[Collins] Re: {Collins] Colpitts oscillator

[Dr.Gerald Johnson]

You could reduce the inductance of the coil by adding a shorted turn near 
the coil. Either a turn of copper wire or a brass or copper washer will do a 
small amount. Adjust its position for the inductance you wish. But that will 
bring up both the low and high frequency points on the dial.

You might work to lower the stray capacity that set the minimum C for the 
oscillator, like being sure the oscillator tube is the right one with small VHF 
geometry, then being sure that the leads from band switch, variable 
capacitor, and tube socket are not pushed neatly down against the 
chassis. You get band width of tuning only by square root (Max C/ Min C) 
and a higher minimum from strays can limit your coverage.  You might check 
that the capacitor rotor plates are centered and that there's no 
accumulation of dust bunnies around the stators to add C (and R).

And you might just have to live with the fact that what the engineers 
designed and drew up for the front panel / dial silk screen were with a 
different part for the variable capacitor than what production bought. I've 
read that Collins had some difficult moving the S-line from prototype to 
production because the manufacturing department wanted longer leads on 
everything, especially the wiring harness and those longer leads made it 
oscillate well. The engineers had to convince manufacturing to use the 
shorter leads of the prototype to make a saleable radio.

73, Jerry, K0CQ, Technical Advisor to the CRA


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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.