[Boatanchors] SX-100 differences

[james.liles]

Good morning Byron:
 Congratulations for your acquisition.  The SX-100 is a wonderful 
placeholder in radio history.  The information that you are searching for 
seems to have been relegated to history as well.  We entered a fairly 
comprehensive article regarding the differences between the SX-100 models 
into the Hallicrafters sites and can no longer find them.  The following is 
off the top of the grey matter and to avoid wearing my index fingers to a 
frazzle.  After reading and evaluating  --- call at 847-571-8420 for a chat.

The SX-100 included the SX-100 MARK1, MARK1A, MARK2, MARK2A and MARK2B.  I 
believe two were short lived and could be considered marketing prototypes. 
Two model changes were simply cosmetic or knob changes.  If you examine the 
knobs from the 1A, 2A, and 2B, you will see a distinct difference.  The 
first upgrade was because the 1A knobs would disintegrate because they were 
built with a very narrow skirt.  The second, 2A to 2B was simply to make the 
radio look different.  Now regarding electronic component changes.  The 
tuning cap and linkage were changed from MARK1A to MARK2 to reduce backlash 
and mechanical instability.  The electronic changes were but a handful but 
made a paramount difference in the way the radio handled SSB and CW.  These 
changes occurred between the MARK1A and MARK2.  The schematic for the MARK2 
can be downloaded from BAMA, --- it includes all of the wiring upgrades.

1.  The feed from the BFO to audio detector is re-routed to the plate side 
of the BFO coil and R66 is changed from 100K to 47K.  Significant affect in 
the presence of strong signals.

2.  The AM/CW-SSB switch is changed to DPST and a .5mfd capacitor added to 
slow AGC decay.  A world of difference when listening to SSB.

3.  R46 and R47 were removed and Pin 9 of V7 grounded to improve AGC 
sensitivity.

4.  R64 was changed to 10meg and a 100pf cap wired in parallel.  Improves 
audio performance.

5.  The I.F. Alignment center was changed from 50.50kc to 50.75kc.  This 
change improves opposite sideband rejection and most importantly nearly 
eliminates the following malady.  While receiving SSB, the BFO is used for a 
pseudo carrier.  It bleeds through V7 to T6, T5 and back to the AGC detector 
causing erroneous AGC and S-Meter readings.

6.  A side note:  The sub chassis does not include any paper capacitors, and 
measurements of other components can be made through the tube socket and 
exit wires.  Don't remove it unless you want the experience or suspect a 
problem with it.

Last thought.  The upgraded SX-100 is a staggering improvement over the base 
model 1A.  You can now leave the RF gain at 10 with SSB sounding as good as 
any radio with superb sensitivity.  Unfortunately mechanical stability can 
only be had with the MARK2 or later.

That's what I remember and if anyone has more information or the original 
notes, please add them.

Kindest regards Jim K9AXN

All content copyright James J Liles  EE


ORIGINAL MESSAGE:

> Hello-
> ??? Can anyone tell me these things regarding the Hallicrafters SX-100 
> versions:
> ??? ??? 1. Is the version (Mk.I, Mk. II, etc.) stamped on the rear of the 
> converter box?
> ??? ??? 2. What are the differences between the first SX-100's and the 
> later ones? Is this documented anywhere?
> ??? I bought a decent cond. SX-100 today and I plan on going through it 
> completely in near future cleaning, replacing capacitors or other parts as 
> needed, and checking alignment. Are there any good websites or sources of 
> information on woirking on the SX-100?
> ??? This was my first decent receiver when I became a general class ham in 
> late 60's. I regret ever selling the one I had then. I finally found a 
> good replacement!
> ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? Thanks, Byron 
> WA5THJ.