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

Thu, 02 Jan 2003 14:46:30 -0500

"Radios in supplementaal spaces" is probably spook-speak for classified
listening posts.  They (whoever "they" are) would want to reduce any
local oscillator or PTO signal emissions that could be intercepted by an
enemy in order to deduce the frequencies being monitored.  I suspect the
screen resistor changes helped reduce oscillator output.  Alternatively,
it was to reduce their output to prevent interference with other
receivers.

David Wise wrote:
> 
> I also installed FC7, and like Jim found that it decreased
> the sensitivity, even after adjustment of the IF gain
> for maximum S/N.
> 
> I removed it.  If I acquired a radio or module
> that had it, I would remove it.
> 
> We've never found the original documentation
> explaining the reason for FC7.  After some thought
> about the target ("radios in supplemental spaces"),
> I believe it was meant for less-important R-390As
> so they wouldn't interfere with more-important
> ones.  It obviously does not apply :-)
> 
> 73,
> Dave Wise
> 
> > -----Original Message-----
> > From: Jim M. [mailto:[email protected]]
> > Sent: Thursday, January 02, 2003 7:27 AM
> > To: Tom Warren
> > Cc: Morgan, Roy; [email protected]
> > Subject: Re: [R-390] Field Change No. 7, R390A
> >
> >
> > I installed the mod. to my R-390a last year to see what would
> > happen.  I
> > found that it decreased the sensitivity noticeably.  By that
> > I mean that
> > a 40 dB test signal now read 30 dB on the carrier meter.  Not an exact
> > test but it was enough to convince me to remove the mod.  I
> > did not look
> > at the more esoteric effects such as harmonics or spurs.  I
> > like a "hot"
> > receiver.  If it were me, I would remove the mod and go back to the
> > original values.  Jim N4BE
> >
> > Tom Warren wrote:
> > >
> > > 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
> > >
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