First, let me say that this is a cheap, old Shimano derailer that's getting junked and upgraded anyway, but I would still like to understand what's wrong.

So, here's the problem: The derailer is not angling up when I upshift. It's staying in the same downward position throughout the whole range of speeds. As such, the jockey wheel is about 6 mm from the largest sprocket and about 46 mm from the smallest. It's making downshifting very difficult in the upper speeds. I'm thinking a spring has failed in the derailer, and as this is one of Shimano's cheapest-made models, there is no way to adjust or access these springs.

Am I right in my thinking, and would there be anything I could do about it if I weren't already planning total replacement?