View Single Post
  #7   Report Post  
Old September 10th 09, 09:48 AM posted to
Richard Knoppow Richard Knoppow is offline
external usenet poster
First recorded activity by RadioBanter: Oct 2006
Posts: 527
Default SX-28 image problems

"petev" wrote in message
On Sep 9, 9:14 pm, "Richard Knoppow"
"petev" wrote in message


Thanks for your responses! This SX-28 is far from stock
and is
extremely over-sensitive. I bought it about a year ago
help me
compare wiring and performance to my 1st SX-28, but this
one was so
modded up, it was no help. The radio works ok, though,
requires a
of knob tweaking to make a signal sound good. I have to
run the RF
gain at
4 or below. I used to think that was a good thing
until noticing all the cross-mod on 40 and 49 meters.
There is a 6SG7 where the 6AB7 1st RF ought to be.
This receiver also has a 6H6 installed where T5 used to
be.(far rear
There is a 2nd 6H6 installed next to T6 (where the stock
6AB7 used to
So thats 2 6H6s! There is an additional switch on the
faceplate for
the noise limiter,
and the original noise limiter pot was replaced with a
There is also a 6SN7 where the stock 6H6 used to be.
The 6B8 that was with T1(1st IF) has been replaced with
(The 6B8 pentode wire in T1 has been removed.)
The S meter was replaced with one labeled "carrier" and
has very fast
It does OK on SSB, but the BFO sometimes has a little
Its been re-capped, but not the RF deck. The dial
calibration is dead-
If you have any thoughts as to what these mods were
I'm all ears. In the meantime, I'm going to replace the
hotter tubes
with the stock originals,
one by one, and see how that affects performance. I
intending to do a
complete restore on this SX-28, having done one already,
and have had
that thrill.
Thanks for your help!

There should not be either a 6SG7 or 6AB7 in the
receiver. Both RF stages are 6SK7s and that may be part of
the problem.
When the SX-28 was designed it was equipped with a Lamb
noise blanker. This was a complex circuit intended to
out the IF when a noise pulse came along. Such blankers
fairly common now but its characteristics were not very
understood at the time so the SX-28 blanker did not work
well. Hallicrafters later offered a modification which
removed the Lamb circuit and replaced it with a stadard
noise clipper. That may account for the extra tube.
Also check to see what was done to the AVC. If its been
modified it may be worth unmodifying it. Note that the use
of high gain tubes may upset the AVC permitting the lower
gain tubes to be overloaded because the high gain tubes
shut down with much lower AVC voltage than the other tubes
need to be linear.
The SX-28 does not have very good image rejection and
tends to have a lot of spurious responsed due, probably,
lack of adequate shielding. Image rejection depends on the
bandwidth of the RF stages. Some receivers simply have
better RF than others. For instance, the Hammarlund
Super-Pro, which also has two RF stages and a 455Khz IF
significantly better rejection than the SX-28 and is
much free of spurs. It also cost nearly twice as much when
I think the noise blanker mod for the SX-28 is included
in the BAMA stuff, I was able to download it somewhere.
There are also differences between the SX-28 and SX-28A,
again, both manuals are available at BAMA and are worth
looking over.
I rather think that Hallicrafters suffered from wishful
thinking in some of their designs. Their aim was usually
make economically priced receivers of good performance but
they were seldom, if ever, top of the class.

Richard Knoppow
Los Angeles

Thanks for your comments. I pulled the 6AG7 and replaced
with a 6AB7
(which Halli changed from the 6SK7 after the 1st year, its
engraved on
socket). And that extra 6H6
should not be there, I pulled it and replaced it with a 6AB7
(per the
socket engraving)
and the cross mod went away. The rest of the mods seem to be
in line
the noise limiter mod that Halli did a service bulletin on
at the time
and offered to rework the
radios because the mod was extensive. Half of the 6SN7 was
to be a
calibration marker, but
that mod wasn't performed on this radio. Its an H-132836
Still wonder about the 6B8 being replaced with a 6SG7. But
the radio
works really well!

The handbook I was looking at seems to have been the
first so it didn't show the tube change. Undoubtedly H made
the change to improve the performance on the top bands where
the 6SK7 is too noisy and has gain problems.
Its amazing that anyone would substitute a 6H6 for a
6AB7 but maybe they just stuck any old tube in for some
Anyway, I'm glad the thing is working well. They sure
are sexy receivers.

Richard Knoppow
Los Angeles