[Hallicrafters] SX71 MOD, a reprise

[George KB2Z]

Hi Phil,

That's an elegant solution. On head scratchers like that, I usually cut
my hand on the pointy hat.

Kudos, George KB2Z

Philip Atchley wrote:
> 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
>
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