Wall transformer amperage
No, the higher amp rating would just mean it can supply more current if the circuit "asks" for it. The voltage rating is the main thing here, and that matches okay.  

The fact that the 500mA power pack blew too suggests something is pretty wrong somewhere.
How did you confirm it blew? Have you tested the output with a multimeter?  

What model charger is it? Are the batteries you are using the correct type for the charger?

If you have a bench supply you could plug it (current limit it to say 400mA) in and see how much current it is drawing. I would test both with no batteries in and batteries on charge.
If you don't have a bench supply with current display you could rig a 24V source up and use a multimeter to test current.
Also, other clues you can check for - see if any components are getting very hot (especially if not loaded with batteries)
With circuit off, test resistance between power and ground to see if it is low. I would be suspicious of anything under 1k or so.