The SX-101A:  Looking at a Classic

Performance, Plots and Comments

The SX-101A was another of those rigs that I wanted as a kid but had no money to buy.  40 years later I ran into a local who had one in good shape for sale and a deal was made.  The receiver sat in the lineup working fine save for a bit of hum in the AF.  And finally one day I pulled it up onto the bench for a cap swap. After the work was done and the alignment complete, I pulled a couple of plots that may be of interest to others.

  

MDS was about -125 dB even on the high bands.  Another surprise.  Yes Virginia, tubes CAN hear!

  

DRIFT

  

Drift was measured by plotting the AF tone variation out of the SX-101A when fed by a Si570 signal generator .  Over the 2 hour period drift covered a range of about 250-275 Hz.  I had purchased the X-Lock, a huff-puff type frequency stabilization system, from Cumbria Designs but it remains a future project.

  

http://www.cumbriadesigns.co.uk/x-lock.htm

  

  

SELECTIVITY

Selectivity in the SX-101A is implemented with a pair of high-Q LC filters.  The curves were a surprise to me; impressive !

  

Before doing the cap swap and alignment, here's what the passband plots for the various filter settings looked like, focusing on the LSB setting.  Click graph for a high-res view.

  

  

After doing the cap swap and alignment (note scales are a bit different).  Click graph for high-res view.

  

  

  

NOTCH DEPTH  

Click graph for high-res view.

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