Have you called the shop that did the work?
Hopefully you are aware of what’s going on here. Some years back Garmin ‘forced’ faa licensed shops to send the sl30 back to them, where it would be fixed for one price fits all (e.g., anything from a fuse to a complete burn out, same price). The price is not cheap. If that’s what happened, send it back. It should work. More likely, you used a non-faa repair shop, which does not have the latest manuals, because Garmin won’t sell them. Since the radio worked okay in the past, now doesn’t in the same airplane, most likely there is an issue inside the sl30.
One thought: you’re not parked close to a high power TV or AM or FM or radar station, are you? That can do strange things. But since it worked before, my bet is something inside the box.