[Milsurplus] BC-9: NETTED! WoooHOOO!

[Hubert Miller]

So are you saying there should be an optimum, design offset?    It sounds to me from the manual, where it explains the compensating C6, that maybe the goal was 
no offset?  Receive = transmit frequency ? 
If there is an offset, by design or by the variability of circuit components and battery state, i suggest that operators were maybe cautioned to all use only the 
upper beat note ( "LSB" HFO ) and to choose a beat note well under 1 kHz. To minimize stations chasing each other across the band. 
As there's no real zero-beat capability, since you can't zero-beat the transmitter against the other station, the net must have been interesting to listen to, with 
stations spread around a narrow range. 
-H 

-----Original Message-----
From: Mike Morrow [mailto:kk5f at earthlink.net] 
Sent: Sunday, February 19, 2017 8:00 PM
To: Hubert Miller <Kargo_cult at msn.com>; milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] BC-9: NETTED! WoooHOOO!

Hue wrote:

> My thinking is that with frequency shift <1 kHz the set is workable as 
> is, even in a net. Your net is only going to be maybe 4 - 5 stations 
> tops, anyway. As long as it's understood the NCS is NOT going to 
> retune ever, but instead defines the working frequency ( as is stated 
> in the "calibration procedures" in the manual ), and as long as any 
> other station can detect that another station is commencing transmit, 
> the receiving station can tune for usable beat pitch. There will be 
> some retuning, but NCS doesn't move, and it works. Also, people can 
> deal with a variety of beat notes, from over 1 kHz down to about 100 
> Hz...

However, Dave says that the offset is a couple of hundred hertz, due to the plate voltage change.

This would NOT be a satisfactory offset, especially since very little drift could make one station completely inaudible to another.

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.

If the above is not true, then there would have been no reason to employ a LP filter instead of an RFC, and no reason to have C6 at all.

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

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.

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.

Mike / KK5F