[R-390] R-390A IF gain setting and S/N measurement reading

[Larry H]

   We have heard for a long time that the optimum setting for the IF gain is important for correct operation and good S/N measurement.  The doc I've seen says that 150 uv input to the IF deck should create -7 volts on the 'diode load' point in MGC mode.  And that 1 to 4 uv input to the balanced antenna connection should result in the same -7 volts.  No problem for a correctly operating R-390A.  

   Now during my recent 6HA5 mixer testing I did a lot of S/N measuring and discovered something very interesting about the IF gain setting.  With no changes to the RX, except for adjusting the IF gain, the S/N measurement readings are better with the IF gain reduced.  And this is not just a trivial amount either, it varied from .4 uv to .9 uv (while keeping the IF gain in a reasonable usable point (IE: not at all close to the extremes)).  This happens on both of my R-390As.

   I believe that the real S/N of an RX is determined by the first 4 or 5 tubes and circuits in the RX, and those after that point really don't affect it (if they are working correctly).   Since the IF gain is in the 7th stage of the RX (3rd stage of IF), changing it should not affect the real S/N.  I have no way to measure it's real affect, but I don't hear any difference.

   So why does it make a difference in the measurement?  And do we really need to consider S/N when setting it?

Regards, Larry Haney