HF Xtal Oscillator Problems

 

I’ve had several KWM-2/2A units that had very low drive.  When finally traced to the source, the HF xtal oscillator was found to be the culprit.  Extensive trouble shooting failed to isolate the problem and I was about to admit I had found my match when by chance in bright light I noticed that the plate choke (2 mH) of  the oscillator tube appeared to be cracked.  All voltages and resistances were normal.  Tuning of oscillator in alignment procedure simply did not provide any reasonable peaking when tuned.  While this is somewhat normal on 3.5 MHz, other bands in a normal unit exhibit very pronounced peak when tuning the trimmer caps.  I finally decided that the choke was indeed faulty and that the crack indicated that the ferrite core had been damaged and all that remained was the inductance of the winding itself.  Replacement with a new choke of either 1 or 2 mH immediately fixed the problem.  In the first instance that I observed this problem I had replaced almost all of oscillator components including a complete new coil and trimmer assembly in the oscillator can.  When I finally fixed the problem, I felt a little stupid because once understood, the answer was obvious — but it surely wasn’t obvious from measurements other than the oscillator output was extremely low.