Satgen61  Junk box equipment for 9600 bps reception  by GM4IHJ 28May 90

Have spent this last week modifying a CB and a Microwave Modules 432/434 
to 28MHz transverter for 70cm 1200 and 9600 bps FSK. I recrystalled the
transverter on 432 substituting a 101.25 xtal for the 101.00 original.
That gave me the Central Scotland Scot2 node on 432.625 out on Ch2 of the
CB. I found that no tweeking of the xtal in the transverter was necessary
to print good copy of Scot2 at 1200 bps taking the output of Rx Data from
the external mike socket of the CB.

Getting Uosat3 on 435.07 MHz was not so easy because of its large doppler
shift. I recrystalled the transverter high position to 101.85 ( was
101.5). This gave me local oscillator at 407.4 which subtracted from
435.07 put Uosat3 out near 27.670 MHz , channel 8. I took the Rx data out
from the CB discriminator to my G3RUH 9600 modem, then went looking for
Uosat3.

What I got was interesting. After several minutes clearly not tuning Ch7
or Ch8, the signal dopplered into tune on Ch7 printing good copy for about
a minute, then I had to wait about 5 minutes before it dopplered through
Ch6, again giving moderately clean eye diagrams and screen copy. Clearly I
had to do something about the 10KHz steps of the CB, if I was to get
continuous tuning. I tried shifting the CB 2nd local oscillator but could
only move it 500 Hz, rather than the 10 KHz I wanted. So I next tried
serial capacitance on the transverter xtal. After some experiment I got
smooth tuning over about 11 KHz using a variable capacitor with slow
motion tuning in series with the transverter oscillator crystal. For this
test I actually temporarily abandonned the CB and took the transverter
output on 27MHz to an R2000 Rx. Tuning this set up onto the Angus beacon
on 432.98 ( R2000 tuning 25.58), I could watch the change in tuning point
on the R2000 as I tuned the transverter oscillator. 

The acid test was then to go back to the CB and try to track Uosat3. This
I did on a strong near overhead pass , with no tuning problems . I started
halfway down the transverter tuning with CB on Ch8 then as I got to the
bottom of the transverter tuning I switched to Ch7 and retuned high on the
transverter, repeating the process for a similar change down to Ch6 , 5
minutes later. My tuning guide was minimum flicker on the TNC DCD light.

I must emphasise  that this transverter CB combination does not work well
on weak signals. The snag is the ceramic filter on the CB 455KHz 2nd IF is
too tight. So my next job is to find a wider bandwidth replacement for
that. Then and only then will I be able to say that I have a first class
70cm terrestrial and pacsat station receiver which cost me just 5.30
pounds for each of the two xtals. The CB came from GM4NUU's attic , and
the transverter has been doing nothing for nearly 7 years. Further
experiments include trying the CB Tx on 70cm terrestrial packet at both
1200 and 9600 bps FSK. Please note if you only want reception the much
cheaper MM 432/434 to 28 receive converter can be used instead of the
transverter. 73 de John GM4IHJ@GB7MAC  28 May 90