20 Hz v.mp4Single wire good NO AUDIO.mp420240124 201400 1.mp4  


The VFO used in the SR-2000/400/400A are the same great instrument.  Solid and accurate when they’re working and like a roof leak, annoying as the devil and hard to fix when they fail.  Since there is no surviving service manual for the SR-2000, we decided to combine all of the information that we have compiled over 30 years and will make it available here.  Because the VFO is the heart of these radios, we will talk about it’s design, some statistics, and how to service the unit in a format that I hope is useful to you.


The VFO implemented in the SR-2000 and SR-400 series transceivers is a CTO: capacitive tuned variable  oscillator.  Another, used by other vendors is a PTO: permeability tuned oscillator. There are advantages and disadvantages for each design.  Both are analog devices requiring some form of intervention to attain linear tracking from end to end of the range as well as from Kc to Kc within the range. The CTO adjustment is via KNIFING the tuning capacitor and the PTO is a series of washers of varying diameter that operate a cam on the tuning gear. Both methods are more an art than science.  I've knifed two capacitors just to see if I could attain a 1/4 Kc deviation from Kc to Kc and end to end and found it could be done but found myself hallucinating as I slept --- it's not trivial.  Is one a better design than the other?  Don't think so, as both can be made to significantly exceed printed design specifications.  Both Hallicrafters and Collins state 100 cycles after warm up.


The Intimate design specs for the SR-2000/400/400A VFO's have never been committed to print. The following information should help to develop a better understanding of that fine instrument and it's behavior.  Because there is a significant amount of data we will break it into sections: drift mitigation: gear train and mechanics: common failures: adjustments: and after production circuit enhancements to correct birdies, spurs, drift mitigation, and transmit/receive offset.




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