Fw: [Hallicrafters] SX-28 Recommended Modifications?

Sun Aug 15 13:31:52 EDT 2004

I replied directly to David but should have also to the reflector since this
may help some of you so I forwarded my reply to David.  Most on the
reflector don't seem to be aware of the two different SX-28 versions and
schematics.

----- Original Message -----
From: "William G Feldmann" <n6py at qnet.com>
To: "David Woodhead" <km5tz at san.rr.com>
Sent: Sunday, August 15, 2004 10:26 AM
Subject: Re: [Hallicrafters] SX-28 Recommended Modifications?

> Hi David,
>
> Glad to hear you on the net Friday night and noticed you post on the
> reflector.
>
> One thing no one I talked to that knew Hallicrafter receivers and even had
a
> SX-28 seemed to know, there are two very different electronic versions of
> the SX-28.  There is an early version with a cover over the RF and mixer
> tubes that is held on with thumb screws and just covers the tubes.  There
is
> a later version with a larger cover held on with four clips which is much
> more common.  I have the early one and had a very difficult time finding
any
> documentation on it cause me a lot of trouble to trouble shoot the noise
> limiter and first AVC circuit.
>
> Be sure you have the right manual for yours before doing any work and
> alignment on yours.
>
> Unfortunately all the places that supply manual copies seem to have manual
> copies only for the late ones.  The early one uses a 6SK7 for both RF amp
> tubes and for the noise amp instead of a 6AB7.  The whole noise limiter
> circuit is different, the detector does not have a shunt limiter, and
there
> is no band switch wafer to change the gain of the first AVC feedback on
the
> two lower bands on the early ones.
>
> I just got a copy of an early manual but it's a very poor copy.  I'm
trying
> to find someone that has a original copy of the early manual to run some
> copies and get a clean copy to some of the manual supplierd for those
having these early SX-28's.
>
> So David first determine which SX-28 you have.  It's very interesting to
> compare the schematics for my early SX-28, the later more common SX-28,
the
> SX-28A and the military GRR-2.  There are great changes between the early
> and late SX-28 but after that the schematic design changes are minor.  The
> largest change between the later one and the SX-28A is resistor and cap
> values to the later stander ones, i.e. 500ohms became 470ohms, the
> mechanical packaging and individual design of components in the RF area,
and
> the band switched resistors for the 1st IF amp.
>
> On my early one it became very sensitive and seems to work very well after
> replacement of all the old wax paper caps.  I used the yellow film ones
sold
> by Antique Electronics.  One simple modification I did which I usually do
on
> these older rigs is to parallel a modern .005uf disk ceramic cap with all
> the film ones I replaced in the RF and mixer circuits.  This usually
> improves performance on the higher bands.  Replacing the caps in the RF
> area, especially the first RF amp is a real pain.  There is a good web
site
> that will help below.  http://www.antiqueradio.org/halli12.htm
>
> After recapping mine and a very careful alignment of the receiver it seems
> to be very hot and has excellent selectivity.  Be very careful to properly
> align the IF to the crystal for good results.  I did nothing to better
match
> the antenna input and the receiver works so well I don't think it's worth
> the trouble.  I use mine with my BC-610E and the antenna input is between
my
> modified BC939 tuner and the BC-610 using a Dow Key TR switch.
>
> Using a good speaker, my SX-28 has the best AM audio of any other of my
> Receivers, the 75A1, modified 75A4, various military like BC-348's and
> BC-312's receivers, and a NC-183.
>
> When I get back to my other QTH where the receiver is, I got the manual
copy
> up here at my second QTH, I need to look for and remove any mods to mine,
I
> suspect some in the AVC and noise limiter.  Also my noise limiter doesn't
> work as well as others I've used.  When I get it back to stock by the
early
> schematic, I can really determine how well the different circuits work and
> if they should be changed to the later design.
>
> If your at the AMI BBQ we can talk some more about our SX-28's.  But first
> check which one you have.
>
> Bill N6PY
>
>
> ----- Original Message -----
> From: "David Woodhead" <km5tz at san.rr.com>
> To: <hallicrafters at mailman.qth.net>
> Sent: Saturday, August 14, 2004 9:53 PM
> Subject: [Hallicrafters] SX-28 Recommended Modifications?
>
>
> > Besides changing out the paper caps are there any other recommended mods
> for
> > the SX-28?
> >
> > Anything published in old QST's?
> >
> > How about lowering the antenna input Z to 50 ohms?
> >
> > Thanks
> >
> > David
> > KM5TZ
> > ----- Original Message -----
> > From: "Lee Bahr" <pulsarxp at earthlink.net>
> > To: <hallicrafters at mailman.qth.net>
> > Sent: Saturday, August 14, 2004 9:30 PM
> > Subject: Fw: [Hallicrafters] SX-71 set screw size
> >
> >
> > > This is not an allen wrench set screw.  It is a Bristol set screw.  If
> you
> > > use an allen wrench, you will screw up the bristol socket if you have
> not
> > > already done so.  You need to get a set of bristol wrenches.
> > > Lee
> > > w0vt
> > >
> > > From: "Tim Kass" <timkass at hotmail.com>
> > > size
> > >
> > >
> > > >
> > > > Hello group, trying to restring the dial cords on the pulleys on my
> > SX-71,
> > > > what fun!  NOT...anyway, need to get the front pulley off in order
to
> > > reach
> > > > the rear one, does anyone know what size that darn allen wrench is?
> > > >
> > > > 73,  Tim  K8WBL
> > >
> > >
> > > ______________________________________________________________
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>