Using the SB-301 on CW

[Stu Lyon]

Ken,

How does 750pf sound for 622 Hz side-tone?

Almost forgot how to do that.

Stu, W6CUX

Reference: Heathkit SB-102 manual, page 147, fig 2-18
5 sections, 36deg phase-shift/section, r=470K, j=-341K, f=622 Hz.

Kenneth G. Gordon wrote:
> On 2 Feb 2007 at 11:45, W6CUX wrote:
>
> <Much good info snipped>
>
>   
>> Now we need an easy way to change the side-tone 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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