[Hallicrafters] SX-100 MARK1A/MARK1B

[Jim Liles]

Some of the differences between MARK1A and MARK1B are 
   1)  The tuning capacitor has a slightly different drive like the MARK2
          I believe that the change was made to enable easy adjustment
          of backlash.
   2)  The 50 KHz IF is aligned at 50.75 instead of 50.50 KHz
          This is real magic for performance in combination with the slow AGC.
          It enables you to run the RF at full gain on CW and sideband.  It
          also produces better sideband separation and audio.  
          Another reason for the change from 50.50KHz to 50.75 is the 
          BFO bleeds backward through the IF transformer to the AGC causing 
          the S-Meter to see a signal that is not there.  To test for this, simply 
          set the BFO to 50.00KHz and switch from AM to CW/SSB.  You will 
          see a significant increase in S-Meter reading.    
   3)  The BFO is tapped off of the plate rather than the grid.
   4)  The AGC for sideband and CW is set for slow rather than fast.

I have seen MARK1B's with some or all of the changes but all
had the tuning capacitor and 50.75 change which should and can be done 
to all SX-100 models.  All the circuit changes to go from a MARK1A to a MARK2
can be done to any MARK1A with the addition of a two pole switch
for the AGC. 

This is probably not the right venue for long descriptions --- hope I have not 
repeated well known info or bored anyone.  A merry Christmas and happy 
holiday season to all.  ---  Regards Jim K9AXN