Chuck McGregor re-drew the SX-71 IF section schematic with notations.
He commented on Antique Radio restoration forum:
"Unlike the SX-42 and SX-62 which run the First IF at fixed gain using a 270 ohm cathode resistor to ground,
the SX-71 returns both the First (V4) and Second IF (V5) cathodes to the wiper of the sensitivity control, through different resistances and changes those resistances for the Second IF
on a band-by-band basis (Switch 1, Section 5 [rear] wafer).
It would be great to find an explanation of WHY, from the guy who designed it ...
CHECK your Second IF (2075 kHz) which is used on Bands 3 thru 5.Photo of converter stagehttp://www.ppinyot.com/hallicrafters2/IMG_1322.JPG
A poor alignment of the Second IF (2075 kHz) OR
Bad mica capacitors (220 pF) in the IF cans will make Bands 3 to 5 nearly deaf.Photo of good cap on second IFhttp://www.ppinyot.com/hallicrafters2/IMG_1325.JPG
IF you only have issue on Band 3, LOOK carefully (magnifying lens) for
contact corrosion, carbon blackening, or broken contact fingers on the band switch wafer.
SWITCH 1, SECTION 5, REAR WAFER.See the higher Second IF (V5) cathode resistance for Band 3, switching in R32 (2700 ohms)??
CAREFULLY check this area of circuit and measure R32.http://pcbunn.cacr.caltech.edu/jjb/Hallicrafters/SX71/SX71-2ndIFa-Chuck.gif
Hopefully you have GOOD IF slugs
Paul Pinyot's restoration of SX-71http://www.ppinyot.com/H/sx_71.htm
Hallicrafters SX-71 Service Bulletin #1950-109 Decrease in Sensitivity of SX-71 Receivers
Scan of original courtesy of Rodney Bunt.http://pcbunn.cacr.caltech.edu/jjb/Hallicrafters/SX71/1950-109_SX-71.pdf
Souping up the 2nd IF in the SX-71
Phil Atchley, KO6BBhttp://www.w9wze.net/df.php?dn=Restorations/SX71.wp
(MS Word version)http://pcbunn.cithep.caltech.edu/jjb/hallicrafters/SX71/SoupUpYourSX-71.doc