[time-nuts] HP 10811 40 Hz low.

ed breya 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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