Hi
Regardless of weather it's a single or double oven, you want the
crystal to be at it's "turn temperature". If you have a BT cut
crystal, that's going to be the highest frequency you find as you move
the oven around. With either a double or single oven, you should be
able to "map out" a parabolic plot of frequency versus temperature.
That's only close to "right" since the electronics contribute
something to the temperature performance. To really get it *right* you
need to vary the external temperature and see what happens ....
On a double oven, the outer oven is set for some "nominal" amount of
inner oven current. The idea is to keep the inner oven controller from
either cutting off or saturating as the external temperature is
varied. Often the inner oven is set to the "low side" of it's power
range.
Bob
On Aug 6, 2009, at 6:24 PM, Jim Palfreyman wrote:
Hi All,
Thanks very much for your interesting replies.
I'm beginning to think I'm not letting it settle between adjustments.
And besides, The thing has been powered off for at least a decade I
would suspect and maybe I'm just watching the crystal adjust to micro
changes in temperature as it warms up. It's been on for about 10 days
with only about 30 min off time in between. I shall leave it on the
bench for a month or two before changing anything again.
Now my next question is to those with double oven experience. The
service manual says the oven is "not field repairable". And there is
minimal description on how to get the oven settings right.
The container has 61.3 C on the side - obviously the point of
inflection for the temperature curve of the crystal. There are two
test points which connect to either side of a thermistor buried deep
inside the inner oven. The label next to it say 432 ohms. The
resistance it should be at the correct temperature. I'm currently
running at 422 ohms (hotter) - that's the temperature that causes
minimal drift in frequency (at the moment - maybe this will change
after further burning in).
The outer oven has no such thermistor. How do I know that is set
correctly? If it is too high, I'd assume it would heat the inner oven
too much and not allow you to drop the temperature in the inner oven.
If it is too low I'd guess the inner heater would have to use more
power and do more work to keep it ok. What is the ideal outer oven
temperature and how is this worked out?
In all this, keep in mind my power rails are running a few volts high
and I'm not sure the effect this is having in the oven.
Regards,
Jim Palfreyman
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