[R-390] VFO squirreliness solved FOR GOOD

[Dan Merz]

Tim,  thank you for taking the time to relay your findings.  I never messed
with the linearization on my Cosmos because it looked messy.  And I don't
recall really cleaning mine like you did when I had it open to take a turn
off,  which might be worth the effort.  As far as "overanalyzing" goes,
this happens.  I'm glad you took the effort.

I wonder how much the linearization scheme improves the linearity.  How
non-linear are the other pto's that don't have this feature? Or how much
correction are you making as you tweak these?  I'll now have to monitor my
pto with a frequency meter to see what I sacrificed by ignoring further
adjustment of mine.  Best regards,  Dan.

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Tim Shoppa
Sent: Sunday, March 05, 2006 10:29 AM
To: r-390 at mailman.qth.net
Subject: [R-390] VFO squirreliness solved FOR GOOD

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.
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