[R-390] R-390 Poor Sensitivity

[Larry H]

Hi Mark,  The RF deck on the 390/urr can be challenging.  The correct injection frequencies need to be calculated for each mixer for each 1mc band.  The first mixer and xtal osc varies by mc band and only used when below 8mc.  You might try a frequency above 8mc, since it is much simpler.  Then above 8mc, the second mixer subtracts the dial frequency from the 2nd xtal osc freq to create the input to the 3rd mixer, which is always 2-3 mc.  So to start with, you should verify that the mechanical alignment is correct, the tuning cams, 1st and 2nd xtal osc.  The operation of the 1st and 2nd var IFs is very important.  The 2nd var IF should always be at it's top (3mc) when the dial is on xx.000.  Inject the dial frequency into the antenna and verify that the 3 caps on the 2nd var IF will peak correctly (not at the end of their range).  For the 1st var IF, I like to use 7.9 mc input for this test.  The 1st xtal osc freq is 10 mc for 7mc ant input resulting in 17.9mc out of 1st mixer.  If all 6 resonant circuits do not peak correctly, that is the first thing I'd go after.  I usually find a bad connection on these cans pins or a bad mica cap inside or a bad xtal or switch connection.  Use deoxit very sparingly.  All 3 for each set are the same, so you can swap them around to see if the symptom follows.  Don't forget that the slugs in the var IF are different than the RF slugs.  Good luck.
Regards, Larry