The CR-293 is the High Stability timebase for the Icom 910H VHF/UHF transceiver. It appears to be a full OCXO with heater and temperature sensor on the 30.2 MHz timebase crystal metal package. This package as well as all the oven and oscillator support electronics is housed in the outer metal package, shown at the right in the above photograph, taken after the PCB was unsoldered from the outer case.
It appears that the intended way to “net” the oscillator is by way of an adjustable capacitor located under the plastic plug and hole in the outer can. The only electrical connections to the assembly are Vcc (13.9 VDC), RFout and Ground as seen in the schematic. The adjustment on the IC-910 board appears to be a drive level adjustment rather than a frequency adjustment.
The adjustment sensitivity of the variable capacitor inside the CR-293 is such that it makes it difficult to set frequency closer than a few ppb, maybe no more than .01 ppm. This is commensurate with the specification for the entire CR-293 which is .5 ppm – presumable over time and temperature but this isn't clarified.
In order to discipline the CR-293 to be precisely on frequency, a way needs to be found to make it into a VCXO instead of only an OCXO. Unfortunately, the very efforts made by the manufacturer to insulate the oscillator's frequency from external influences make it difficult to do this. Varying Vcc over 10-14 VDC appears to have no more than about 1 ppb effect on frequency. Since this is less than the specified stability, another way needs to be found.
It might be possible to vary the oven temperature and achieve the desired frequency steering. However this technique is relatively slow and wouldn't allow much servo loop (PLL) bandwidth. It appears that the best way to modify the CR-293 to become a VCXO is by paralleling a small varicap with the existing variable capacitor. This capacitor might be no more than a reversed bias silicon diode.