+5 khz switch or 10kc slider
If you have a radio with sideband
The PCB in this radio is a Grant (PC-409)
40 channel radio,
and you'd like to
be able to jump 5or 10kc up,
then this is
just what you need!
but this same modification will work in others
just as well. It's just a matter of using the
correct crystal. In a sideband radio it's best
not to use the 10.24, since the radio might not
TX and RX on the excact same frequency. It's not
really noticable on AM, but for sideband it would
be. The best way to do it is use the crystal next
to the varactor diode. The varactor is the diode
controlled by the cklarifier. As you turn your
clarifier, the voltage on the varactor diode changes,
which in turn creates a change in capacitance. This
change in capacitance is applied to crystal, usually
one that is used in all modes.
Although in some radios there are seperate
crystals for seperate modes, in that case there
is still one clarifier, but it would control two
(or 3) seperate varactors and therefore 2 (or 3) seperate
crystals. I had an old sears radio like this, one
varactor for AM/USB, another one for LSB. Then
there is the 120 channel 3 band Grant export, it uses
2 different crystals with an MB8719 to get the 120
channels, and each crystal has it's own varactor.
Most of the old 23 channel radios from the early 70's
or older used a very simple varible capacitor as the
clarifier control. This type of control slides on
both TX and RX. Back then, this was allowed by the FCC,
and this type of clarifier worked very well.
Since the old varicap clarifiers work so well, why
not try to fit one in a more modern radio? I tried
this on a Grant, it did work well. I was able to slide
up a full 10kc, and it was stable. Another plus was that
I could still use the original clarifier as a fine control
and the varicap as coarse control. There was some draw-
backs to doing this though. First, although you can slide
up 10kc, it would be better if you could slide 5kc up and
5kc down, but I was not able to do that. So the new
varicap had to be turned almost all the way to the left
instead of being in the center to be on channel frequency.
The other drawback was how the hell are you supposed to
mount this thing and make it look deccent? I have not
figured that out yet. Other than those two issues, I was
surprised how well it did work. And cheap too, you can
pull one out of an old AM/FM clock radio as I did. Or
maybe from an old car radio with the varicap right on the
tuning control knob, that might make it easier to mount.
Below is a 5kc jump switch. It works on the same principal
except that this radio has 3 crystals tied to a single
varactor. That's only because the radio was modified for
an extra 40 above and 40 below by removing the 11 mhz crystal
that was by the varactor and running wires from the vacated
holes in the board to a rotary switch. Whenever you run wires
to a set of crystals, there's going to be a slight frequency
change. That's why the disc capacitors are on the rotory switch,
to correct the frequency. Running wires to a crystal will
lower the frequecy a little. If the wires get too long, then
the clarifier will be unstable. Try to keep wires as short
as possible.
Above is a picture of the 5kc mod installed. Of coarse
you could use the clock-radio tuner instead of a disc
capacitor. In this picture I am using a ceramic trimmer
on one of the crtstals. I would have liked to use 4 trimmers
and no disc capacitors, but those trimmers are hard to find.
Below are some other ideas you could try.
Do you have a question or comment?
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