[R-390] Autopsy of a bad PTO.
Barry Hauser
[email protected]
Sat, 28 Jun 2003 10:19:20 -0400
You're probably right about Plan B, and a replacement is on the way. Since
the present one would be a goner anyway, it might be worth experimenting
with epoxy as I first suggested. The entire space under the bridge would
have to be filled to increase the surface area of the bond. The surfaces
getting the epoxy - top of ring under the bridge and underside of bridge
would have to be roughened up to provide grab. Most epoxies are fairly heat
resistant. Wouldn't take much to try it.
If you are going to try to fix that one, Phil, I would suggest pre-setting
any of the 48 linearity screws to remove any "cliffs". A key question has
to do with how well you were able to reshape that bridge. The flex would be
fixed with the epoxy. I suppose you could dress up the surface of the
bridge if there are some bumps and ruts in it.
You could try re-spot welding it -- maybe with a high current battery and a
couple of makeshift electrodes. Or maybe not. ;-)
There's always Plan B.
Barry
----- Original Message -----
From: "Jim Miller" <[email protected]>
To: "Barry Hauser" <[email protected]>; "Phil Atchley" <[email protected]>;
<[email protected]>
Sent: Saturday, June 28, 2003 9:56 AM
Subject: Re: [R-390] Autopsy of a bad PTO.
> I don't want to sound too negative, but if that bridge is broken, I'm
afraid
> it's time for plan B (new PTO). That is spring steel which won't solder
> well at all. Even if repaired, it may never be as smooth and repeatable
as
> an undamaged one.
>
> Jim
>
>
> ----- Original Message -----
> From: "Barry Hauser" <[email protected]>
> To: "Jim Miller" <[email protected]>; "Phil Atchley"
> <[email protected]>; <[email protected]>
> Sent: Saturday, June 28, 2003 9:28 AM
> Subject: Re: [R-390] Autopsy of a bad PTO.
>
>
> > Hi Jim:
> >
> > Sorry ... I didn't notice your earlier post(s) in reply to Phil. Thanks
> for
> > the new URL. Those missing photos help make it clearer about the action
> > with the "footbridge" riding and straddling adjacent screws. The
> > overlap/staggering of the linearity screws is intended to avoid jumping
> > which could otherwise cause up-down variations in frequency as the unit
> > rotates. (and how an abrupt jump from one screw to the next could cause
> > problems.) The photos on your site seem to be clearer and larger as
well.
> > Even clearer to me that the little ramp or bridge should be rigid, not
> > flexible. If the metal will take to soldering, Phil could solder the
> broken
> > weld and perhaps fill the void under the bridge (no boat traffic) with
> more
> > solder. Of course, it would have to be reshaped as well as possible
first
> > and the final result deburred and lubricated with a small amount of
> > non-running grease.
> >
> > Barry
> >
> >
> >
> > ----- Original Message -----
> > From: "Jim Miller" <[email protected]>
> > To: "Barry Hauser" <[email protected]>; "Phil Atchley"
> <[email protected]>;
> > <[email protected]>
> > Sent: Saturday, June 28, 2003 8:38 AM
> > Subject: Re: [R-390] Autopsy of a bad PTO.
> >
> >
> > > 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 -----
> > > From: "Barry Hauser" <[email protected]>
> > > To: "Phil Atchley" <[email protected]>; <[email protected]>
> > > Sent: Saturday, June 28, 2003 7:28 AM
> > > Subject: Re: [R-390] Autopsy of a bad PTO.
> > >
> > >
> > > > 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
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > _______________________________________________
> > > > R-390 mailing list
> > > > [email protected]
> > > > http://mailman.qth.net/mailman/listinfo/r-390
> > >
> > >
> >
> >
>
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