts to exhibit drifting
and requires frequent retuning to keep on frequency.
I recently replaced a few with modern TCXO DIP 20.000 MHz oscillator
packages; and I dissected one of the defective original ovenized crystal
packages.
It seems to be a very simple circuit. The crystal is heated by what
appears to be a plastic package transistor, and the temperature of the
crystal is monitored by a thermistor that is epoxied to the crystal case. I
surmise the thermistor adjusts the forward bias on the transistor
accordingly.
In the unit I opened, I discovered that the thermistor was no longer
attached to the crystal case; thus the thermal tracking would no longer be
accurate. I'd guess this could be fixed using a tiny dab of heat
conductive epoxy or other adhesive?
I'm just passing this along in case someone cares to experiment and report
back on whether this is a viable fix or not.
regards
Pete
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