Fixing should; require two changes. Change the crystal to 22.0 MHz.
Change the coil and capacitor on the PTO mixer output for 18 Mhz to one
that tunes to 27 MHz. Maybe change the counter initialization.
Right now the tuned circuits at the output of the PTO mixer tunes to
19-21 MHZ for 28-30 MHz output with no added shunt capacitors. For
proper 18 MHZ that double tuned transformer needs to tune to 27 MHz.
That means either replacing all the coils and capacitors, or replacing
the 47 PF fixed capacitors C22 and C23 with inductors to raise the
resonant frequency of the coils. Depending on the stray capacitance, the
existing coils are 4 to 5.5 Microhenries. To tune to 27 MHz the
effective inductance needs to be about 1.7 uh. Shunts of 2.5
microhenries may make the change, but will probably change the mutual
coupling so the output is low on 27 MHz. The added shunt coils might
need to be coupled like the originals. Then such a circuit has the
possibility of creating unwanted passbands that might prove to be
annoying or spur producing. Else backing out the slugs or taking off
turns until they tune to 27 MHz and then adding padding for all the
other bands including 10 meters where there are no trimmer capacitors
now might work. Filching and cobbling in a PTO mixer board from a
Corsair would work too.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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