[time-nuts] Z3801A OCXO manual trimming

[Tom Curlee]

Since the Z3801A is being discussed, I thought I'd ask about an issue I'm having with my unit.  I use my Z3801 as my working lab standard for the usual pieces of RF test equipment.  In the past year or so I've had the unit drop out of lock and go into standby mode.  Resetting/cycling power would bring it back into lock for a while, but it generally got worse and now stays in hold over mode.  LH (thanks Mark Sims!!) reports that everything is operating normally except that the it has a PLL unlock.  The one highly suspicious item is that the DAC is at 99.996902% - full output.
The unit shows it has over 94.5K hours run time, so I suspect that the OCXO has aged to the point that the EFC can't pull it into lock.  I disassembled the OCXO to see if it had a trimmer capacitor like the standard 10811 units.  After removing the outer case and foam insulation, I see that the outer heater is one of the thin printed circuit serpentine heaters on what I think is Kapton.  That would need to be peeled off of the case to either get to the hole for the trimmer (if there is one) or to remove the cover for the inner case.
What I don't understand is the purpose of what looks like another coil or heater wrapped around the Kapton printed circuit heater stuck to the inner case.  This second coil/heater is 2 layers of 1/8" thick red foam wrapped completely around the inner case, with fine copper wires wrapped over each layer.  At least I think there are wires on each layer.  This whole second heater is taped down and I don't want to dig any further until I know a bit more about what I'm doing or find that there isn't a trimmer on the double oven 10811 oscillators.
Any ideas on the purpose of the outer heater (or whatever it is)?  Does the double oven 10811 have a manual trimmer that I can adjust to bring the oscillator back to the center of the EFC range?  Assuming that I can manually adjust the OCXO back into adjustment range, will there be any issues with the Z3801 performance, things like phase noise, short term stability, etc?
Any suggestions will be most appreciated.
Thanks,
Tom