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

[David Stinson]

Subject: RME Receivers / Preselectors: That Bad
Tuning Drive

Those of us who have worked with an RME receiver
or pre-selector are familiar with the problem of
the tuning reduction drive.  Typically, the drives
are worn and they feel and move like one is
turning the shaft of a stepper-motor.  This, plus
incorrect setting of the backlash gear makes
tuning the higher bands of an otherwise nice RME
receiver, "interesting."  Here is the reduction
drive, removed from the receiver.
https://photos.app.goo.gl/39N5w0nsYEzo6qxI3

The brass cylinder drives the gear directly.  The
steel shaft provides the drive reduction.  Note
the black wear stripe on the steel shaft just
above the brass part.  If you hold the drive and
turn the either shaft, it feels like a stepper
motor.
However, if you hold the "ears" on the bottom and
compress the steel shaft:
https://photos.app.goo.gl/cy0cyjosmfSCU6Tq1
to the point the black wear mark is inside the
brass cylinder, the action becomes smooth.
Therefore, the "stutter" action is related to this
wear.

In order to disassemble and investigate the
mechanism, one can place a large towel on a table
(to catch bits that want to fly away).  Remove the
"C" clamp from the drive. 
https://photos.app.goo.gl/WloEBn5rs8LwATIJ3

Hold the drive vertical with two fingers on the
"ears" at the bottom.
https://photos.app.goo.gl/KKctcfeWp3ApBO9E3

Compress the steel shaft downward, then press down
on the ears.  The drive will come apart.
https://photos.app.goo.gl/XT8Wwbe67zcO84RI2

This photo illustrates the parts, left to right-
the steel reduction shaft, the spring the shaft
compresses, the brass direct drive shaft with its
three ball bearings and the mount for all of it:
https://photos.app.goo.gl/UXJplumPllv9Sr1u1

Had to soak it all in acetone, then naptha,
followed by a gentle application of very fine
emory cloth to finally get all the dried grease
out of everything.  Also, the first time I
disassembled the drive, I neglected the towel and
one of the ball bearings leaped from the bench and
descended into the abyssal darkness, there to
remain forevermore, amen.  Took the remaining
balls to a local bearing supply house and they
sold me dozen for like 15 cents each.
If one reassembles the drive (put it all back in
order, then compress the steel shaft and it will
pop back together), without the tension spring and
holds a small amount of back-tension (simulating a
much weaker spring), the drive operates smoothly.
Therefore, if there is a way to reduce the tension
of the spring, the drive might be returned to
service.

The spring is an interesting device.  One end has
a concave "anvil" insert, which is a bearing
surface for the round bulb on the end of the steel
shaft.  With a caliper (thank you, thank you, Mike
Hanz), measured the spring uncompressed at 12mm
and fully compressed at 10mm, so there is only 2mm
of "throw."  The spring is quite stiff.
https://photos.app.goo.gl/jYIMdpBstKmSwN2Z2

Submit this to the community for suggestions- "No
going back" from any modification of the original
spring and finding a replacement will not be
possible outside of stripping another drive.  A
more common spring will not have the "anvil."
How can we substitute for the original, "too
strong" spring and thus return the drive to
service?  Your thoughts solicited.

TNX OM ES 73 DE Dave AB5S