[Milsurplus] BC-9: Answer One Question, Get Another.

[Hubert Miller]

Dave, it's counter-intuitive as can be, isn't it?  The 'compensating capacitor' is always in circuit. And you'd think the components of filter F would pretty well isolate the tuned circuit -
including tube plate and grid - from the power supply.  I looked at the filter components using the Shure Bros. cardboard swizzle, and the 23 uH chokes are about 500 ohms and
the 0.001's are about 50 ohms each. Not perfect isolation, but you'd think good enough?  I am eager indeed to learn how well this 'compensating capacitor' works, assuming a good
one is in the circuit. Hey - a funny thought just struck me - if that compensation idea has merit, you could try a mica compression trimmer across the battery here, and supposedly
adjust for exact compensation. 

You have certainly contributed an outstanding brain-puzzler to the group!
-H 

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Monday, February 13, 2017 7:19 PM
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] BC-9: Answer One Question, Get Another.

The mystery of the frequency shift appears to be
solved:
https://goo.gl/photos/Cp3hyLwEJmX3Ua3M7

But that brings-up another question:
Just how does this freq. compensation work?
The manual is available as a PDF at:
https://drive.google.com/file/d/0B1zXYo8XYQS8MmgtNHVsektvT1k/view?usp=sharing

A simplified schematic is on page 19 (PDF paging).
A full diagram is on page 20 (PDF paging).

It appears that the compensation cap, C6, when the rig is keyed, is connected one side to the Cathode circuit of the RF stage and the other, in series with the .001 cap in the Plate circuit filter, to the center of the Oscillator tank at the junction of
the C1 caps.   So the effect is in
the cathode circuit?  Anyone understand
what's going on here?

73 Dave AB5S