The F-50's reverb isn't tube driven, so you can eliminate tubes as the problem entirely.
Have checked the reverb tank itself? Open it up, and look for disconnected springs, or anything obvious like that. The only foolproof test here is to connect a known-good tank, though.
Another thing to do is a simple continuity test on the cables that connect the tank -- one of those came bad in my Lonestar.
If the problem isn't with either of those, then, by elimination, it has to be within the chassis. With tube-driven reverbs, I sometimes test the driver circuitry by grounding out the return jack against the chassis (it's quicker than swapping the tube). It makes the amp sound bad, but tells you whether it's working or not. I don't know whether or not this is safe to do with an op-amp or whatever drives the reverb in this amp, though, maybe someone else can confirm this.
