[Collins] Colpitts oscillator

[Richard Keller]

I have an oscillator circuit for an RF generator with an output which is
low in frequency but all attempts to tweak it have failed. It uses a
Colpitts shunt fed oscillator using several switchable coils in parallel
with a ganged 400pf air variable cap, comprising the tank circuit. The cap
is arranged wiith both rotors to ground and stators which are capacitively
coupled to the grid and plate of the oscillator tube.

The rear section of the variable is shunted with a 12pf variable trimmer
cap for alignment of the dial. The trimmer cap at minimum capacitance is
still not enough to align the dial (32 khz low at 5 mhz) so I removed it.
Obviously the oscillator is the victim of too much capacitance. I cannot
track down the excess capacitance. There must be a theoretical fix I am
missing. Is it as simple as adding a capacitor in series with the ganged
variable; if so, what value should I start with and should it be in the
front or rear of the tuning cap, keeping in mind the "Q" and circulating
current (sustained oscillation) of the tank?

I bring my technical dilemma to this group of willing, knowledgeable people
as an alternative to the other Collins group who have dictated that no
message maybe posted which does not directly apply Collins equipment. That
is a sad scenario because there is such a great wealth of fine,
intelligent, experienced individuals who could be so valuable to the ham
community.
Thanks and 73, Dick KF4NS
St. Petersburg, FL 33714 USA
Keep the glow!