R390A/Cosmos PTO Alignment

Contributed by:      John Harvie
                                 jbharvie@erols.com

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.

 So do not take it as anything but my individual hands-on experience.

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 - You have been warned!

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

With that:

I "tuned up" both a Collins and a COSMOS PTO on one of my R390A receivers  about 9 years ago.  Roughly this is what would be along the line of what  I did recently for the COSMOS unit. (not knowing any better this turned out to be a very interesting educational experience)

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.   Bottom line, 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.

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

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:

The R390A 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

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:

Be gentle
 

Remove and open up the PTO
Remove the outer PTO can, heater and insulation (without  trashing the thermal insulation)
Unsolder where necessary

Once open, 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 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 PTO frequency output on the counter

Check the output v/s 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:

You will need to have a lot of time and patience. With care,  extremely good performance with the COSMOS PTO.
 

...My memory may have missed some items along the way...

"Your mileage may vary"

Best regards to all in cyber space!

John

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