[R-390] RE: COSMOS PTO

[Laird Tom N]

>From the WC9M archives:

Hope this helps,

Tom Laird WC9M
------------------------------------------------------------
The following is an old post from JB Harvie........

Let me start by saying that the alignment of A COSMOS was for me a tough
difficult and stress producing procedure. More than likely there are 
experts out in the group who will take exception to this "simple"
posting. 

Candidly speaking I suggest that anybody who is considering this closely
assesses the performance of your PTO PRIOR to any opening up and
set-screw
adjusting, even though the performance might be off and you might be
tempted 
to do so the as-is performance might just be as good or better than the 
following intervention you can likely make.

For a learning experience, on a spare unit, or if you are bold GO FOR
IT!

I "tuned up" both a Collins and a COSMOS PTO on one of my R-390A
receivers
about 9 years ago. This is what would be along the line of what I did
for 
the COSMOS unit.

It was a real pain in the rear and took me a lot of time but in the end,

(no pun intended) performance was significantly improved. Today the
receiver 
still operates no more than one tick (200 Hz) on all the calibration
points.
  
With enough time and some care and some special tools you can work on
the 
COSMOS PTO, results are dependent on your overall setup, equipment, time

and finesse.

You must have a good stable and accurate frequency counter or some other
means
of measuring the PTO output to say 10Hz precision.

Some tips:

I found I could obtain better results by slightly loading the output of
the PTO
under alignment into a non inductive load.

Raise and maintain (for 4 hours minimum prior to any sensitive work) the
PTO to
a warm temperature by placing it under a 75 watt light bulb, mounted
overhead,
adjust the distance from the bulb to the PTO so that the unit runs at
about the
(indoor) receiver in-use operating temperature.  Try to keep the
temperature 
stable while you are working on the unit. If you must stop mid-stream
you may
have to back track and do it again, keep the light (heat) on at all
times while
you are working on it. If you keep the door closed on an inner work room
I 
found the temperature variations are within a couple of degrees. 

As you start to get into the PTO guts diagram everything you see as this
is 
what will become your owner / operators and recovery manual for your
PTO.

TAKE YOUR TIME

Mcmaster Carr sells adequate precision 6" and 8" diameter dial face
plates with
reasonable precision index marks for around $10.

Mount this dial to an Aluminum backing plate which has been drilled for
the 
shaft diameter (0.185 inches) and secured (epoxy is OK) to some sort of
a 
shaft clamp, mounting close to center (0.005") is somewhat important.  
Once your happy tighten up the clamp onto the PTO shaft, this gives you
a 
good dial reference for making repeatable angular displacement
measurements. 

The 8 inch diameter dial, though bigger does a little better job to
resolve,
measure and enable a better than 400hz shaft angular result.  If desired

a 10 turn counter (with lock) can be added to the alignment mechanism 
(such as the 400 series from Kilo International - Digi-Key page 376 in 
catalogue Q983 for $15.75 part number 412KL-ND)

PTO basics
PTO provides range of 3.455 to 2.455 mHz span

         therefor:
         10 shaft rotation = 3,600 degrees = 1,000 kHz
          1 shaft rotation = 360 degrees   = 100 kHz
          
          1/2 turn         = 180 degrees   = 50 kHz    
          1/4 turn         = 90 degrees    = 25 kHz
          1/8 turn         = 45 degrees    = 12.5 kHz
          1/16 turn        = 22.5 degrees  = 6.25 kHz
          1/32 turn        = 11.25 degrees = 3.125 kHz

          1/64 turn        = 5.625 degrees = 1,562.50 Hz
          1/128 turn       = 2.813 degrees =   781.25 Hz
          1/256 turn       = 1.406 degrees =   390.625 Hz
          1/512 turn       = 0.703 degrees =   195.31 Hz

       * Operational Limit = 1.79 degrees   = 500 Hz
       * (Reasonable)      = 1 degree      = 277.78 Hz          
       * (Obtainable)      = 1/2 degree    = 138.89 Hz

Per TM 11-856A paragraph #150:
..Total End Point error on PTO not to exceed 500 Hz....(or about 2
degrees
of shaft rotation)
      
To proceed:

Remove and open up the PTO

Remove the outer PTO can, heater and insulation (without
trashing the thermal insulation) 

Unsolder where necessary

Be gentle

Once it is opened up examine the PTO closely, look for damage or
excessive
lead screw wear.  Lubricate the lead screw if absolutely necessary with
an
acceptable lubricant.

Mount the PTO to a suitable stable test fixture, mount and insure that
the 
dial face is securely mounted and rotates perpendicular to the PTO
shaft.
Verify the turn counter works (if used) 
Insure you can gain access to and rotate the set screws. I had to make
my 
own set screw driver as the set screws are real small! One option is to
obtain the smallest possible driver and stone it down.
 
Mount and check that the reference pointer is secure and that the dial
clears
the sharp pointer by giving the dial a full turn.  

You can place a fixed magnifier lens and lamp to assist you in seeing
the 
lines as needed but with a 8 inches dial a 1/2 degree spacing is quite
readable.

Connect up the power to the PTO, 

Connect the PTO output to a load resistor and the frequency counter.

Bring up and maintain the (exposed) PTO to a "warm to the touch"
condition
with the lamp.

Turn on the PTO

Look for a frequency output.

Check the output vs. shaft rotation and move the shaft to get to the
high end

Double check the output range (its still not to late to turn back :)

As you rotate the PTO you will see how the 40 or so adjustment screws
are
mounted in an perimeter array and what they do. These screws act to set
into 
a precise position a metal ring on which rotates a coil plunger. The
plunger
depth is "modulated", by the degree to which the set screws deflect the
metal
ring. These plungers are what "trims" the precise PTO output frequency.
As you rotate the PTO shaft the 40 or so set screws are need to be
"adjusted" to precisely establish and define the mechanical plunger
depth 
(position) so that the desired precise frequency output is achieved. 

Keep track of which and how much you turn the set screws - make a
diagram 
and keep good notes.

If you mess up take a break and start over.
Always check the end to end (10 turns) as well as the 1 turn
performance.
After you are completed, turn the PTO off, let it cool off overnight,
maintain
the lamp in position,  turn the PTO back on, let it heat soak for
another 4 
hours and (re) measure the results.

Settle for a reasonable performance level and call it good enough

Reassemble with care

Bottom line is:
You will need to have a lot of time and patience however you
can obtain extremely good performance with the COSMOS PTO.