[R-390] setting the PTO endpoints (long)

[wli]

Over the last 10 years, there have been many posts about the PTO. Half dealt with the non-linear PTO, which is clearly a challenging and time-consuming task. The others have been more elementary: dealing with setting end-points.  Chuck Rippel correctly points out that setting the endpoints is probably the most important thing to do to get these receivers functioning as they were designed to operate. 

Recently, I set the endpoints in my 62 Capehart PTO. What follows depends on using a counter, and avoids dropping the front panel. 

The Y2K, Navy, and Army manuals use a different approach that does not require a counter, but I felt that approach was too cumbersome. Furthermore, they drop the front panel in their method.

I used the wisdom contained in previous postings to this list to try to get everything everyone said on one piece of paper. There were valuable tips contributed by all. To that end, I documented each step in my task... trying not to forget anything or goof it up.  I managed to get my PTO to cover 1000.2KC over exactly 10 turns, so I was happy.

So here is how I did it:

1. From top of receiver, disconnect mini-BNC plug P217 and drop it thru the chassis hole to the other side (the same side as the PTO)

2. Disconnect P116 mini-coax from rear panel

3. Remove J116 from rear panel  (this is the IF out BNC adapter)

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

5. Tune the KC dial to (+) 000 

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

7. Remove Oldham spring using a fine forcep clamp (may have to rotate the KC dial a few degrees so that you can get at both ends of the Oldham spring). Write down the Veeder Root reading at this point and lock the KC dial.

8. Loosen the Bristo screw on PTO shaft clamp, 

9. Remove both plain 6-32 screws off of back of triangular metal support of PTO, to release the back of the PTO subchassis off of the main chassis. Do not disturb the single rear green screw.

10. Loosen both front green 8-32 captive screws

11. Wiggle the PTO subchassis and  draw it straight backwards 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˚. Rest it on small plywood shelf so the rear bracket sits down in its compartment, and the front rests on the chassis cable loom. This exposes the front of the PTO which is now tuned to about 2455KC, the high end

13. Attach the now loose J116 BNC-to-mini-BNC adapter to the mini-coax with P217 from the PTO chassis, in order the mini-BNC cable can be mated up to a standard sized BNC connector

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

15. Remove the large hexagonal adjusting screw cover behind Z702, pulling it out thru one of the holes on the front PTO support bracket. This cover screw has a very shallow thread, and comes out in 2 turns.

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

17. Using your hand, twist the Oldham disc on the PTO shaft until the counter reads exactly 2455. Using the Oldham spring post as a point of reference, mark the front of the PTO plate with a pencil as your starting point.

18. Now manually rotate the Oldham disc on the PTO shaft exactly ten turns. Watch the counter frequency rise as you turn. At exactly ten turns the counter should read something close to 3455KC. say, within 20% 

19. Now turn the tiny adjusting screw with a fine screwdriver until the counter reads exactly 3455KC

20. Go back approximately ten turns until the counter reads exactly 2455KC.. 

21. Your pencil mark will now not line up with the spring post, so erase the old pencil mark and make a new one.

22. Now advance PTO exactly 10 turns and see where you end up

23. Readjust the PTO so that its output is again 3455KC

24. Repeat as often as necessary until 10 turns gets you from precisely 2455KC to 3455KC.(this took me 6 cycles of fooling around)

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

26. Unlock the KC dial

27. 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

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

29. Rotate the KC dial until the Veeder-Root reads (-) 000 and lock it

30. Stick the middle Oldham coupler disc to the one on the chassis side with a blob of grease

31. Carefully rotate the chassis-side Oldham coupler clock-wise until it can mate up precisely with the PTO coupler disc, at which time you can slide the entire PTO subchassis forward........ this step is to mechanically synchronize the PTO to the rest of the RF gear train.

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

33. Rotate the KC dial up 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 mechanically engaged. 

35. 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. 

36. Tighten the Bristo spline clamp on the PTO shaft

36. Secure things by tightening down both green screws, and reinstalling both 6-32 screws. 

37. Now that the PTO is physically reinstalled, do a final check 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.

38. If it does not, go back and redo things until step 37 is met

39. Disconnect the counter, and the mini-BNC to BNC adapter.  Plug P217 back into the RF chassis, remount J116 and P116.

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

41. You are done