[Hallicrafters] SX71 MOD, a reprise

[Philip Atchley]

Hi All,

Sheesh, if I'd have realized what a firestorm all this was going to create I 
would have kept my mouth shut, or rather, my fingers off the keyboard!

Anything that I submit or come up with, whether on my web site or via Email, 
is for the full benefit of the boatanchor community.  The SX71 "mod" (which 
I first posted in the form of an Email and which was subsequently copied to 
the HCI site with my approval) that I came up with was 'slightly' more than 
just the cathode resistor, I also changed the screen bias etc. of the second 
stage.  Still, all pretty simple stuff.  All that the HCI folks did was copy 
my email to the Web-site and add some advertising pictures to make it 
interesting.

As far as I'm concerned, the written part that I produced can be printed and 
may be freely used anywhere so long as there isn't any monetary compensation 
(though if it's copied to any web-site etc. I'd appreciate credit).  So far 
as I know, the PICTURES ARE HCI PROPERTY and any needed permission should be 
definitely be obtained to use them.

Since I originally wrote the article, and only parts of it made it's away 
"around the circuit",  I'm re-posting a copy here from my own archives. 
Enjoy!

<Pasted in>
I decided to take a '2nd' look at the SX-71.  While it played quite well I 
never felt like it was doing all that a receiver with 3 IF stages should do. 
Front end seemed sensitive enough, but overall gain and AGC action, while a 
little better than 'most' receivers with 2 IF's like the Hammarlund HQ-100, 
Halli SX-110 etc, just didn't seem to really have the "punch".  After I 
overhauled Don's National NC183D that had 3 IF's and a circuit design quite 
similar to the SX-71, that opinion was further reinforced.  In that set I 
had to change a couple resistors (per factory recommendation) to decrease 
the gain slightly as it was unstable.

So anyway, I sat down with the schematic and tube manual to see if I could 
determine why it's gain seemed so modest for the tube lineup.  Didn't take 
me long to spot it.  The 2nd IF stage had it's tube cathode biased very high 
and an unusually low screen Voltage on the screen grid.  So, I dug out ye 
old trusty soldering iron and set the cathode resistor and screen Voltage to 
the same level that the 1st and 3rd IF stages use, which is a normal and 
expected value.  Voila! Gain and AGC action came up to my expectations.  It 
won't affect overall sensitivity/noise figure as that is mostly determined 
by the RF amplifier and first mixer stages.  What it does affect, especially 
on the higher frequencies is how well the AGC performs and how high you need 
to turn the volume on the weak stations.  Before you always had to 'crank it 
up' on weak stations on the 17 & 21MHz bands, which is why I nearly always 
used the Ameco Nuvistor preamp over 15 MHz.  The thing really sounds good 
now and that is without the preamp!

The SX-71 is claimed to be the first double conversion set on the market. 
They also wanted the selectivity and perhaps a 'little' extra gain that an 
extra IF amplifier gave them.  I can think of no explanation of why the 
receiver was purposely 'tamed down' by crippling the 2nd IF and putting 
coils that are sub par for the BCB band in an otherwise hot set.  Perhaps 3 
IF stages were so rare back then that they felt the "little" added boost was 
sufficient, while now days we are used to all receivers having several AGC 
controlled stages that give superb AGC action.

<end of article>

73 de Phil,  KO6BB
DX begins at the noise floor!

THE BEACONEER'S LAIR:  http://www.geocities.com/ko6bb/
MY RADIO-LOGS:   http://www.geocities.com/ko6bb/Logs/
QSL GALLERY:       http://photobucket.com/albums/y123/KO6BB/
Merced, Central California,    37.3N  120.48W  CM97sh


----- Original Message ----- 

> Let's see if I have this sequence right-
>
> Someone discovers that changing a resistor in a stage leads to better 
> performance and posts this for everyone's benefit.
>
> Someone else posts a message indicating that this mod is similar to one 
> found in a pre-written article. That article is identified with a link for 
> everyone to see.
>
> The author of this earlier article contacts the list administrator to 
> protest that someone has hijacked his material without giving him credit.
>
> Right so far?  OK-so if I discover a mod for a radio and post it for 
> everyone's benefit, not knowing that someone else has "discovered" this 
> earlier and written an article on it then I am contravening "copyright" 
> laws?
>
> Horse puckey!!  Let's get real!  I know Americans are a litiginous society 
> but let's not get carried away here.  This is ham radio, not the great 
> Blackberry steal.
>
> Bill, VE3NH