The horse still isn't moving no matter how hard we kick!
The shared crystal is for the 10 and 28 MHz bands.
In the Omni C the crystal is 4 MHz to mix the 5 MHz vfo to 9 MHz where
its second harmonic roars through the mixer. Often LO spurs roar through
stronger than the desired mixed output. Not quite so in this case,
though 17 dB down from full signal is not acceptable performance to the
FCC, especially since it may be out of band. If you tune the Omni C
receiver to 18.000 give or take a little you should hear that birdie
really strong.
I have the schematic of the oscillator mixer board for the Omni C (and
the manual for my Corsair II). Changing the crystal looks to be a real
pain. First of all the new crystal needs to be 22 MHz, well above the
highest frequency used otherwise. Then it needs to be a FUNDAMENTAL
crystal, not an overtone and likely the oscillator constants need to be
changed. Then there's an IC mixer that's pretty clean going to a simple
double tuned transformer. Its tuned to the highest LO band alone then a
couple band switch sections add on variable capacitors to tune to the
lower bands. The highest resonant frequency for the Omni C is 20.5 for
low side injection on 29.5 MHz band. The lowest is 10.5 MHz for the 160
meter band (high side injection). The transformer has to tune on up to
27 MHz for high side injection for 18 meters. Means everything tuned has
to change and it might not have enough bandwidth on the low bands.
Impossible, not quite, but a lot more work than finding a later radio.
One might make better progress looking for an oscillator-mixer board
from a later radio, such as a Corsair 1 that also has band switch
information for the external PA.
The oscillator-mixer board for the Corsair II doesn't have the band
switch information and rather than running one coupled coil up and down,
it has individual double tuned band pass filters for each needed
oscillator output band. That way they don't interact when doing an
alignment and the coil size can be selected to me more appropriate for
each center frequency giving a response curve closer that predicted by
the engineer.
I have no idea whether a later oscillator-mixer board would fit in the
box and fit the band switch stack. And I doubt I'm going to find out.
The Omni C should be a decent receiver on 18 MHz, so long as that LO
second harmonic doesn't kill the mixer. But it will be a very dirty
transmitter without drastic changes. Its really hard to remove a spur at
18.136 in the output while transmitting on 18.068, or at 18.336 while
transmitting on 18.168 with any trap or filter. As hard as getting this
dead Clydesdale to move by merely kicking!!!
The only other possibility for getting the Omni C cleanly on 17 meters
is to use an external transverter and that adds more mixer birdie
possibilities and probably doesn't use the PA so it needs one of its
own, plus a low pass filter plus a TR relay... Possible but generally
not considered practical by most.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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