[R-390] Squirrely PTO & SJC update

Tue Feb 21 10:35:07 EST 2006

Funny how some of us seem to experience the same type things at about the 
same time.....at times.  (confused yet?)

I have been fighting with my Cosmos PTO for the last week of available 
evenings.  Same problems...got the jitters with reversing direction or even 
when fine tuning at times. Tap on the PTO cover with a small screwdriver 
handle and the het tone would bounce all over the place. (most noticeable 
with the bfo on and using a heterodyne tone for discernment)  I've had the 
thing apart so many times it's crazy.  I finally completely removed the 
heater and associated wiring with no intents in putting any of it back.  We 
don't use them anyway and I see no need for them to remain.

I even put the thing back in the radio with all covers removed so I could 
move things around with a tuning tool with the radio tuned to a carrier and 
bfo on.  All seemed to center around the rear connection of the tuning slug 
to the lead screw and anti rotation hardware.

I had remembered reading Rogers post but didn't remember the details. (went 
back and looked this morning)

Here is what I did.

I considered fully removing the tuning slug but had concerns about getting 
the slug back in sync with the compensating ring and it's already pretty 
close adjustments.

I pulled the rear end plate with the slug fully in that direction and 
removed the tiny screws (had to drop one on the carpeted floor), the top 
brass ring, the lead screw to slug tensioning nut and finally the inner 
brass ring.

It appears that the slug is not actually threaded but probably has some type 
of threaded fixture on the opposite end as well.

I cleaned and scuffed the brass rings and the lead screw tensioner fingers 
(for lack of a better description).  I washed them with spray contact 
cleaner and scrubbed them with a scotch brite pad to remove any oxidation 
and re-washed.  All was re-installed on the end of the tuning slug.  There 
is a metal ring with a couple of arms that go to a piece of square stock 
that is used to keep the frictional forces from rotating the tuning slug. 
(it is attached to the slug)  It has twice or three times the number of 
threaded holes needed to attach the brass rings.  Pick a set that don't 
interfere with the tabs on the tensioner once set as tightly as desired.  I 
torqued mine down a bit more than it was when I disassembled it to improve 
electrical contact with the lead screw. (with fingers, not tools)  I have no 
fear of it damaging the core as it is springy and should prevent any damage. 
The top ring and screws lock everything in place and ground everything by 
virtue of the shaft ground on the front of the PTO...which was also removed 
and treated to the cleaning and scotch brite pad treatment....shaft 
included, re-tensioned and re-installed.

I feel no added friction in tuning but the thing is dead on now...no more 
jitters period!  I washed all lubricant off the lead screw ( I had placed 
there)....it looked like stainless steel and the frictional components 
appear to be brass or copper so I consider it to be self lubricating....and 
obviously the RF ground at that point is very important.

I was watching the slug movement before disassembly and there was no 
perceivable backlash even under high magnification so I don't think that was 
too much of a contributing factor in mine.  I could push slightly on the 
parts mounted on the end of the tuning slug and it would jump around in 
frequency like crazy but I couldn't see anything actually move so I 
attribute it to poor electrical contact....

It's all reassembled and back in the radio....minus the heater stuff!  It is 
a joy to tune!  Very smooth sounding....  Now to fine tune all those little 
screws at the 100cps points...while in the radio!  Still planning that one!

Right now this SJC survivor tunes and hears as well as any radio in the 
shack and that includes an Icom 756 Pro II and a hot rod Leary SP-600...all 
sharing the same antenna through a military active multicoupler.  The Pro II 
needs preamp 1 on to keep up...the Leary is still the winner with best 
recovered audio and top notch sensitivity. (nice front panel IF gain 
control)

Improved audio is next.....more power, less distortion...then on to adapting 
look alike meters with 100 ohm inputs.

Remember this is the "lets see what is possible within the scope of the 
technology" radio....never intended to be a museum piece!

So far no extra holes...no modifications that would prevent module swaps 
beyond 12V tubes to eliminate the ballast tube and solid state stuff limited 
to rectifiers in the power supply and replacement of the selenium which had 
failed.  All molded paper caps replaced.  New filter caps and a few 
resistors replaced. Will need to go back and check more resistors now that 
some of them have been flagged by others.  Am running a 6BZ6 in 1st RF amp. 
Can't measure any additional sensitivity but it sounds much cleaner, a bit 
quieter and is every bit as sensitive as original.  Half turn removed from 
end point coil in PTO for end point adjustment.

Stay tuned...more to come for those interested.....

Cecil...

----- Original Message ----- 
From: "KC8OPP Roger S." <kc8opp at yahoo.com>
To: <R-390 at mailman.qth.net>
Sent: Wednesday, February 15, 2006 5:14 AM
Subject: Re: [R-390] Squirrely PTO

> 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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