[R-390] revised PTO endpoint adjustment (long)

[wli]

Hi. Have revised the previous post thanks to the comments received. Here is version 2.0
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PTO End-point adjustment
W. Li

1. From top of receiver, disconnect mini-BNC plug P217 and drop it thru the chassis hole to the bottom side. This coax carries the PTO output.

2. Disconnect P116 mini-coax from rear panel

3. Remove J116 from rear panel

4. Tune the MC dial to 7MC (I did this in anticipation of further RF work)

5. Tune the KC dial up to (+) 000

6. Turn receiver over so it is up-side-down on the bench

7. Remove Oldham spring using a fine forceps clamp (may have to rotate the KC dial a few degrees so that you can get at both ends of the Oldham spring). Tie a string on the spring before it flies off.

8. Do not touch the KC  dial again until  step 28. Consider locking it.

9. Remove both stainless steel 6-32 screws off of the trapezoidal metal support bracket on the back of the PTO.  This support secures the rear of the PTO to the chassis. Do not disturb the single green captive screw.

10. Loosen both front green 8-32 captive screws. The entire PTO subchassis should now be free to slide aft.

11. Wiggle the PTO subchassis and  slide it aft about an inch to free it from the KC shaft. The Oldham coupler will come apart and the center disc will fall free... save it. You now have one Oldham disc attached to the KC shaft, and the other  Oldham disc attached to the PTO shaft, and the middle one loose somewhere.

12.. Tilt PTO chassis upwards 45 degrees. Rest it on small plywood shelf so the rear bracket sits down in its compartment, and the front rests up on the chassis cable loom. This exposes the front of the PTO . Note: the cable in the back with the blue plug carries power to the PTO and is left attached.

13. Use J116 to adapt the PTO's mini-BNC to a standard BNC jack.

14. Hook up your frequency counter to J116 using a short BNC cable

15. On the front face of the PTO, remove the large hexagonal adjusting screw cover behind Z702. This cover screw has a very shallow thread, and comes out in 2 turns. Store it with the small Oldham spring for later.

16. Plug in the receiver and counter and allow an hour or longer to warm up and stabilize the electronics.

17. Using your hand, slowly twist the Oldham disc on the PTO shaft until the counter reads exactly 2455. Look at the counter to figure out which way to turn. Note: the PTO has its own stop, which can be easily damaged. GO SLOW.

18. Using the Oldham spring post as a point of reference, mark the front PTO plate with a pencil as your start point. 

19. Now manually rotate the Oldham disc on the PTO shaft exactly ten turns. Watch the counter readout rise as you turn. Stop at exactly ten turns lining up the spring post with your pencil mark. The counter now reads something fairly close to 3455KC. 

20. Without any further rotation of the PTO, turn the tiny PTO adjusting screw with a fine screwdriver until the counter reads exactly 3455KC. 

21. Go back about ten turns, stopping when the counter reads exactly 2455KC..

22. Chances are your start point will not align with the spring post, so erase the old pencil mark and make a new one. Changing one end-point alters the other.

23. Now hand rotate the PTO shaft exactly 10 turns the other way and see how close you end up to 3455KC. Chances are you will be off a bit.

24. Readjust the PTO adjusting screw again, so that the counter reads exactly  3455KC

25. Repeat steps 21-24 as often as necessary until 10 turns gets you from precisely 2455KC to 3455KC.(this took me 6 cycles of fooling around). Note all the PTO adjusting screw tuning  is at the 3455KC end. All you do at the 2455KC end is redraw your pencil mark.  

26. OK, now the PTO has the correct frequency range of 1000KC in exactly ten turns of the PTO shaft

28. Be sure the KC dial is unlocked. Gently turn the KC dial down as far as it will go until it is stopped by the 10-turn stop. The Veeder-Root  should now read between -963 and -972. If it does not, adjust the ten-turn stop until it does (another issue entirely).

29. Rotate the PTO until the counter reads exactly 3455KC

30. Rotate the KC dial up a few turns until the Veeder-Root reads exactly (-) 000, and lock it in place.

31. Stick the middle Oldham coupler disc to its mate on the PTO shaft with a blob of grease. 

32. Replace the large cover screw you took off in step 15. 

33. Now loosen the Bristo screw clamp nearest the KC dial, so that the Oldham coupler nearest the front may rotate on the KC dial shaft to mate precisely with the Oldham coupler disk of the PTO shaft. Since the KC dial is locked, only the Oldham disc and its clamp will rotate. Take great care NOT to rotate the PTO shaft. You can tell if you do inadvertedly rotate it, because the counter will show a change in frequency.

In some cases, there is no room to access the Bristo clamp on the KC shaft. Plan B is to loosen the Bristo clamp on the PTO shaft, so that it can rotate to mate with the other Oldham disc. Here again, you must not allow the PTO shaft to rotate whilst you fiddle with the Oldham disc. There is no way to lock the PTO shaft, so great care must be taken not to move it. Again the counter will detect any rotation of the PTO shaft at this time.

34. Now you can slide the entire PTO subchassis forward to mechanically re-engage the Oldham coupler........ this step is to mechanically synchronize the PTO to the rest of the RF gear train. This will require minute fiddling of the now loose Oldham coupler disc as the fit is very close.  Take your time in getting all the Oldham discs to mate.

35. Loosely start both 8-32 green screws in, to stabilize the PTO chassis

36. Now that the coupler is together, both the PTO shaft and the KC dial shaft will rotate together. The KC dial is at  (-)000 and the counter shows 3455KC.

37. Unlock the KC dial now.

38. Rotate the KC dial up slowly and gently until the spring posts are accessible for you to replace the Oldham spring. Note that the PTO shaft will also rotate the same amount since the coupler is now mechanically engaged.

39. Wiggle the PTO such that both PTO and dial shafts line up, and so that there is a small gap visible between the Oldham coupler discs. At this point, if things are tight, you can gain an extra mm by loosening the single rear green screw. 

40. Tighten the Bristo spline clamp on the shaft of the KC dial

41. Secure things by tightening down both front green screws, and reinstalling both rear deck 6-32 screws.

42. Now that the PTO is physically secured, do a final electrical check on the PTO with the counter. Turn the KC dial to (-)000. The counter should read exactly 3455KC. Now run the KC dial up to (+)000. The counter should read exactly 2455KC. This is the whole reason for setting the PTO endpoints.

43. If you can not get exactly 1000KC in ten turns, go back and redo things until you can.

44. Disconnect the counter, and restore the mini-coax interconnections.

45. Final check: tune in WWV, it should come in exactly at 5MC, 10MC etc

46. You are done
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