[time-nuts] OCXO Adjustment Question

[WB6BNQ]

Hi Fred,

It sounds like you are turning it too far and too fast and expecting too much
from it.  In old items, like what you have, those adjustments are fairly coarse
and required a very slight touch, then a wait and see, then another very slight
touch, etc.,.  It sounds like you are turning a physical capacitor.  If it is the
kind that has two little ceramic discs and a small screw type head for
adjustment, often the variable ceramic disc part would become lose where the
screw head would not really be fully attached to the disc causing backlash.
Oscillators that only have a single adjustment like that are not at the high
quality end of things.

When you make a mechanical adjustment there will need to be some time for the
mechanically induced stresses, along with some electrically induced stresses upon
the quartz blank to dissipate before the final results of the adjustment will be
realized.  All of the above is assuming no temperature change.

The time frame of the Cushman 5110 was back when very high quality oscillators
were very expensive and for the intended purpose were not really necessary.  The
Cushman only needed a given amount of accuracy to meet the requirements over a
certain time frame to qualify for adjusting commercial radios to FCC
specifications.

With that said, to improve the time base of the Cushman, you should consider
replacing the internal reference oscillator with one of the modern oven
controlled 14 or 8 pin IC size oscillator packages.  They are way above what is
in the Cushman, of very small size and can be adjusted via a resistive ten turn
potentiometer to a much higher degree and will hold its frequency to a much
closer degree.

You do need to be careful which one you select as most of the small IC size
oscillators do not have a voltage tuned connection.  On eBay, you can find ads
for a Marion 89A which is the kind you would want for such a project, that is
assuming the base reference of the Cushman is 10 MHz.  If it is lower there a
ways to use the Marion and achieve the needed frequency without a lot of space
consumed.

Of course you could disassemble the Cushman oscillator and replace the variable
capacitor.  That would improve things a little but no where near what can be done
with the Marion 89A.

Hope this is helpful,

Bill....WB6BNQ


Frederick Bray wrote:

> This might be slightly off-topic, but probably there is a time-nut who
> knows the answer.
>
> I am trying to adjust the 10 MHz OCXO in a Cushman 5110 service
> monitor.  I am using a frequency counter driven by a GPSDO.  Perhaps
> someone can educate me about a couple problems I am encountering.
>
> I tried making small incremental adjustments but after I am done, the
> frequency drifts several Hz and then re-stabilizes at a new value.  When
> I make further adjustments, I notice strange behavior. For example, if I
> initially turned the adjustment clockwise to increase the frequency, it
> will now decrease if I turn it clockwise and increase if I turn it
> counter-clockwise.  On the next adjustment, it will reverse again.
>
> Is there some correct procedure to adjust an OCXO?
>
> Many thanks for any suggestions.
>
> Fred Bray
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