[time-nuts] GPSDO won't lock: OCXO aged out?

[ed breya]

Scott, it looks like you have already proven the tune voltage can go 
much higher than specified, and sufficient to get in-range for 10 MHz 
ideal. The currents noted show there's probably a pull-down resistor on 
the tune line - maybe 75-100 k. The reverse leakage of the varicap diode 
should be in the nA range cold, and even hot at oven temperature, I'd 
think only up to a few uA or so. You can get a better idea of what's in 
there by doing the same test immediately after cold power-up.

You can avoid any issues about doubling the EFC sensitivity by adding a 
fixed DC offset instead of higher gain. The DC reference added should of 
course be stable and clean, and so should the opamp, the associated 
resistors, and its power supply, so you won't have to worry about PSRR.

Ed