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

[Larry H]

   In one of my previous posts on this subject I let you know that the additional gain of the three 6HA5 mixers is too much and that the IF gain adjust could not compensate for it. The higher gain does not allow the normal S/N measurement (in MGC mode) to work correctly. The 4th IF is being swamped and turned into a limiter.  I've also discovered that it could adversely affect the operation of the filters.  In order to adjust for this, the gain must be reduced at the 1st IF stage.  I tried a few different circuits, but nothing worked until I tried a simple cathode follower.  It uses a 6C4.  Here's a link to the diagram:
     

     https://s31.postimg.org/xn1tybe8r/IMG_7219s.jpg

My 1st iteration of this circuit did not include the 18k resistor on the output.  What I found is that tuning the input of the filters was not effective because of the low impedance on the output of the circuit.  Now they tune well.  The sensitivity measurements are still very good.  

   I've been using the 3 6HA5 mixers with this 'gain reducer adapter' for a few weeks now, and it's working quite well.  The IF gain adjust provides very good adjustment range and the traditional S/N measurement is working correctly.

Regards, Larry Haney