[R-390] Autopsy of a bad PTO.

[Barry Hauser]

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