[R-390] Re: R-390 Digest, Vol 25, Issue 12

[Miles Anderson]

Barry,

You are borrowing a lot of trouble!

(1) The values of the capacitors inside the PTO can are absolutely critical
in setting the linearity of the PTO tuning. If you don't get the total
capacitance exactly  right, you may be able to align both ends, but the
errors will be concentrated in the middle. Thus, at the "500" point you are
either going to have a huge amount of positive error or negative error
(depending on whether you have too much total capacitance or too little) and
this is going to be beyond the ability of the screws in the Cosmos pto to
compensate. It's 100 times more of a problem with a Collins "stack
compensator" type of PTO.

(2) All, repeat all, the capacitors inside the PTO can are temperature
compensating. If you don't get the right delta C/delta T the PTO is going to
drift on warm-up.

(3) Lead lengths and component dress are critical. They affect the total
capacitance. Also, they affect the total series inductance, and the slug
tuned compensating coil is tiny. Too much lead length, too much inductance
and you are going to have to unwind turns on the little compensating coil.

Bottom line:  Change components inside the PTO can only when the tuning is
non-linear. I really don't think any of these components are going to affect
the output so long as the PTO is actually oscillating, which yours is.

Miles, K2CBY