[Hallicrafters] Phil's SX-71 mods (was) RE: Sloper and SX-71 (was) Re: An AM antenna

[Philip Atchley]

Hi,

> There was an article on how to soup up the SX-71 some time ago. I hope
> that the author is reading the mail.

Yep, I'm here.  The replacement coils worked very well.  The mod is/was
documented on the HCI website.  In fact, I'm listening to a fairly low power
Ranco Cordova (NOT LODI as I inadvertantly said in another email) station on
650 KC (120+ miles) right now and it sounds like a local! Los Angeles
stations (260 Miles) in the daytime also sound like locals!  Must be all
ground wave.

I also rebiased the 2nd and 3rd IF stages to bring the gain up some.  It
entailed
replacing the cathode resistor and changing the screen resistor to bring the
gain up some.  I don't recall the exact values I used, and at this time it's
a "real" pain to pull the receiver out of the operating position and
dis-assemble.  Next time I have it apart I'll try to remember to document
it.  The overall effect of the mod was to increase the IF gain to the point
that the set performed much better at the higher frequencies where
previously AGC action was rather poor.  Of course it did nothing for front
end noise figure, but it improved the set to the point where I no longer
needed a RF pre-amp or had to turn the volume up higher.

Somebody mentioned a position on the bandswitch that switched in different
cathode resistors for the different bands.  My SX-71 DOES NOT have that
switch section, which is probably why the mod helped so much.  I HAVE seen a
similar
circuit on the National NC-183D that uses different cathode resistors on the
different bands.

Below paragraph was excerpted from a posting I just made today.

SX-71 on the BCB band.  Mine was nearly dead on that band with very poor
performance, even after re-capping and overhauling it.  It turned out the
BCB input coil was fried.  For the BCB I replaced BOTH the antenna input
coil AND the RF amplifier interstage coil with a set of ADJUSTABLE universal
coils from AES Arizona.  The results was that I was able to adjust the
tracking of both coils for the BCB with GREAT results.  The set is one hot
firecracker on he BCB band now!

73 from the "Beaconeers Lair".
Phil, KO6BB


> Happy Thursday to all.
> There was an article on how to soup up the SX-71 some time ago. I hope
> that the author
> is reading the mail.
> Basically it was to replace the ant and rf coils on band one with
> universal replacements
> which have slugs to tune the low end (600 kc) so as to track the
> oscillator stage.
> These coils will mout in the same place as the old, non slug tuned coils
> withthe slug adjusting screw on the chassis top.
> This mod increased the sentitivity a bunch and did help with some images.
> It may be more sensitive to overload on strong signals.
> BUT the overall will be very good.  These coils are avaiable from Antique
> Electronic Supply
> in Tempe AZ. .A change in the front end rf amp to a lower noise tube and
> corresponding cathode resistor may also be in order.
> Jack
>
> On Mon, 3 May 2004 08:48:05 -0700 "RJB" <rjb at lynden.com> writes:
> > Phil
> > thanks for the response. I saw your article "Soup up your SX-71" in
> > which
> > you mentioned some mods to the 2nd IF. Two questions, if i may
> >
> > 1 - what exactly did you do to the cathode connection on this tube
> > (6SK7
> > IIRC)?? The cathode resistor is (i think) 1K, going to a switched
> > set of
> > resistors, presumably to keep gain constant from band to band. Did
> > you
> > replace this with 270ohm to ground??
> >
> > 2 - what mods did you make to the screen supply?
> >
> > 3 - Did you ever check your electrode voltages?? mine are all over
> > the map,
> > even after replacing all resistors and all tubular caps :(
> >
> > 4 - Have you had your SX-71 repainted?? Mine works well, but is
> > ugly. (if it
> > had engraved lettering i would do it myself :-)
> >
> > thanks in advance for any help
> >
> > Bob