[R-390] R-390A Cosmos PTO repair and adjustment

Sat Aug 27 20:49:56 EDT 2016

Great stuff Larry...this will probably end up in the "Pearls".

Cecil
K5DL

Sent using recycled electrons.

> On Aug 27, 2016, at 7:31 PM, Larry H <dinlarh at att.net> wrote:
> 
> I went through a Cosmos and got it working quite well.  I'd like to share my experience with you.  There was a lot of great information online, and I studied it carefully.  Thank you all very much who contributed.  I like the design when the linearity mechanism is in good condition and working correctly.  If not it can be a big head ache.  Opening it up is like any other PTO.  Before you do, check the end play on the main shaft.  There should be none that you can feel.  If there is and it goes unrepaired, you will have forward/backward tracking issues, and probable warbles trying to zero beat a station or trouble tuning in CW or SSB stations.  You should also check the rotational freedom and smoothness of the shaft.  You need a slight resistance, but it must be absolutely smooth from end to end.  If not, it will have the same tuning and wear issues.
> 
> To help you locate a couple key components,  look at these photos: 
> 
> P1. https://s22.postimg.org/ul7jtkbgh/IMG_5889s.jpg 
> P2. https://s16.postimg.org/56s9uzjyt/IMG_5899s.jpg 
> 
> The EP adjust is the slotted screw on the right.  The linearity screws are on the left right above the tuning shaft.  Note that the hole is centered above it.  One screw should be centered in the large hole with the 3 visible, at each 25 kc point on the dial.  The centered one is the currently active one.  Do not adjust the ones on either side.  This is the one pressing on the metal spring that is pushing on the spring loaded core of the linearity adjusting coil.  As the KC knob is rotated 25 KC, the next adjusting screw will become centered.  There are 2 rows of screws on the disc.  One from the top row will be centered, then one from the bottom row, etc.
> 
> Once it's open, here's the 3 inductors I saw:
> 
> P3. https://s3.postimg.org/4gbsaf01f/IMG_5589s.jpg 
> P4. https://s21.postimg.org/ikdk0v2zb/IMG_5594s.jpg 
> 
> The short one is the EP adjustment on the bottom.  The one right above it is the linearity compensating coil.  It is a spring loaded core controlled by the adjusting screws.  This is the linearity mechanism:
> 
> P5. https://s16.postimg.org/7h6y7xl4l/IMG_6112s.jpg 
> P6. https://s21.postimg.org/oc1n30u07/IMG_6115s.jpg 
> 
> The first one shows the red phenolic button the cam actuates to move the core inside of the linearity inductor.  The second shows my screw driver manually moving the core by pushing on it.  I'm making sure it is moving freely and has good spring resistance.
> 
> The next photos show the PTO opened up at the cam operating area.  It's the round flat metal spring with the cam on it on the opposite side from the red button in the first photo.  The cam is moved by the position of the adjusting screws shown in the second photo. 
> 
> P7. https://s9.postimg.org/twbll51ov/IMG_6162s.jpg 
> P8. https://s13.postimg.org/4n7xow1o7/IMG_6164s.jpg 
> P9. https://s13.postimg.org/rn875v6dz/IMG_5978s.jpg 
> 
> I believe that the linearity adjusting screws can turn a total of 5 turns before falling out.  Be very careful that they don't fall, as it is difficult to get them back in.  You will probably have to take it apart.  The third photo shows a close up of the adjusting screws.  When they are unscrewed as far as possible CCW, they will stop on the disc that holds them due to the flat head being larger than the threads.  I used a jewelers screwdriver to adjust them, but be careful to not damage the splines.  If one is stuck, you will need to use a .033 inch 4-spline wrench.
> 
> Here's pictures of the inside showing the moving tuning core:
> 
> 
> P10. https://s21.postimg.org/cbroii8vb/IMG_6106s.jpg
> P11. https://s12.postimg.org/807j8rn99/IMG_5597s.jpg 
> 
> The photo P10 shows the linearity inductor pulled away from the frame.  You will also see 2 flat compensating caps.  They are very fragile.  Be very careful with them.  In order to bring the EP adjustment into range, one of these needs to be reduced in value by 1.5 to 2 pf.
> The next one shows the split nut on the end of the tuning core.  It has a spring clamp around the end of it to maintain the correct pressure on the shaft.  The purpose of this mechanism is to eliminate end play between the core and the tuning shaft.  This is a critical point for minimizing forward/backward tracking alignment and creating smooth frequency change when the tuning shaft is turned.  It needs to fit just right.
> 
> P12. https://s15.postimg.org/91cx5zk4b/IMG_6158s.jpg 
> 
> The next two pictures show the flat ring retainer held in place with three round head screws, has been removed in order to unscrew the split nut a little.  I did this in order to check the tightness and play on the shaft.  The nut can be removed for adjustment, if necessary.
> 
> P13. https://s21.postimg.org/z3i7lf7nb/IMG_6152s.jpg 
> P14. https://s4.postimg.org/fzl51ljot/IMG_6155s.jpg 
> 
> 
> If you need to remove the core from the shaft, you might loose synchronization between the core position and the linearity disc position.  If this positioning is lost, you will need to go through the linearization from the start.
> 
> It is imperative that the metal guide arms and the base for the split nut attachment plate are rigidly attached to the core.  If not, it needs to be repaired or replaced.  One guide arm is rigid and the other has a spring attached to it in order to keep a reasonable amount of pressure on the aluminum guide bar (it should be lubed very lightly with a very thin slippery lube (not oil)).
> 
> After it's in good condition and before you put the covers back on it, see if you can operate it on the bench.  A good test is to hook up your counter to it and put forward and backward pressure on the core to see if it will move.  Don't worry about the actual frequency, as it will change 40 to 50 cycles when the cover is put on.  It should move about 7 to 10 cycles and then go back to the original reading when pressure is released.  Do not use a lot of pressure, but enough to move it at least 1 cycle.  If it moves about 15 cycles and goes back to the original setting, it's probably ok.  Ideal would be about 7 cycles.  If it does not go back to the original reading within 1 or 2 cycles, repair is needed.
> 
> Also check the guide arm operation.  Rotate the tuning shaft slowly and quickly in both directions and make sure the arm contacts do not move away from the guide bar.  If they do, repair is required.
> 
> Happy Cosmos'ing.
> Regards, Larry
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