[R-390] Squirrely PTO

[Jim M.]

The Oldham coupler between the PTO shaft and the front gears should have a 
spring between the two posts.  This is on the outside front of the PTO where 
it's shaft meets the front panel mechanisms.  If you don't have a spring 
there, you will get backlash.  The warble can be caused by either a loose, 
corroded or missing bracket on the front of the PTO that presses against the 
PTO shaft to ground it.  It's a little right angle bracket and it needs 
tension to effectively ground the shaft.

Jim M.
----- Original Message ----- 
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, February 14, 2006 8:36 PM
Subject: [R-390] Squirrely PTO


> OK, you guys may remember my dissected PTO. It's mostly back together,
> but now I notice something else:
>
> If I tune continuously in one direction, then the frequency is nice
> and smooth.
>
> If I reverse and tune continuously in the other direction, then
> the frequency is nice and smooth after a small fraction of a turn.
>
> But... for a small fraction of a turn (say a few hundred Hz which
> would be like a degree or so) if I wiggle it back and forth it sounds
> "squirrely". Seeing how this only happens when I reverse direction,
> I'm guessing this is some form of backlash.
>
> "Squirrely" means that if I'm tuning through a carrier it does
> not smoothly go up and down, but jumps around by like 100 Hz
> or so from smooth. In the "reversing zone" I can turn the knob a bit
> with no noticable frequency change, and then it jumps a lot (maybe
> 100 Hz).  After the reversing zone it seems to be nice and smooth.
>
> Pulling the covers off and looking inside I see no obvious anti-
> backlash spring. There is a ring holding tight the PTO slug to
> the threaded shaft, and maybe that's supposed be under tension
> to make the assembly be anti-backlash?
>
> What is the "official" anti-backlash mechanism for this PTO? It's
> a Raytheon-refurbed Cosmos by my best reckoning.
>
> If I go to my other 390A (also a Cosmos PTO) the behavior is not
> nearly so squirrely. There may be a very small amount but it's
> possibly my wrist :-).
>
> Or is it not anti-backlash, but something that the right grease
> cures? I've rebuilt Ten-Tec PTO's and the usual cure for everything
> there is a few new plastic parts and magic grease. I didn't really
> understand what I was doing but I was just following Ten-Tec's
> directions. My feeling is that plastic pieces and grease is not
> the answer for my 390A's PTO, but I will appreciate corrections!
>
> Oh, as to overall PTO alignment, after I "jumped a turn" the range
> was correct and with a little tweaking of the endpoint and a couple
> of the setscrews it's nice and linear, to within a few hundred Hz
> end-to-end. I do seem to be at the very very end of the endpoint
> adjustment range: if I need more range, is the endpoint inductor
> the inductor that folks remove a turn from? I remember hearing about
> the procedure but didn't really relate to it, having never torn into
> the PTO when I was reading!
>
> Tim.
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