[Boatanchors] Re Hallicrafters SX-71 Oddities?

[Duane Fischer, W8DBF]

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.