Hi, and welcome to RCU. You've done a lot of "process of elimination" already to find the problem and it sounds like a radio problem to me. If you are using a rx pack on the vehicle then you can try new AA batts. If it uses the main subC 7.2 pack only then just make sure it is charged. The tx (transmitter) should have new AA batts in it (and possibly some lights to show charge?)

If tx and rx batts are new/good/charged then look for loose crystals/connections. Hopefully it WAS a working vehicle?

Your charger should be able to do your nicad and nimh battery both. It may be possible that the charger is bad and you just have a dead battery. The battery should be quite warm when charged and take about 30 minutes for a 1500 mah battery to charge.