I was originally recruited to replace an Amana tech rep. Hell, I didn't even know Amana had an HVAC line at the time
I can work on anything because it is all operated on the same basic principals. I was a Carrier and Rheem/Ruud dealer for years and did some repping for Rheem/Rudd equipment. Not much out there in resi/light commercial I haven't seen.
Oh, and before getting into the business end of our trade I designed, manufactured and installed commercial and industrial duct systems.