[R-390] R-391 positioning head

[Barry Hauser]

Hi Paul & list:
>
> I've got an R-391 multi-turn positioning head in my hand, and have a
> couple of questions.

Put it down verrrrry slowly and back away!  Just kiddin'

>
> After soaking it for awhile in kerosene and cleaning the mud wasps nest
> out of various parts of it, it seems to behave itself and be in reasonable
> condition.

How can you tell -- I've got two 391's with not-quite-working autotunes.
Still haven't gotten around to fixing them.  But the description in the
manual is a duesy.  The R-105/ARR-15 is similar and a bit easier to follow.

>
> However, the locking pin does not snug all the way up to the big tuning
> knob.  Is it normal for some R-391's to do this?

That much I can tell you -- yes -- the wingnutty head on the locking
key/screw should not snug up to the knob, there's some space there.  It
doesn't work by locking it there, but inside someplace.  You should be able
to see that with it apart.

>
> Also, I've noticed that I can get it into a position where the main tuning
> knob is locked (this is with the unit out of the radio), and I can see
> that the pawls on one of the levers seem to be what is hanging it
> up.  With a little careful jiggling and twidling, I can free it up, and
> then it appears to resume a reasonable behaviour.  Is this, too, normal,
> and just an indication of how the mechanism works?

This is where I get lost.  Looks like the mechanism is a job for a
locksmith -- it's what I'd expect to find in a complicated (8-number?)
combination lock for a safe.  While you have it out, I'd suggest
experimenting with different lubes.  I don't know if the usual Mobil One
would be good here.  The pawls are all sandwiched together and the surface
drag of a regular lube might mess it up.  Also, it's the type of situation
where, should the oil attract the slightest bit of dust, it might cause the
pawls to stick.  Might be an application for that very thin silicone lube or
even graphite.  I suspect either the wrong lube or hardened lube is the
reason why many of the autotunes don't work.  However, there could be
secondary damage as a result of jamming that occurs when they stick and
allow the autotuner to run "past the end" and crash against the 10 turn
stop, or worse.

>
> Thanks for any advice.  I know this is a delicate part, and am being very
> careful not to force anything.

Jot down some notes as you go along -- might help the rest of us. (Got a
digital camera?)

Did you check the other modules -- the controller with the rotary switches &
"1-8" dial, relay, etc?  Be sure to check that transverse drive shaft (worm
drive).  There are four or five sintered bronze ("oilite") bearings pressed
into the casting.  On one of mine, the two on the left were so badly worn
that the mating gear bounced off the motor drive gear when it started up.
Little puddle of bronze dust underneath the bearing is a telltale.  See if
they're fairly snug.  I think there are all sizes of these bearing
available, but might be tricky to press them out and new ones in.  I
wouldn't want to hit that casting with anything too hard.

Barry