:(My TS 520 worked great when I put it away some time back...
Sitting dormant often causes problems when switch and control contacts become dry, corroded and no longer make proper connections.
Switches and Controls that are in use get "excercised" -- which can keep the bad stuff from happening by the mechanical wiping action -- but sitting without that excercise can often stop all sorts of functions from working right.
So the very first thing to do with any piece that you wish to bring back to operational life is simple enough: Use a good Cleaner/Lube spray such as DEOXIT on every switch and control, taking the time to turn each one full travel lightly while the spray is still wetting the contacts in order to get some mechanical wiping action, which cleans the contacts.
If the rig has an AC power switch, do not spray into that one, because the lubricant can cause more problems there, causing problems to the contacts due to AC sparks when the switch is thrown. But all other controls are good candidates and should be cleaned and lubed like that.
This should be done before any other attempts at troubleshooting or decisions about component replacement or the dreaded Golden Screwdriver re-calibration.
Quite often is the case that cleaning controls is all the "repair" that is really needed in these cases.