Collins 390 PTO calibration for newbies

[iw5ci]

I would like to align the pto linearity in my collins 390 non A.

From start to end of any mhz segment i have 3 - 4 khz of disalignment on the mechanical digital readout.
My 390 mounts a Cosmos PTO.

I would like to do this procedure, but without doing a mess. Is it simple? how to do it?

[w3jn]

It's pretty easy, but a lot of gynmastics taking the PTO in and out.  Set the receiver to 7+000 MHz.  Align the PTO shaft using the KHZ adjust so that the Oldham coupler slot is vertical.  Remove the PTO per the manual (release the backlash springs with a needlenose pliers, making sure they don't fly away, loosen the green captive screws).    The PTO will now be loose, remove it and catch the Oldham coupler as it falls away.  Remove the plug covering the endpoint adjust on the PTO, revealing a flat tip screwdriver adjust inside.  Note that if you have a Cosmos PTO there will be 2 screws or nuts - one is the linearity adjust and the other is the endpoint adjust.  DO NOT mess with the linearity adjust screw unless you want big headaches.

Use the proper sized *plastic* alignment tool and adjust it 1/2 turn in or out (doesn't matter).  Reinstall the PTO in the radio (don't bother with the springs just yet).  Putting a bit of grease on the Oldham coupler will allow it to stick to the shaft.

Now see if your adjustment made it better or worse.  Turn on the xtal calibrator.  Adjust close to 7+000 for zero beat.  Now tune down to near 7-000 and see where the zero beat falls.  Better or worse?  You should have EXACTLY 10 turns of the KHZ dial (ie 7.002 to 7+002, or 6.998 to 7.998) for EXACTLY 1 MHz of dial readout.  If it's worse, remove the PTO again and adjust the screw 1 turn the OPPOSITE direction you turned it the first time.  If better, but not exact, remove the PTO again and turn it another 1/2 turn in the direction you turned it the first time.

Repeat until it displays ExACTLY 1 MHZ zero beat to zero beat.  If you can't get the mechanical zero adjust to stay in range, loosen the shaft coupling on the PTO and adjust the PTO shaft while holding the KHZ knob steady.

When done, replace the nut covering the endpoint adjust, replace the PTO in the radio, and replace the anti backlash springs.  It's a lot of messing around taking the PTO in and out unless you can fashion a tiny adjust tool to adjust the PTO while it's in the radio.  I did that once but it was pretty frustrating, I just ended up doing it the "hard" way.

[k4kyv]

I recall with mine that it is not too difficult to make this adjustment without removing the PTO.   I haven't worked on it or used it for years, so I would have to look inside to  remember exactly how to do it. I seem to remember using the same special home-made tool that I use to make the adjustment with the 75A-4.

Maybe some web searches would turn up more information.

[w3jn]

Indeed, it'd be much less time consuming to fashion the right tool to avoid removing the PTO a dozen times. 

[k4kyv]

Also, the procedure described in the R-390A manual works for the 75A-4, better than the procedure described in the A-4 manual.  The 390A procedure involves extrapolating the adjustment (~80% IIRC) beyond the zero-beat point, which often allows one to hit it dead on the first try.  The method in the A-4 manual is to adjust the slug to zero-beat, then to tune back to the opposite end of the range, re-zero and to keep repeating the procedure until the best linearity is achieved.  When you  zero-beat with the slug, and then re-tune to the opposite end of the band to re-set the calibration point, that throws off the adjustment you just did, so the whole process has to be repeated a half dozen or more times.

Also, since most PTOs are not perfectly linear over the entire range even when the end points are precisely set, I have found that you get better overall calibration accuracy across the dial scale by using the +100 kc and +900 kc points for the end-point adjustments, rather than 0 kc and +1000 kc as the manual recommends.

[w2emn]

The Cosmos has two adjusting screws, both require the removal of a larger screw which seals the enclosure.  The end point screw is in the upper right as you face the front of the unit, and partially hidden behind the transformer can.  The screw access directly over the shaft is actually 42 little screws used to adjust the linearity, and should not be messed with unless you have a LOT of patience. 
The end point can be adjusted as described above, providing you find a small jewelers screwdriver to fit the screw.  In some cases the adjustment has run out of range due to drift in component values, in which case you need to disassemble the PTO and take a turn off the adjustment coil.
For adjusting the end point, I'd go with the remove and adjust approach.  I've done the linearity, and I cut down a small screwdriver so I could do the adjustment while the PTO was in the receiver.  It was a real chore, using a small mirror to see where to put the screwdriver, as I adjusted 42 screws several times each.
Good luck
Ed  W2EMN