[R-390] R-390A s/n improvement 6HA5 mixer

[Larry H]

I mentioned in my original post on 4-19-16 that with 6HA5's in the 1st and 2nd mixer places, the usual sensitivity measurement (S+N/N) nolonger worked.  This was only a problem in MGC mode.  Well, here's what happened.  The gain of the 6HA5 is considerably more than a 6C4, so when 2 or 3 6HA5's are used, the signal level at the 4th IF is way too high  (even with the IF gain turned all the way down).  This causes the 4th IF to go into saturation way too early and turn into a limiter (thus reducing the audio output and the ability to obtain a 10 db increase in S+N for the measurement.  The solution I chose is to reduce the gain of the 2nd IF by adding an 18K ohm res in the cathode circuit unbypassed.  This reduced the overall gain to where the usual sensitivity measurement (S+N/N) now works.  This also makes it easy to set the overall gain to 1uv input = 7v on diode load.  All is now good again.  The only measurement I've made so far is on 9.9 mh and it's .65 uv.  The measurement on 9.9 mh with the 3 - 6C4's using this method was 1.5 uv.   I'll report more measurements once completed.
I tried numerous different ways to reduce the gain of the 6HA5's, but could not find a way without eliminating the gain in S/N ratio they provided.
Regards, Larry