On 2018-03-03 3:56 PM, Tom Curlee wrote: > 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. You might want to read the tear-down info here: http://www.realhamradio.com/GPS-oven-journey.htm . The author suggests that this strange structure isn't a heater, but is just a way to thermally isolate the inside from the outside. Seems a bit extreme, but HP sometimes did things like that. Note that right at the bottom of the page, there's a link to another article where the fault is diagnosed and repaired. > 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? According to the tear-down, yes it does. > 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? AFAIK, it's just a 10811 with extended EFC range, so I don't see why tweaking the tuning would compromise anything. Ed