[time-nuts] Ancient OCXO in scope calibrator.

David C. Partridge david.partridge at dsl.pipex.com
Sun Sep 7 09:56:25 EDT 2008


I'm having some problems with an ancient Bulova OCXO in a Tektronix 184
'scope calibrator.   

This is an crystal mounted on an octal valve base with a heater winding
wrapped round it, and a common or garden variety thermostat which is
supposed to switch at 75 celsius.   I've not measured the switching
temperature, as I don't have the means but the outer case does get fairly
warm (40 plus degrees?).   The oven is turning on and off.

It is used in a circuit with a 7587 Nuvistor tetrode (yes it's a valve/tube
circuit).  The crystal is connected cathode to G1 with a trimmer capacitor
of 3-12pF.   The signal at the cathode is supposed to be about 70V p-p.

I'm measuring the frequency of the signal after the transformer stage which
couples it to the first stage of a countdown board.

For the first minute or so after turn on from cold, it sits below 10MHz and
is fairly stable and climbing as the oven warms up, and I can adjust the
frequency up towards 10MHz with the capacitor (but not all the way), then
suddenly, at the stage where it is starting to look as if it will soon
stabilise at about the right frequency, it jumps to way over 10MHz and the
lowest I can get it down to with the capacitor is about 10.0003xxx MHz where
xxx is not very stable at all - in fact it can vary up to to 10.0005xxx and
down to 10.0002xxx.

I've tried freeze spray on most of the components round there to no effect.

If I try to probe the signal at the cathode of the nuvistor even with a high
impedance active probe with a P6201 with a 100x attenuator (about 1pF
loading IIRC) the oscillation just drops dead.

Now for calibrating 'scopes, it doesn't need to be any more accurate than it
is (30ppm) - but ...

Do any of the collected mavens have an explanation for the behaviour, and
recommendations for fixing things?

Cheers
Dave




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