[Boatanchors] Re Hallicrafters SX-71 Oddities?

Tue, 1 Oct 2002 00:25:57 -0500

I have a December 1, 1950 service bulletin #1950-109 that deals with
"Analysis & Remedy For Sudden Decrease Of Sensitivity In Model SX-71
Receivers".   It is signed by N. J. Cooper General Service Manager.  Picked
it up just in case but have never needed it.

It mentions 4 causes:

1. leakage between B+ and AVC circuits across terminal boards of IF
transformers T-7 or T-8.

2. Gassy RF or IF amplifier tubes.

3. Intermittent 220 mmfd. condensers, connected across IF transformer
windings.

4. Intermittent short in oscillator coil T-21, resulting in unsatisfactory
receiver operation on bands 3 to 5.

Its an old one but if anyone is interested, drop me a note and I will see if
I can get scans of it that are readable.  Might take a few days, work has
been pretty busy lately but then I guess that's what it is all about.

My 71 is perceptibly no different than my SX-100s and other BAs or the
"other kinds" (non-glowister types) which I have.

BTW:  I will be out of town until Thursday night.

Gerry

Collecting & Restoring since 1959
Gerald Steffens P.E.
Rochester, MN  55904

----- Original Message -----
From: "Duane Fischer, W8DBF" <[email protected]>
To: <[email protected]>
Sent: Monday, September 30, 2002 4:58 PM
Subject: [Boatanchors] Re Hallicrafters SX-71 Oddities?

>
>
> Phil Atchley, KO6BB, is an excellent technician and knows his radios. What
> follows are some observations he made about the SX-71. Please post your
replies
> to the entire list. Thank you.
>
> 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!
>
> I 'think' that I figured out Hallicrafters strategy in the design of this
> receiver and probably others in their lineup.  They wanted to market a
> double conversion set. The SX-71 is claimed to be about 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.
> However, they probably "did not" want to market a set that performed
> better, or as good as their top line models that sold at higher prices!
> I could be wrong in that but it is the only reason that I can think of to
> explain why the set would be purposely 'tamed down' by crippling the 3rd
> IF and putting coils that are sub par for the BCB band in an otherwise
> hot set.
>
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