Re: {Collins} 75S-3B Question

On Nov 23, 2015, at 10:44 AM, james.liles@xxxxxxxxxxx wrote:

> Good morning,
> I asked the following question a while back and received no answer.
>   QUESTION REGARDING THE 75S-3B:  I believe C301 is hand selected in final to mitigate drift.  Should be
> several values available to choose from.  Does anyone have the values or a complete set of
> those Temp comp caps used for C301?   


I did not want you to get no answer again, though I can’t answer your question except offer a couple of suggestions and thoughts:

The values available for the C301 selection might have been a standard set - many capacitors were and still are offered in a standard list of values.  A clue as to the values might be available from manufacturers listings or catalogs of the time.  The Electronics and Electrical Master book comes to mind. I think that there are standard values in use now for resistors of particular tolerance: that is the 5 percent resistor range has different values than the 1 percent ones, as far as I know.  And most likely the same applies to capacitors.

I don’t know of a source now for the temperature compensating capacitors, though they may well be avaialble.   An online search reveals that Vishay may make such capacitors.  No doubt a further search at suppliers web sites would reveal some.  (I checked surplus sales of Nebraska and did not find them. Also, I did not find them at RF Parts, Inc.)

Not long ago I wanted to find a high coefficient capacitor of 3 pF for use in the BC-779 receiver I have here.  The company published a modification to the oscillator section to correct for the drift inherent in the radio as it was designed.  I did get a couple of caps that will have about half the tempo factor suggested in the article of the time, and will install them as I overhaul the radio in the near future.

The whole art and craft of temperature compensation of oscillators is little practiced today, I suspect.  Especially with modern crystal controlled frequency synthesizers, there is little need for it in new equipment.  But for sure the Collins equipment we use did have that sort of thing built in.  No doubt the production and testing process included steps to select the right capacitors to match the oscillators being made.  

There is a neat design trick to allow for screwdriver adjustment of temperature compensation: a differential air variable cap introduces more or less of the capacitance of two tempo caps of differing coefficients into an oscillator circuit. The arrangement keeps the total capacitance added to the circuit constant as the adjustment is varied.  Instead of testing and selecting and soldering in the right cap followed by re-alignment, the extra air variable and tempo cap allows for one step adjustment after the temperature changes.  The extra cost would make this arrangement found only military equipment, though (or maybe General Radio class test equipment).

Note that the LM frequency meter of 1930’s design (and possibly the later BC-221) includes a bimetallic operated inductance trimming disk in the main oscillator inductor.  The LM was used on flight lines that might vary from *way* below freezing to ungodly hot, and the thing had to be about right no matter what.

I look forward to information other list members might have.


Roy Morgan
K1LKY Since 1958

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