[R-390] What is the difference between 390 and 390A

[John KA1XC]

Hi,
I just browsed both Collins engineering reports in the Reference section of
the R-390 FAQ page http://www.r-390a.net/  and they didn't say anything
about that. However Final Engineering reports listed the reasons for the
double RF as image rejection and minimization of "spurious responses". Their
methodology involved putting both RF amplifiers under AGC control which then
kept the signals to the 1st mixer low enough. Later during the cost
reduction redesign they found that using a single RF stage with the newly
available 6DC6 tube provided just as good AGC control. It's a good read.

I have heard of some WWII receivers that were specially built to minimize
the osc leakage through the antenna input for the reasons you mentioned, the
National brand comes to mind but I could be mistaken.

John



----- Original Message ----- 
From: "B Riches" <bill.riches at verizon.net>
To: "John KA1XC" <tetrode at comcast.net>; "R-390 reflector"
<r-390 at mailman.qth.net>
Sent: Monday, April 11, 2005 4:03 PM
Subject: Re: [R-390] What is the difference between 390 and 390A


I thought the two rf stages were to keep the local osc from being heard by
others.

Regards,

Bill Riches, WA2DVU
Cape May, NJ
  ----- Original Message ----- 
  From: John KA1XC
  To: R-390 reflector
  Sent: Monday, April 11, 2005 3:42 PM
  Subject: Re: [R-390] What is the difference between 390 and 390A


  Bob,
  in addition to what's already been said, for *your* particular BCB DXing
  application I would rate the R-390A preferable to the R-390, but not
  overwhelmingly so.

  The 1st mixer is the first weak link in the RF chain, and the R-390 has
two
  stages of  gain ahead of it which put a much hotter signal into the mixer
  thus decreasing its intermod performance. The reason for 2 RF stages was
  image rejection, not sensitivity, and as you know when listening to the BC
  band it is full of power house signals that need to be dealt with.

  The A's are also much easier to find, fix, and maintain, and it doesn't
need
  to be a Collins manufacture unless you seek bragging rights.

  The sharp mechanical IF filters in the "A" are also excellent for
filtering
  away adjacent channel signals.

  John