[Hallicrafters] RE: SX-28A "deaf" on higher bands

[Ken Kaplan]

Dave et al,

The circuit you are probably trying to remember is the S-9'er as originally described in the 
May 1956 issue of CQ (pg 21-22). A later article titled Improving the S-9'er was published on 
August 1956 in CQ (pg 20). Although the S-9'er wasn't designed specifically for the SX-28, it 
can improve it at higher frequencies. The method involved substituting a low noise 6BK7A 9 
pin dual triode for the standard pentode RF amp (i.e. 6SK7 in the SX-28A). Its main benefit is 
the 20 meters and above. As mentioned in the article, signal-to-noise ratios are determined 
largely by front-end noise at higher frequencies. The method described is easily reversible. I 
haven't tried it yet in my SX-28A only because I haven't gotten roundtoit.

There is also a very good article called Updating the SX-28 that was published in the May 
1959 edition of CQ (pg 48).

If anyone wants a copy of any of the above, holler. I can scan them.

73 Ken Kaplan kb7rgg

> Many years past, during my novice days, I had an SX-28A and I remember
> replacing the RF stage with a plug in circuit that used a 9 pin tube (I
> believe it was a 6BZ7 but I'm not sure).  I can't remember the details for
> the replacement circuit but I'm sure it came from a CQ or QST article (that
> would have been my only source of information at the time).  It was built in
> an aluminum "minibox" with an octal plug and the bottom and the 9 pin socket
> on the top. It made all the difference in the world on 15 meter CW.  The
> stock receiver was "deaf as a post."  Someone else is bound to remember it.
>
> Dave Cohen, N7TC
>
> -----Original Message-----
> From: hallicrafters-bounces at mailman.qth.net
> [mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Roy Morgan
> Sent: Monday, April 17, 2006 9:52 AM
> To: Roger Basford; hallicrafters at mailman.qth.net
> Subject: Re: [Hallicrafters] SX-28A "deaf" on higher bands
>
> At 04:27 PM 4/14/2006, Roger Basford wrote:
> >...I've just bought a very nice SX-28A receiver... things are very quiet 
> >on the HF bands, falling away to very little on 20 metres.
>
>
> Roger and others,
>
> The 6SA7 mixer tube used in the SX-28's was followed by a better one later 
> on. A plug-in mod is to replace the 6SA7 first mixer with a 6SB7 for better 
> performance on the upper one or two HF bands.  You may need to do a small 
> bit of alignment, but it should work better at the higher frequencies as is.
>
> (In Rogers case, apparently *all* the HF bands are weak, so that points to 
> other difficulties.)
>
> I cannot remember if the 6SB7 is built with the low loss base (tan instead 
> of black) or not.  If not, I also don't know if there is a 6SB7-Y which 
> would be the low loss base version.
>
> My SX-28's are some ways from overhaul, so I would be glad to hear results 
> from  anyone who tries this change.
>
> Roy