Sx-28 broadcast band weak

[kc2etm]

Any suggestions i have an sx-28a all bands
work great except broadcast band
i hear one local station very faintly adjust cap
for the band didnt help much
thanks
fred

[WQ9E]

Check the primary of the antenna coil, it may have been taken out by lightning.

I believe only one RF stage is used for the broadcast band on the SX-28.  I know that the followup SX-42 and 62 are that way but it has been awhile since I went through my SX-28.  Check the wiring/switching to make sure the antenna is connected to the input coil properly for this range. 

With only one range not working, if it isn't due to an open primary in the input coil then a switch issue is most likely followed by damage to another coil for that range.

[KB2WIG]

Roger Rodger,
 the BcB skips the 1st rf and goes to the 2nd rf amp - the 6SK7 --

F,

try a clip lead to pin #4 o' the 6sk7; this will bypass T13, and as R said, this probably will give you mo information to go on.....

klc

[KM1H]

It could also be the output coil fried by high V-2 plate current from a leaky cap. Due to wire size the BCB is the most prone to go.

Carl
KM1H

[kc2etm]

Well if i clip a lead or even touch a screwdriver to the output
side of the coil that goes up topside to the varible
tuning caps it wakes right up i could get stations
60 miles away on bcb wth screwdriver i can also touch
the connection topside and it works any other pins
i touch on that coil do nothing but i tested them with
the meter and the coils are not open also get continuity
through the ant connection to the input side of the coil
so that probably rules out the band switch
thanks
fred

[kc2etm]

So i noticed the higest band setting is really weak now to
it was working well before does this bypass the first rf stage to
maybe the second rf tube is flaky
sorry for the broken text i use my phone

thanks
fred

[KM1H]

It uses the 2nd RF amp only below 3mc.

You need to follow the schematic and do the voltage and resistance tests that are in the manual.

Carl

[kc2etm]

Ok i found the problem a coil was broken on both sides
i was able to repair it but. I forgot to mark direction it came out
when i removed it which coils should be the primary
the top one is the two heavy coils
the bottom coil is large coil of very fine wire
which coil should go to the antenna input
thanks
fred

[KB2WIG]

F,

here are a few links you may find of interest.

http://w5ami.net/cpg1419/thumbnails.php?album=16


http://antiqueradio.org/halli12.htm

http://xoomer.virgilio.it/egirland/sx28/restor-log/restoration%20log.html

When I restored my '28, I made several detailed drawings.  I did the Rf sections on a 1:1 scale; each 'box' on a seperate piece of paper. Now, with the cost and availability of digital cams this would seemto be the way to go. I also decided to replace almost everything. I got the manual, made up a list of all the R's and C'c and purchased them from Mouser...

replacing the parts. fun times........

klc

[kc2etm]

Ok the problem is fixed but the upper band still weak
no better then my s-20r is there any mods
to improve it or should i just throw in a solid
state preselector on that band
thanks
fred

[KM1H]

No alignment, weak tubes, and no sunspots all add up.

Carl

[kc2etm]

It has all nos tubes and they all tested good
i did the alignment acording to the manual
well as best i could follow im trying to compare
to the local fellows i hear on my kenwood
so no sunspots needed on that just wondering if this
is the standard performance for this band
like i said about the same as my s-20r all other
bands are far better then the s-20r
thanks

[kc2etm]

So i found a mod of swapping out the osc tube and the mixer for a 6sb7y to improve upper bands
has anyone done this

[Jim, W5JO]

The SX 28 is not and probably will never be super hot, but preform pretty well.  Is there a problem  with the input link to the first coil or a bad contact on one of the switch wafers?

[WQ9E]

If you want to be serious about receivers you need to have a decent calibrated signal generator and make sure everything is working as intended before modifying anything.  All it takes is one resistor that has drifted high to create a tremendous drop in performance.  A problem in the oscillator circuit (weak tube, out of spec component) is most likely to show up at the higher end of the frequency range.

There was a set of updates put out by one of the Hallicrafters engineers for the SX-28 long after it was out of production.  I believe it was reprinted in a column in CQ and if I can find my copy of it I will scan and post it here.  I guess I am the "anti-mod" since I did not do the updates to my SX-28A but I did look through the article.

[N3DRB The Derb]

If you want to be serious about receivers you need to have a decent calibrated signal generator and make sure everything is working as intended before modifying anything.

word. without a consistent repeatable signal source you really wont be able to tell anything or figure what the real results of your mods are.

[KM1H]

The SX-28 does not like to be run on a 50 Ohm feedline. Run it thru a 4:1 or better still, a 9:1 broadband balun. A preselector just makes noise. My SX-28 doesnt set the world on fire on 10M but its better than most 1RF stage alternates using octals. A NC-240D with 6SG7 and 6SB7Y swaps blows it away.

Changing the 2nd RF to a 6SG7 will help a bit and a 6SB7Y mixer a bit more. Its now limited by the noise generating 6L7 slug in the 1st IF. As mentioned if ANY resistors in the RF or IF are out of tolerance performance will suffer and it all starts on the highest range.

Carl
KM1H