I recently sent back an N-scale Northern 4-8-4 that I purchased from the early 1980's. It reached a point were it was working very poorly (not that it ever worked that well). I was instructed by Bachmann customer service to send the old loco back with $15 and a note. About three weeks later they sent me back a brand new unit because they could not do anything for the old one (had no 1980's parts). It was cheaper for them to send a new unit.
I know the Northern is not their 'Spectrum' line, but assuming your unit can't be easily fixed, they will send you a new one which may be a Spectrum if they don't have any of the older ones left.
To be sure, I'd recommend that you contact Bachmann and explain your situation. They are very good about standing behind their products.