[R-390] Squirrely PTO

[KC8OPP Roger S.]

Tim,

Just last week I had the same problem with a Cosmo PTO
that I had disassembled for maintenance and cleaning. 
When I check a PTO I use an Excel spreadsheet to plot
measured freq at each 100KHz point against desired
freq, and problems like this really show up on the
graph.  I started with about 300Hz difference, after
some fixing managed to get it to over 800Hz.  After
taking the PTO apart more times than I can count I
think I have found the answer.

The lead screw is threaded into the tuning slug, at
the end opposite the oldham coupler are two arms that
reach out to the frame to keep the slug from turning. 
The bracket that holds the arms also has a washer with
four tabs.  The four tabs are for backlash tension. 
Riding on that washer and threaded on the lead screw
is a castled nut that you can rotate to
increase/decrease tension.  

You may have to remove the tuning slug to clean and
free up the castled nut.  I thought about removing the
screws that hold this assembly to the slug, but now
would recommend against doing that.  I was able to
clean things up and re-lube in place.  With all covers
off, I would move the PTO both ways to a pre-set point
and measure the off-set.  Each time I would rotate the
castled nut a little to increase tension (decrease
backlash).  I stopped when the backlash was in the
50/100 Hz range.  I did not want to put too much
strain on the threads inside the tuning slug.

You do need to look at the oldham coupler also.  I
changed to a stiffer spring which helped remove some
of the backlash there.

Hope this helps.  I did take a few pictures but don't
have anyplace on the web to put them.

73's
Roger
KC8OPP


--- Tim Shoppa <shoppa_r390a at trailing-edge.com> wrote:
> 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.
>

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