[R-390] Field Change No. 7, R390A

[Tom Warren]

Anybody got clues to the intended and non-intended effects of Field Change
No. 7 to the R390A?  This is the change where R702 (screen resistor on the
PTO) and R210 (screen resistor on the 1st crystal oscillator) are changed
from 56K to 220K, presumably to reduce radiation to the outside world (so
the enemy can't listen for your PTO freq or your 1st oscillator freq and
then bomb in that direction).  My guess is that the first consequence is to
reduce the output level of each of those oscillators.  Secondly, there may
be some harmonic reduction from each of those oscillators.  However, what
else happens?  That is, is the sensitivity of the receiver or the intermod
performance reduced by the modifications?  Also is the birdy response
affected by the modifications (e.g., less harmonic output from those
oscillators leads to fewer internal mixer products).  Any other effects that
folks have definitely found?

My reason for asking is that I'm working on a Motorola PTO with the 56K
screen resistor and a later Cosmos PTO with the 220K screen resistor and
wondering if I ought not to "re-convert" the Cosmos PTO to have a 56K screen
resistor.  R701, R702, and R703 are out of spec on the Cosmos anyhow, so I
will probably change them out, but if a better choice for R702 is 56K
considering that I'm not worried about the enemy direction finding on my
oscillator outputs, then maybe that's what I ought to do.

For Tom Bridgers and Al Parker, yes, the Motorola PTO is from the Charlie
Taylor (hello Charlie, if you're out there) 390A.  AND I'VE GOT THE CHARLIE
390A MOSTLY WORKING. HOORAY AFTER ABOUT TWO YEARS.

Tom, W4PG