[Boatanchors] RME Receivers / Preselectors: That Bad Tuning Drive

[David Stinson]

Found a solution to the "stepper motor" action of
the RME tuning reduction drive (see earlier post).

Decided against trimming the spring; no going back
if that didn't work.  Finding a different spring
seemed daunting; have no idea how to even define
the specifications of such a spring.   After
thinking about it, got an idea from what Stonewall
Jackson would say: "If ya caint flank'em on the
right, flank'em on the left."

The spring seats in a "well" in the bottom of the
brass shaft, where it holds (too much) tension on
the inner, "slow tune" shaft against the ball
bearings.  
https://photos.app.goo.gl/UXJplumPllv9Sr1u1
If we deepen the well just a bit, that will have
the same effect as shortening the spring and, if
it doesn't work, one can get some tiny ball
bearings from the local bearing supply to replace
the removed metal.

A 9/64th inch drill bit fit the well nicely;
that's 3.57mm- don't know if there is a Metric bit
that will fit.  Alas, I do not own a drill press
or any other tool with which one could make a
"precise" cut.  I have a bench vise, a hand drill
and eyeballs.  The idea was to deepen the well
about 1.5mm, leaving just enough tension on the
spring to keep the action working without
hard-driving the ball bearings into the (invisible
to my eye) wear defects that were causing the
"stepper motor" action.  Put marks on the drill
bit for "start" and "stop," and proceeded slowly.
After the cut, thoroughly washed-out all the
shavings, lubricated and re-assembled the drive.
And, of course, I'd cut just the tiniest bit too
deep.  There wasn't quite enough tension from the
spring for reliable action from the "slow" shaft.
Removed the spring and, using one of those very
fine wire cutters they sell at WallyWorld in the
"craft" section, very carefully "pinched" between
two of the coils, effectively "stretching" the
spring by about half a millimeter- just a tiny
bit- then reassembled.

The drive action is not perfect- you can still
feel some wear- but it's minor and the drive
action is greatly improved from the
"bind-stutter-bind-stutter" with which we started.
It is entirely serviceable.  If one had precision
tools and a few drives on which to experiment,
doubtless the "right" well extension depth could
be found to balance the drive action.  Look
forward to installing this one in the RME-45 to
see how it does.  Likely to be awhile, as my "Play
radio" time is very limited right now.  To give
you some idea- this RME drive project has sat on
my bench for about 4 months.  I finally got tired
of looking at it.

Aside:  Cannot stress enough the use of a big,
lint-free towel covering the bench when working
with these drives.  Those ball bearings and that
spring will magically run away if given one
chance.  Getting those little bearings all back in
the brass shaft with my fat fingers would be
maddening on a flat surface.

GL OM ES 73 DE Dave AB5S