[Hallicrafters] Need help on documentation for a SX-28

[Bill Feldmann]

Hi All,

I'm new to the reflector and hope this is the place to find some information
on my SX-28.

I purchased a fairly clean SX-28 about a month ago and have completely
recapped it along with aligning it.  When doing the alignment of the noise
limiter I noticed that transformer T5 for the noise amp is different than
the manual copy I have and also could not find C55 to tune the 455kc trap
for the limiter.

I next started to trace the circuit and found it wired much different than
shown in the manual.  It seems to have a coil and some caps in the line from
the noise detector to V5 the stage controlled by the noise detector.  There
are also some caps with the coil that makes it look to me like some kind of
a low pass filter.  Also the noise amp uses the first AVC amp as a preamp
which is different than the manual.

On further inspection of the radio I found some different resistor values on
SW8-14 the switch that changes the first IF amp tubes gain on different
bands.  Also there is no SW8-15 to change the first AVC gain to the grids of
the RF amp tubes.

I next looked at two other SX-28's a friend has and they are wired exactly
like the schematic in my manual copy.  My thought then was maybe someone
modified mine but on inspection of all the different parts they look like it
was factory wired that way.  I did also notice the two other SX-28's had the
hole for the coil only mine has for the noise amp output but no coil there.
I now wonder if mine is some type of prototype SX-28 or a very early one
that is very different than most.

I wonder it any of you have run into SX-28's so different from their manual
and if someone would have a schematic for my receiver if it's stock or where
I could get a copy of one.  Also if anyone knows the history of these
differences.

The S/N of mine is H-120864 which seems like a high number.  Also mine has a
small cover held by thumb screws over the RF, mixer and oscillator tubes.
Except for the noise limiter it seems to be working fine but I would like to
find some factory documentation on it.

Bill N6PY