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 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.
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.
Click graph for high-res view.