SB-102 on MARS frequencies.

[Kenneth G. Gordon]

I and a friend, Bob Preston W7DPG, used SB-101s for AFMARS 
when we did 'phone patching into SEA during the Vietnam war 
days.

We used an external heterodyne oscillator and fed its output 
into the grid of the normal, internal heterodyne oscillator tube.

The external heterodyne oscillator I used was one of those OX 
modules made by International Crystal, IIRC. I mounted mine 
inside the SB-101 as near as possible to the normal heterodyne 
oscillator tube.

We mounted a miniature SPDT switch right on the circuit board 
to switch the grid of the normal heterodyne oscillator tube from 
the normal batch of crystals to the external oscillator's output, 
then adjusted the signal level at the output of the external 
oscillator so that the output of the internal heterodyne 
oscillator's cathode follower showed the same levels as the 
crystals would provide.

We found that the SB rigs would easily tune-up over the entire 
500 Khz band segments above and below the normal ham bands 
with no special alignment needed, except on 40 meters, where 
the normal bandwidth of the driver grid and plate coils over 
that segment is BARELY wide enough for the 40 meter band 
alone.

We used a 23395 Khz heterodyne oscillator crystal to operate 
over the range 14.5 to 15 Mhz, and 22395 Khz rock for 13.5 to 
14 Mhz,  with the bandswitch in the 20 meter position, and 
12895 Khz rock for 4 to 4.5 Mhz with the bandswitch set on 80 
meters.

As I remember it, there was a bit of falling off of output at the 
extreme ends of the band segments, but we didn't operate there 
anyway. 

Using this method, we preserved the accurate frequency 
readout of the normal Heathkit system, and avoided any 
problems with the bandpass filter. Our method also made it 
extremely easy to switch the rig back to normal ham-band 
operation by throwing one switch. What we had was essentially 
a "poor man's Collins KWM-2A", which operated on exactly the 
same principle.

This scheme worked very well for us for several years. Between 
the two of us, we made over 20,000 phone patches in that 
period. We used the SB rigs to drive modified BC-610s as amps 
to around 3.5 KW output at maximum. 

Bob later bought a new KWM-2A to replace his Heathkit. I 
couldn't afford to do that and stuck with the SB-101.

BTW, to arrive at the correct heterodyne oscillator crystal 
frequency you would need, take the lower band edge of the 
segment you wish to operate on and add 8895 Khz to it.

I.e., if you wanted to operate over the band 20.5 to 21.0 Mhz, 
add 20500 to 8895, which tells you to use a 29395 Khz rock, and 
set your bandswitch to 15 meters.

I hope some of you find this information useful.

Ken Gordon W7EKB

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