[time-nuts] Efratom FRK - no 10Mhz oscillator

ed breya eb at telight.com
Tue Dec 20 13:21:18 EST 2011


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







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