How can you tell if the cap. is bad? I don't have a receiver pack to test that either.

You could add another one. Make sure its a fast one, the types used in computer power supplies are the ones you need.

A simple test would tell.. (or you must have got an C tester, but this requires removing the cap from the speedo..)

Usually, the caps heat up when in use. If one cap is significantly warmer than the other, then the other is bad or disconnected. When I reconfigured my mgm 120, one of the capacitors came loose and I lost all servo control under acceleration and throttle control was way out of sync.