[R-390] R-390A frequency stability

[Larry H]

It was finally time to check the frequency stability of the R-390A that I
am working on and not that it is bad, but was disheartened to find that it
took 6 hours to stabilize.  I was checking the frequency stability against
the 5 MH WWV, so was using all 3 converters and found that it had drifted
about 300 hertz.  Although this is the allowable limit, I decided to see
what's going on.  What I saw was all 3 oscillators were about equally
responsible.

The problem with the 1st osc was that the oven was not working.  I replaced
it with a functional one and within 7 minutes it had stabilized to a 10
hertz range (controlled by the thermostat).  It was easy to tell that it
was not working as it was chassis temperature, not the 75 degrees
centigrade on the label.

Now the 2nd crystal oscillator situation becomes a little more of a
problem.  It has the same symptoms as the 1st oscillator - as one would
expect, it took a very long time to stabilize (about 5 hours).  So, I did a
little test - I disconnected the heater oven from the VFO (the 2nd crystal
oscillator and the VFO's heater are on the same line) and turned the 'oven'
switch on the back to on.  Of course, I measured the circuit first to be
sure there would not be any big surprises.  Well, it seems to be working as
desired - 10 hertz stability after 7 minutes.  The oven heats up to about
75 degrees centigrade and stays there.

I plan to improve the VFO stability with an appropriate NTC cap, as I have
done this in the past.  I expect to end up with a fairly stable VFO without
using it's oven (as that is too risky).  I'll let you know how it all turns
out.

What do you think about turning on the oven for the 2nd crystal oscillator
deck?

Regards, Larry