[R-390] Autopsy of a bad PTO.

[Phil Atchley]

Hi Jim, Barry et al...
I'll try answering all the messages posted here at once.

First, thanks Jim for providing the alternate URL for the file download.
Yes, I had previously downloaded that one along with a gazillion files =
on
the R-390A and the Cosmos in particular. =20

That "Journey" was one of the factors that motivated me to dig deeper =
into
the Cosmos at hand.  However, as you discovered, the picture I really =
needed
of the "footbridge", perhaps more correctly called an "arch" was =
corrupted
and unusable here.  (I still think that "footbridge is a better =
description
as the screws march across it in succession).

NOW, the bad news.  As I mentioned, this unit's "arch" had been =
"crushed",
probably by some ham handed "mechanic" who thought everything needed to =
be
torqued to 50 ft/lbs 8^((  Why else would some of those screws been =
spread
out like they were?  Instead of a nice arch or ramp, what I have here is =
a
ramp that has been crushed, with a severe "crink" on either side of the
peak, a rather flattened peak with a deep dimple (comparatively =
speaking) on
the inner half closest to the shaft. All this plus the broken spot welds =
on
one side makes for a very rough and unreliable ride for the screw heads. =
=20

Now, if the ramp was intact and just had broken spot welds I could =
probably
fill the entire area under the "bridge" with JB Weld or similar and it'd =
be
rigid and good forever!  The only way to fix this one would be to fill =
said
void with JB Weld or similar and then rebuild the ramp and shape it with
some more JB weld.  However, I don't think that would be a very reliable
long term "fix" as the JB is an adhesive, not designed to use as a
"bearing".  The constant (and I do mean constant) friction with the =
screw
heads would wear it down quickly making for a continuous change in
linearity!!=20

No, I'm afraid this one is destined to be an "organ donor" unless I can
eventually find a replacement for the spring steel ring.

ONE ADDITIONAL NOTE:  This also explains the Frequency "jumpiness" that =
I
experienced while trying to set the linearity screws.  I didn't mention =
it
but I kept seeing the frequency "jump" back and forth erratically while
setting the various screws.  This got worse as I progressed up the band,
though I'm not sure why that would be except that perhaps the linearity =
coil
has more affect at one end of the band than the other (makes sense to =
me).
I really thought maybe it was the "shaft grounding" though I had cleaned =
it.

  =20

73 de Phil, KO6BB
DX begins at the noise floor!
k06bb@elite.net
Merced, California
37.18N  120.29W  CM97sh

-----Original Message-----
=20
I just noticed some of the photos in my Cosmos PTO article on Dave's =
page
have become corrupted.  If anyone's interested, the original article is =
at
the following url.  Copy anything you want.

http://home.att.net/~jamesmiller20/cosmos.htm

I tried emailing Dave but his email bounces.  I guess I like the Cosmos =
over
the Colling PTO because it appears to be relatively easier to linearize
despite Phil's nightmare.  I spent over a week (couple of hours a day) =
going
through the stack (trial and error) in a Collins PTO, taking the cover =
off,
tweaking, putting the cover back on, over and over.  I can see why the
inventor of the Cosmos approach thought he had a better idea.  Sounds =
like
someone in a past life got heavy handed with a screwdriver and jammed =
some
of the little screws.

----- Original Message -----

> Hi Phil & gang:
>
> I dunno, there was a time there would have been several posts with
detailed
> "been-there-done-that" posts to assist you.  I've held back on =
replying
> simply because I've never been that deep into a Cosmos.  Soooo.. .in =
the
> absence of any experienced replies .... (maybe some were off-list?)
>
> Somehow, I'd also assumed that with all the downloading you mentioned =
you
> had certainly come across Dave Medley's pages or another site with Jim
> Miller's "A JOURNEY THROUGH THE COSMOS (PTO)".
>
> The direct URL on Dave M's site is http://www.davemed.com/cosmos.html
This
> includes detailed photos, though there's something wrong with one or =
two
of
> the photos.
>
> Gonna try to shed some light on this, but again, bear in mind that I =
have
no
> hands-on, so take it for what it's worth.
>
> >Since there is another PTO coming I went ahead and took this one =
apart
all
> >the way down to the corrector disk to see if I could determine the
failure
> >mode (not sure I can get it back together correctly 8^)
>
> Somehow, I'm not surprised.  I always take your "quit" and "throw in =
the
> towel" declarations with a grain of salt. (about the size of a =
football
;-)
> While you were tearing into that PTO, I was probably searching for =
that
web
> article (or copping some Z's).
>
> >Anyway, I was
> >right.  The 48 tuning screws are rotated one by one past a round =
spring
> >loaded thingy made of spring steel that has a small piece of steel =
that
> >looks kind of like a "footbridge" spot welded to it.  As the screw =
for a
> >particular segment of the frequency range (25 KHz steps) walks over =
the
> >"footbridge" it presses down on it and pushes the slug under it =
further
> into
> >the coil.
>
> The web site photo shows this.  The spot welded piece looks more like =
a
> triangle, so I'm not sure what you mean by a footbridge.  (Maybe like =
the
> ones in Japanese gardens and Central Park?) Check out the photo for =
what
the
> shape of it should be.
>
> >The "footbridge" was damaged in two ways.  First it had a BIG dent in =
it
on
> >one side that would make screws riding on that side of the bridge
(they're
> >staggered) not press down as hard on the bridge. Secondly, the spot =
weld
> >that holds one end of the bridge is broken loose so that it has lost =
some
> of
> >its "stiffness" and positive action.
>
> >In trying to fix the "dent" in the bridge I'm afraid that I also =
weakened
> it
> >some more as spring steel doesn't take kindly to dents and trying to
> >straighten them out.
>
> From the photo, it would seem that bridge is supposed to be rigid, not
> flexible, even if it's made of the same spring steel as the ring. =
(Just a
> manufacturing convenience, vs. a solid piece of something, also =
avoidance
of
> dissimilar metals. Also, spring steel is fairly hard and less prone to
> wear.)  Wouldn't it be solid if that other weld weren't broken?
>
> >The third problem I found was that some of the screws had apparently =
been
> >turned "counterclockwise" too hard, spreading the slot and screw too =
wide
> to
> >thread down into the disk far enough to reach the damaged bridge.  =
These
> are
> >the culprits that probably had no adjustment affect as they wouldn't
reach
> >the "footbridge".  These are very tiny screws that are really only a
screw
> >shaft with a "slot" in the end and a flat head on the backside to =
"walk
> >across the bridge".
>
> Can't you turn them back down again while it's apart, to make sure =
they'll
> make it through?  Sounds like what you are describing is a false =
bottoming
> out of the adjustment screws.  The "no adjustment" effect might also =
be
due
> to the "bridge" being out of shape, partly flattened and flexing =
flatter
due
> to the broken weld on one end and/or the dent.  Actually, if that =
bridge
or
> arch was flexing, that would have messed up your linearity adjustments =
and
> may have contributed to the other problem.
>
> >NOTE:  A couple articles that I read said that there was a flexible
Teflon
> >ring that these screws rode on forming kind of a cam.  This one had =
no
such
> >ring, only the spring steel thingy that the screw heads walked =
across.
>
> Was that a Cosmos PTO in the articles?  May have been a different mfr.
>
> Again, keeping in mind the nature of the source, (not
> been-there-done-that) -- here's some partially lame advice:
>
> From the photo, it looks like the bridge should be solid, not =
flexible.  I
> imagine the screws should just catch the bridge on the rise, not at =
the
base
> near the spot welds.  If so, then you should be able to repair the =
thing
> with epoxy, or perhaps a solder repair, if the spring steel will take
> soldering.  One way would be to fill the void in the bridge -- =
basically
an
> "arch support".  Reshape the arch as best as possible, using the photo =
or
> another unit as a guide.  Cut the corner off a piece of sheet metal --
> aluminum -- to make a small triangle support with a slightly rounded =
apex
to
> fit the underside of the bridge and secure the whole business with =
epoxy,
> using a small clamp to hold in place until it sets.  Basically -- an
> orthothic shoe insert.
>
> Then fix the screws somehow, if they actually need fixing.  The photo =
on
the
> website shows that quite a few screws are either not present or in the
fully
> backed out positioin.
>
> There's another thing.  I was a bit confused as to how this setup =
could
work
> if the screws are riding up and down the arch, with gaps in between,
causing
> reciprocating action.  That would mean that it would be nearly =
impossible
to
> have smooth action in terms of degrees rotation to frequency.  The key =
is
> that the adjacent screws are straddled so that the tip of the =
bridge/arch
is
> always riding on a screw or none at all.
>
> But -- what that means is that the screw setting should end up with
> reasonably smooth transition from one to the next.  If there were too =
much
> of a differential from one (or a pair) to the next, it could jam or =
cause
> roughness.  I would think that extreme differences between adjacent =
screws
> might be indicative of a problem elsewhere -- worn lead screw or
something.
>
> Also -- does it seem possible to run a screw down far enough to =
actually
> crush the bridge piece (or snap a weld)?
>
> The photos on that site are of fairly high resolution.  I blew up the =
side
> view of the "bridge" and it certainly appears to me that it should be
fairly
> or completely rigid, not flexible, if both welds are intact.  It's
basically
> triangular in shape, but with a smooth bend at the apex. With one weld
> broken, it will flatten under pressure and you won't get a reliable
> adjustment.  That's my theory anyhow.
>
> Of course, as I'm typing the last of this speculative tract, Phil is
waiting
> for the epoxy to cure or soldered the thing, or maybe hand carved a =
new
ring
> and "footbridge" out of some spring steel in his junk drawer.
>
> "Throw in the towel"  -- yeah, right.  We know you better than that. =
;-)
>
> Barry




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