I currently use 6 6646 white transformers on my analog layout (4 for 3rd rail, 2 for overhead). About once every year or two, one will fail, usually with the rheostat going bad but the fixed terminals still working fine. This tells me that the average useful life of a new transformer is about 10 to 12 years. I didn't have this issue with the older blue transformers. The only reason I replaced the old ones is that I don't want to fry the electronics on newer locos, which can happen using the old style transformers.

Has anyone else noticed this issue of a relatively short life for these units? Can anything be done, or do I just have to resign myself to replacing them periodically?

Sounds like a plan. Only problem is that they're sealed units. What's the best way to open one without causing undue damage?

Thanks, I'll give it a try

Actually, the fix turned out to be easy. I just bent the brass tab attached to the control knob down by a millimeter or two to improve the contact. Unit works fine now.