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Old September 9th 09, 06:55 AM posted to rec.radio.amateur.boatanchors
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Posts: 12
Default SX-28 image problems

I have a problem with strong stations imaging under adjacent stations.
For example, China on 5950 also shows up under a station at 5985 and
one at 5935. Also a strong station on 9985 will also show up under WWV
on 10mHz. The images always seem to show up under adjacent stations,
never on their own. Can anyone suggest what may be wrong or out of
alignment with this radio? Thanks!

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Old September 9th 09, 02:42 PM posted to rec.radio.amateur.boatanchors
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First recorded activity by RadioBanter: Jul 2006
Posts: 241
Default SX-28 image problems

Hi:

That sounds like a cross-modulation problem, rather than images. Images
would appear 910 kHz away.

Cross-modulation would occur in the rf stages or in the first mixer. To
determine if that is the problem, reduce the signal strength and the
cross-modulation will go away.

The SX-28 should not be cross-modulating under those conditions and should
be able to handle fairly strong signals. Often, the culprit is that someone
has modified the set for "improved sensitivity", in which case, they made
changes to make the S-meter read higher. Frequently, this involves changing
rf amplifier tubes from remote cutoff types to sharp cutoff types, or
changing resistors or bias voltages.

Check the circuit to make sure it is stock. Also, check the caps and
resistors in all of the circuits, especially in the rf, mixer and avc
circuits.

Also, realignment of the rf stages may help, especially if the tuned
circuits amplify the offending signal more than the desired one.

73, Colin K7FM


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Old September 9th 09, 04:12 PM posted to rec.radio.amateur.boatanchors
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First recorded activity by RadioBanter: Oct 2006
Posts: 527
Default SX-28 image problems


"COLIN LAMB" wrote in message
m...
Hi:

That sounds like a cross-modulation problem, rather than
images. Images would appear 910 kHz away.

Cross-modulation would occur in the rf stages or in the
first mixer. To determine if that is the problem, reduce
the signal strength and the cross-modulation will go away.

The SX-28 should not be cross-modulating under those
conditions and should be able to handle fairly strong
signals. Often, the culprit is that someone has modified
the set for "improved sensitivity", in which case, they
made changes to make the S-meter read higher. Frequently,
this involves changing rf amplifier tubes from remote
cutoff types to sharp cutoff types, or changing resistors
or bias voltages.

Check the circuit to make sure it is stock. Also, check
the caps and resistors in all of the circuits, especially
in the rf, mixer and avc circuits.

Also, realignment of the rf stages may help, especially if
the tuned circuits amplify the offending signal more than
the desired one.

73, Colin K7FM

I think checking the tube types is a very good idea. If
someone replaced the originals with later high gain types it
may be the cause. These tubes look attractive because they
have lower noise and higher gain than the originals but do
not work well in circuits not designed for them. The SX-28
uses a 6SK7 in both RF stages. The later but similar SX-32
uses a 6AB6 in the first RF position. This was a remote
cut-off tube designed for TV RF and IF use and has much
higher gain than the 6SK7. Another possible tube is the
6SG7, also a high transconductance tube, similar in
performance to the 6BA6 miniature tube. This one was used in
the S-40 series. Someone may have been inspired to stick one
of these guys in. Beside the higher gain these tubes require
a different bias voltage so the AVC won't work right.
Actually, there are other tubes which can cause even
more trouble such as the 6AC7, a very high transconductance
sharp cut-off pentode meant for video amplifier use. A tube
like this will tend to cause the AVC to operate at a slight
AVC bias which will cause overloading in the other
controlled tubes. Even a 6SJ7 in the first RF stage will
cause this trouble.



--
Richard Knoppow
Los Angeles
WB6KBL



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Old September 10th 09, 01:35 AM posted to rec.radio.amateur.boatanchors
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First recorded activity by RadioBanter: Mar 2009
Posts: 12
Default SX-28 image problems

Thanks for your responses! This SX-28 is far from stock and is
extremely over-sensitive. I bought it about a year ago to 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, but requires a
lot
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
right)
There is a 2nd 6H6 installed next to T6 (where the stock 6AB7 used to
be)
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 switchless
type.
There is also a 6SN7 where the stock 6H6 used to be.
The 6B8 that was with T1(1st IF) has been replaced with a 6SG7.
(The 6B8 pentode wire in T1 has been removed.)
The S meter was replaced with one labeled "carrier" and has very fast
ballistics.
It does OK on SSB, but the BFO sometimes has a little warble.
Its been re-capped, but not the RF deck. The dial calibration is dead-
on.
If you have any thoughts as to what these mods were trying to
accomplish,
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 really wasn't
intending to do a
complete restore on this SX-28, having done one already, and have had
that thrill.
Thanks for your help!

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Old September 10th 09, 03:14 AM posted to rec.radio.amateur.boatanchors
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First recorded activity by RadioBanter: Oct 2006
Posts: 527
Default SX-28 image problems


"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 to
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, but
requires a
lot
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
right)
There is a 2nd 6H6 installed next to T6 (where the stock
6AB7 used to
be)
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
switchless
type.
There is also a 6SN7 where the stock 6H6 used to be.
The 6B8 that was with T1(1st IF) has been replaced with a
6SG7.
(The 6B8 pentode wire in T1 has been removed.)
The S meter was replaced with one labeled "carrier" and
has very fast
ballistics.
It does OK on SSB, but the BFO sometimes has a little
warble.
Its been re-capped, but not the RF deck. The dial
calibration is dead-
on.
If you have any thoughts as to what these mods were trying
to
accomplish,
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 really
wasn't
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 blank
out the IF when a noise pulse came along. Such blankers are
fairly common now but its characteristics were not very well
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 will
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, to
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 has
significantly better rejection than the SX-28 and is pretty
much free of spurs. It also cost nearly twice as much when
new.
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 to
make economically priced receivers of good performance but
they were seldom, if ever, top of the class.



--
Richard Knoppow
Los Angeles
WB6KBL





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Old September 10th 09, 05:29 AM posted to rec.radio.amateur.boatanchors
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First recorded activity by RadioBanter: Mar 2009
Posts: 12
Default SX-28 image problems

On Sep 9, 9:14*pm, "Richard Knoppow" wrote:
"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 to
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, but
requires a
lot
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
right)
There is a 2nd 6H6 installed next to T6 (where the stock
6AB7 used to
be)
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
switchless
type.
There is also a 6SN7 where the stock 6H6 used to be.
The *6B8 that was with T1(1st IF) has been replaced with a
6SG7.
(The 6B8 pentode wire in T1 has been removed.)
The S meter was replaced with one labeled "carrier" and
has very fast
ballistics.
It does OK on SSB, but the BFO sometimes has a little
warble.
Its been re-capped, but not the RF deck. The dial
calibration is dead-
on.
If you have any thoughts as to what these mods were trying
to
accomplish,
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 really
wasn't
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 blank
out the IF when a noise pulse came along. Such blankers are
fairly common now but its characteristics were not very well
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 will
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, to
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 has
significantly better rejection than the SX-28 and is pretty
much free of spurs. It also cost nearly twice as much when
new.
* * 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 to
make economically priced receivers of good performance but
they were seldom, if ever, top of the class.

--
Richard Knoppow
Los Angeles
WB6KBL


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
the
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
with
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 (1941).
Still wonder about the 6B8 being replaced with a 6SG7. But the radio
works really well!
Pete
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Old September 10th 09, 09:48 AM posted to rec.radio.amateur.boatanchors
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"
wrote:
"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
to
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,
but
requires a
lot
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
right)
There is a 2nd 6H6 installed next to T6 (where the stock
6AB7 used to
be)
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
switchless
type.
There is also a 6SN7 where the stock 6H6 used to be.
The 6B8 that was with T1(1st IF) has been replaced with
a
6SG7.
(The 6B8 pentode wire in T1 has been removed.)
The S meter was replaced with one labeled "carrier" and
has very fast
ballistics.
It does OK on SSB, but the BFO sometimes has a little
warble.
Its been re-capped, but not the RF deck. The dial
calibration is dead-
on.
If you have any thoughts as to what these mods were
trying
to
accomplish,
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
really
wasn't
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
blank
out the IF when a noise pulse came along. Such blankers
are
fairly common now but its characteristics were not very
well
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
will
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,
to
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
has
significantly better rejection than the SX-28 and is
pretty
much free of spurs. It also cost nearly twice as much when
new.
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
to
make economically priced receivers of good performance but
they were seldom, if ever, top of the class.

--
Richard Knoppow
Los Angeles
WB6KBL


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
the
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
with
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
(1941).
Still wonder about the 6B8 being replaced with a 6SG7. But
the radio
works really well!
Pete

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
reason.
Anyway, I'm glad the thing is working well. They sure
are sexy receivers.



--
Richard Knoppow
Los Angeles
WB6KBL





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