[R-390] R-390(A) gain distribution

[Tim Shoppa]

I grew up on 70's era QST's and ARRL handbooks with W7ZOI
receivers.  These are extremely simplistic and generally don't
have any more gain than needed.  What gain there is almost
always comes after the mixer.  (Except for maybe the high bands
where there'll be a RF amp on the front of the receiver).

Obviously the R-390/390A had a different set of critera and were
working with differently-capable components.

One criteria that I understand immediately is that you don't want
any of the local oscillators leaking back out to the antenna in a
military setting.  So the RF stage before the first mixer is a necessity.
It just so happens that there is plenty of selectivity around it too.
I'm just a few blocks from a powerhouse MW broadcaster and I
never have intermod problems in my R-390A!  So even though there
is an active stage before the first mixer I've never had problem with
IMD there.  This is one point where the W7ZOI designs have to
deviate: the transistors just never have nearly the dynamic range
of a tube.

But 4 stages of 455kHz IF... why so much gain there?  Most receivers
of the era had 2 (cheapie) or 3 stages and do just fine.

How much gain is typically the 3-mixer chain capable of?  Each mixer
gets fed from the AGC control line, right?

It's been a while since I looked at the Collins cost reduction report...
is there any thought given in there as to eliminating an IF stage?

Tim.