David C. Partridge wrote: > 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 > > Dave
Need a few more circuit details. The crystal current wont be very high unless their are other components connected to G1. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
