Some technicians install and troubleshoot avionics equipment on fixed wing aircraft or helicopters. They're responsible to see that everything works properly and none of it interferes with other electronic devices on board. When a device fails, they remove it and send it to a shop where a bench avionics tech performs repairs. In a small facility, a tech may work on the aircraft and in the shop, but in large operations the jobs are separate.
Bench techs may work on navigation and communication radios, autopilots, computers, and even large mechanical assemblies like radar antennas. A large shop may work on hundreds of different items. Technicians are often specialists within one or two areas. The job requires excellent troubleshooting skills and the ability to do intricate, high-reliability soldering on delicate components. Good bench techs have patience, steady hands, and excellent vision. Modern circuit boards are densely populated with tiny integrated circuits. Much of the soldering is done with binocular magnifiers or microscopes.
System trouble-shooters are technicians with years of experience and a tremendous depth of knowledge about commercial aircraft. They may not know the intricacies of the individual components that make up the system, but they know how those parts interact and sometimes counteract one another. Line avionics techs rely on them for accurate diagnosis of difficult problems. There's always a deadline. An aircraft has to fly again shortly, so it seems that these trouble-shooters are constantly on the run.