eHam Forums => Boat Anchors => Topic started by: N1RND on January 19, 2013, 06:26:31 AM

Title: Need help with Halli. SX-71
Post by: N1RND on January 19, 2013, 06:26:31 AM
I'm in the process of restoring an sx-71.  When I got it, it had virtually no reception above the broadcast band and the S meter did not function.
I did the usual, replaced the lytic and paper caps, out of spec resistors, cleaned all switches and pots, checked tubes, etc.
Went through the alignment also.
The radio now has sensitivity and the S meter works on all bands, except band 3.
On band 3 the meter stays all the way to the right like when the radio is off and I can only hear my signal generator with no band noise.
I'm wondering if its something in the bandswitching circuitry.

Title: RE: Need help with Halli. SX-71
Post by: AC5UP on January 19, 2013, 07:37:25 AM
Hang your DC voltmeter on the AGC line and measure the voltage with a moderately strong station tuned in. Tune to a quiet spot on the same band and note the AGC voltage with that condition. This will tell you in which direction the voltage swings for strong signals and weak signals.

Switch to the band that's dead and do the same. Betcha' the AGC thinks Radio Habana is next door and it needs to go deaf.

Figure out why.

Title: RE: Need help with Halli. SX-71
Post by: W9GB on January 19, 2013, 12:53:45 PM
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 stage

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 IF

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.
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.

Hopefully you have GOOD IF slugs
Paul Pinyot's restoration of SX-71

Hallicrafters SX-71 Service Bulletin #1950-109
Decrease in Sensitivity of SX-71 Receivers
Scan of original courtesy of Rodney Bunt.

Souping up the 2nd IF in the SX-71
Phil Atchley, KO6BB
(MS Word version)

Title: RE: Need help with Halli. SX-71
Post by: N1RND on January 22, 2013, 02:09:31 PM
Thanks for the help.  I'm kind of busy with other things right now, but when I get time I'll get back to trouble shooting the radio.