I may not have mentioned that I think I figured out the underlying
problem with this OCXO lies with the trimpot in the oven control
circuit. (I believe that someone mentioned this as a possibility, so I
can't take credit for finding it on my own.) Some of the other things I
did helped, but adjustment of the trimpot was the final issue. If the
pot is misadjusted, the oscillator will wander all over the place as the
oven temperature swings wildly.
I drilled a second hole in the Ovenaire case so that I could access the
pot. I also monitored the current to the unit with a DVM. When the
oscillator started wandering, I tweaked the trimpot until the current
increased, indicating the oven was activated. I kept adjusting it to
the point that the frequency varies by 0.1 Hz on my GPSDO counter.
Perhaps it is less than that, but I don't have enough digits to say. (I
know that for final adjustment, I will need to use a scope but the
counter does let me see if the OCXO is wildly unstable.)
Now, it is clear that the adjustment is very touchy. I am going to
remount the assembly in the Cushman tonight or early tomorrow and see if
handling it has made it unstable. If so, next weekend I will tear it
down again and figure out the value of the trimpot and the number of
turns. If I can find a trimpot with more turns, I will buy it.
Otherwise, I will just get an exact replacement.
All of this has caused me to realize that the Ovenaire is probably still
quite usable for what it was designed to do. Although I considered
installing the MV89 in its place, the Cushman power supply isn't
adequate to handle it.
Thanks again to all.
Fred Bray
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