[Boatanchors] Re Hallicrafters SX-71 Oddities?

Tue, 1 Oct 2002 18:25:25 +0000

Hi Gerry et al...
Yes, somebody sent me a copy of that service bulletin.  HOWEVER, I went
through it and checked all those items.  They WERE NOT the cause.  As I
said, gain seemed a 'little' better than the average set with 2 IF
stages, just not what I felt it should be for the lineup.  

Front end sensitivity was good but AGC action lacked on the higher bands
indicating low overall gain (I hate having to crank up the volume control
on those weak stations).

Whatever the reason, I'm now very pleased with the way the set performs. 
It has more than sufficient gain and it DOES NOT overload on strong sigs.

It will never rival my R-390A in performance but it sure does sound good
for program listening.  And my "Bay Area Oldies Station" in Santa Cruz
sounds so much more listenable too!  (I'm well outside their service
area).

73 de Phil  KO6BB
Loving home provided for wayward Boatanchor Receivers.
R-390A '67 EAC.  HALLICRAFTERS SX-71 ('53) & 5R10A ('53).
REALISTIC  DX-160 ('78).  HOMEBREW  Longwave CW receiver.
Merced, Central California, 37.18N  120.29W  CM97sh.

> 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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