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

[Mike Morrow]

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 pasz 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.

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.

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

>> Maybe I should count myself lucky that, with 
>> the help of our outstanding community of 
>> "Smart People," the radio is alive and kicking
>> 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"?  If the former, then the obvious answer must be ... New Zealand! :-)

Mike / KK5F