Using the SB-301 on CW

[Kenneth G. Gordon]

On 2 Feb 2007 at 11:45, W6CUX wrote:

<Much good info snipped>

> Now we need an easy way to change the side-town oscilator to 600 Hz.

I am assuming you mean in the SB/HW transceivers: if so, it is actually 
quite easily done. I will dig out the necessary info as soon as possible 
and post it here.

I have not yet gotten my SB-301/401 pair on the air, and have not 
examined the schematics too closely, so if there is some means of local 
sidetone involving the audio stages, somewhat similar to that used in 
the SB/HW transceivers, then perhaps the method used for the 
tranceivers would work for the pair.

In the tranceivers, a PrintedElectronicCircuit (PEC), which consists only 
of some resistors and capacitors in a simple phase-shift network is 
used to select the sidetone oscillator frequency. This PEC, or a home-
made version of it is very easily made. Thus the fixed audio tone is 
easily changed.

I have an article somewhere in my stack of info wherein someone did 
just that.

Also, by judiciously rewiring the mode switch in the SB/HW 
transceivers, a different CW crystal can be added to the BFO that will 
result in any offset you wish. Then you must change the sidetone 
frequency to match so you can accurately "zero-beat" a desired station.

And, BTW, the FILTER does not determine offset or the pitch of a 
desired signal for CW: if the signal is centered in the passband, then 
the BFO, and only the BFO, determines the audio pitch.

To prove that I am correct, use any receiver with a variable frequency 
BFO, such as an R-390(*) or an SP-600, or any other. 

Set the selectivity at a fairly narrow setting, turn up the audio so you 
can hear the band noise "hiss", then vary the BFO. 

With the BFO centered, i.e. zero-beat (assuming the receiver is 
properly aligned), you will hear a very low pitched hiss, and the tone 
will rise in pitch on either side of zero-beat as you adjust the BFO 
frequency to either side of zero-beat.

At zero-beat, the BFO frequency is centered in the passband of the IF 
filter, and as you adjust it, you are simply moving the BFO's frequency 
to one side or the other of the IF passband.

Therefore, to change the pitch of the signals centered in the passband 
of the IF filter, all you have to do is change the frequency of the BFO.

Ken Gordon W7EKB

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