I have some M100s, which are similar, I think. A while back one of them wouldn't lock, although the oscillator didn't stop. I found a leaky high-K ceramic cap in the OCXO control loop. I don't have the correct schematics, so was just shotgunning stuff. I would suspect those parts and any tantalum caps in that section. I don't think they used any aluminum electrolytics in this mil-grade gear. VCOs can stall from inadvertently reversing the polarity on the varicap diode, causing it to forward bias. In some circuits this is possible, which turns the varicap from a high-Q capacitor into a load resistance on the oscillator. You may want to check the loop conditions (not in a lockup/reversed mode) and make sure the actual diode tuning signal is in the right range before blaming the crystal. If not, you can force it and see if the XO starts up. Ed