[R-390] R-390A Changing VFO PTO EP adjusting range

[Larry H]

As we know, the aging of our PTO's has often caused the End Point adjustment to fall out of range and can no longer set the VFO range to 1 MH in 10 turns.  A relatively easy solution is to remove 1 turn of wire off of the adjusting coil, thus reducing the overall inductance and bringing it back into the 1 MH range.  This solution is probably OK on a Collins PTO, because its adjusting coil has 5 turns of wire on it originally.  Removing 1 turn only reduces the adjusting range by 20%.  This should still provide enough adjustment range for another 30 - 40 years.  The question is will someone have to go into it again in the future and make another change so that it can be brought back into adjustment.  Maybe.
As for the Cosmos, we all know that it's adjusting range is not near as large as the other PTO's.  That's because they added another coil in series with the original 2 in the Collins.  The 3rd inductor is necessary to allow for external linearity adjustment (see a few other write ups by Jim Miller, Tom Marcotte, John Harvie and myself) about the Cosmos).  In order to use the same capacitor scheme as in the Collins, the inductance of the EP inductor had to be reduced (thus reducing its range of adjustment).  So the EP inductor in a Cosmos only has 3 turns of wire on it, making its range smaller than the Collins.  Unfortunately, I've done 2 Cosmos recently where removing 1 turn would not bring it back into range sufficiently, and the range is also reduced by 33%.  It would have been barely usable (it was within 1 KH), but that's really not good enough.  And, in a few years, someone would have to go back in and make another fix in order to obtain 1 MH.  So instead, I put the 1 turn I removed back on and reduced the capacitance.   I know that the 2 10 pf flat wafer caps in there are extremely fragile, so I very carefully added an 82 pf N200 in series with 1 of them.  This reduced the total capacitance by 1.5 pf, making the EP adjustment work quite well again.  I did not see any side effects from doing this.
Regards, Larry