[Milsurplus] BC-9: Back to Head Scratching.

[Kenneth G. Gordon]

On 18 Feb 2017 at 14:51, Mike Morrow wrote:

> Dave and Ken wrote:
> 
> >> No matter what "frequency compensation" cap
> >> I use, I can't seem to net the thing closer
> >> than about 3 KC.  Going below about 220 pFd
> >> actually makes it *worse.*  Duh......
> >
> >Well, since this exercise began, I have been wondering about "F"...
> 
> The low pass filter F with 46 uH of inductance and 1000 pF of
> capacitance (750 kHz -3dB cutoff frequency) does not present all that
> great an impedance at the BC-9-A 4600 kHz design operating
> frequency...and certainly less at the 3600 kHz being tried. 

Agreed. I keep thinking, "Why bother?"

> I believe that the imperfect RF choke characteristic is a deliberate
> design feature for introducing a frequency offset from receive to
> transmit by using the circuit change of C6 (key closed) vs. R5/C5 (key
> open) to affect the oscillation frequency of the RF VT-1. 

Again, I suspect you are correct. We still have no real definitive idea of how the VT-1 works 
in that circuit.

> What would be interesting is to insert a common 2.5 mH RF choke in
> series with F,

Absolutely!!!!!

> knowing that will present sufficiently high RF
> impedance (even at 3600 kHz) that the circuit changes key-open to
> key-down before F would then have no effect on the RF at the
> oscillator plate.  Any frequency shift after the 2.5 mH choke is
> installed should be due only to the change in VT-1 plate voltage.  The
> results from this experiment would be interesting. 

I completely agree. I would suspect that the offset between receive and transmit would be 
MUCH reduced by that single item alone.

> >> again after almost 100 years.  I even had the 
> >> correct Hi-Z 12,000 Ohm headphones.  
> >
> > Gee....where did you get those?
> 
> Get what..."Smart People" or "12,000 ohm headphones"?

The 'phones.

>  If the former, then the obvious answer must be ... New Zealand! :-)

Ha ha! :-)

Sure.

Ken W7EKB

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus