[Milsurplus] BC-9: NETTED! WoooHOOO!

[Kenneth G. Gordon]

On 19 Feb 2017 at 23:00, Mike Morrow wrote:

> The circuit design using a low pass filter instead of a simple RFC
> confirms that there was purpose to it, with that being the setting of
> a controllable offset by what happens at the output of the filter
> affecting the RF stage at the input to the filter.  C6 exists in the
> circuit to affect the RF stage through the filter when the key is
> closed.  Appropriate selection of the value of C6 results in the
> desired nominal offset on key closure. 

Yes.
 
> But the proper design and performance of the LP filter and C6 are
> frequency dependant, and would never have functioned as designed to
> create a modest offset when the frequency of operation is as grossly
> different (23 percent) as 3570 kHz is to 4600 kHz.

To me, THAT is the most important change in the operation of this rig from its original use.

>  That's why I
> suggested earlier that testing at the design frequency of 4600 kHz
> should be tried.

Yes!

>  Only that would have allowed a valid test of the
> ALL-original circuitry without any modifications, to determine if the
> resulting offset was satisfactory...i.e., better than just 200 Hz if a
> simple RFC had been used, and less than the 3000 Hz that the testing
> at 3570 kHz (where the LP filter presents less RF attenuation) showed.
>  I believe that what was unsatisfactory at 3570 kHz would have worked
> OK at 4600 kHz.

Again, yes!
 
> So, while a ham-usable offset has been obtained with the RFC, a real
> test of the complete original circuit design has not been conducted at
> the frequency for which the circuit was designed.  To establish the
> appropriate circuit at 3570 kHz, the LP filters would need redesign to
> lower the cutoff frequency from the original 750 kHz to about 600
> kHz...and then the appropriate value of C6 to create the desired
> offset would need determination by experiment. 

I totally agree. THe LP filter is the key component here, IMHO.
 
> In short, the RFC is not a circuit improvement, but just a circuit
> simplification that leaves the set stuck with an offset that is too
> low for service use. 

What it amounts to is a "ham kludge" to get the rig to work on a ham band! 

Ha ha! :-)

Well, I want to build a clone and see what I can achieve here. Should be fun.

Ken W7EKB

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