Cable tension systems are very tricky to engineer, and Arecibo was built quite a while ago. If you use fixed, non-adjustable cables, you still need to tension them again after installation to allow for settling to balance tension on all the wires. I don't know much about Arecibo, but having strain gauges on each wire, some can be built into the assembly, would allow monitoring and possible automatic tensioning systems to balance the loading. Even in a system of many cables, the load shifts with wind and geological shifting of foundations, and sometimes for brief periods a few of the many cables take the majority of the load leading to differences in strain or stress.
