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

[Jim M.]

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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