[R-390] VFO squirreliness solved FOR GOOD

[Tim Shoppa]

OK, I went one step further and think I've solved all squirreliness
issues with my two Cosmos VFO's for good:

The pics on the net show the lead-screw and slug. What may not
be clear is that the bottom of the slug is threaded, and the top
(above the arm assembly) is threaded separately.

Backlash is removed by taking the three tiny screws off the top of
the arm assembly. You will find two rings (top with three holes,
bottom with six holes) and a threaded piece in between them.

I removed the threaded piece, cleaned and polished the rings and
the threaded piece (they seem to be copper or a copper alloy and
had a good amount of green-red crud on them), tighten the
threaded piece back down, and reassemble the ring back on top.

How tight? Well, I don't think I tightened mine down any more than
they originally were, but mostly removing the crud did the job.
This is not something you tighten with a wrench! Finger-tight
seems to be more than enough. Make it too tight and there will be
too much "drag" as you turn, I guess (I didn't try tightening them
that much.)

I am now deep in the math of optimizing the linearization. It
is a rather interesting thing with a number of controls. Of course
there are the 40-some little linearization screws, but the effect of
these screws is also affected by the overall adjustment of the slug
in the linearization coil (for MOST RANGE, you adjust this slug
so that it is JUST ENTERING the coil region; for LEAST SENSITIVITY
you want to pick an area where the end has ALREADY ENTERED the
coil region.).  The endpoint adjustment comes in too (and while
setting end-to-end to be 1MHz is a good place to start, for
optimal adjustment the number may be a kHz higher or lower
depending on where the linearization deviations are, if they're
in the middle or at the ends.) To make things just a little bit
trickier, the linearization setting is MOST SENSITIVE at the
3.455MHz end, where the fraction of inductance provided by
the linearization coil is the largest, and it is the LEAST SENSITIVE
at the 2.455MHz end, where the fraction of inductance provided
by the linearization coil is the smallest.

I am PROBABLY OVERANALYZING the linearization issues. Maybe I should
just tweak the little screws more rather than graphing all this stuff!

At least the squirreliness is gone. Before, when I tuned the VFO
through while listening to the 2.455 through 3.455 MHz end on a
nearby receiver, I could clearly hear some funny wiggles going on.
Now, it's smooth as silk!

Tim.