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

[Michael Bittner]

I have revised my schematic diagram to correct the connection of the cold 
end of the compensating capacitor (C6) and the location of switch S2.
https://goo.gl/photos/KrLEzh7pdHXfneNC9
Leaving any capacitor permanently in-circuit across an active battery seems 
like poor practice to me, but at least it does make a cleaner path from C6 
to the rest of the circuitry.  Also, I recall Dave S. measured the four RFCs 
of Filter F as 1 mH each rather than the 23 uH given in the manuals.  In any 
case, I agree that there seem to be too many obstacles in the paths from the 
compensating capacitor to the frequency determining LC components.  The 
suggestion for putting a trimmer cap across C6 to see if it actually affects 
frequency seems like it would be a useful experiment.
Mike, W6MAB





----- Original Message ----- 
From: "Hubert Miller" <Kargo_cult at msn.com>
To: <milsurplus at mailman.qth.net>
Sent: Monday, February 13, 2017 8:14 PM
Subject: Re: [Milsurplus] BC-9: Answer One Question, Get Another.


> 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


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