[time-nuts] HP 10811 40 Hz low.
eb at telight.com
Fri Dec 16 12:44:43 EST 2016
I had a reference problem on my Z3801A years ago, and ultimately found
that the opamp that controls the 10811 oven temperature was bad. I think
it turned out there was a bad batch of certain date codes. Replacing the
IC with an equivalent type fixed it right up, with no other changes or
Also note that the the ovens are sequenced to avoid drawing too much
power from cold-start. As I recall, the inner one has to get up near
operating temperature to get things running ASAP, then its status signal
enables the outer oven driver (a switched-mode converter on the Z3801A
power supply board) to start - it takes longer to stabilize. I think the
inner oven should take maybe 20-30 minutes tops, to reach the spec
range, and the outer one gradually catches up over maybe an hour or two.
The crystal is very sensitive to temperature until it gets to turnover,
so the frequency can seem way out of whack compared to its normal
operating point. The oven temperature doesn't have to be very far off to
cause this, so a failure in the inner oven control that throws it out of
range could cause problems.
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