[Collins] Re: {Collins] Colpitts oscillator

[Richard Keller]

Wow! What a fine refresher. I was looking back at my electronics school
notes also from 40 years ago (Whew, can't believe it has been that long)
and you did a  great service by reminding me of the intricacies of the
shunt fed Colpitts. I also learned some new things that only someone of
your experience would present.

I did as you suggested for reducing stray C and the difference was small. I
give high probability to your suggestion that the silk screening may be the
biggest culprit. I noticed that the ganged capacitors are slightly
different in value. The feedback path is 410pf and the signal path is 400pf
but I believe that is by design for proper feedback function.

Thanks so much for the help and willingness to do the back and forth
discussion. Your knowledge and experience is invaluable to us who still
need a teacher from time to time.
73, Dick KF4NS
St. Petersburg, FL 33714 USA
Keep the glow!

----- Original Message -----
From: "Dr. Gerald Johnson" <geraldj at ispwest.com>
To: <rkeller at ij.net>
Cc: <collins at mailman.qth.net>
Sent: Friday, September 03, 2004 10:48 AM
Subject: Re: {Collins] Colpitts oscillator


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