Is there any way to drop 230V AC to 200 V AC by using resistor? I want to drop voltage before applying to a 12 0 12V 500mA transformer. If yes what will be its rating?
One way to do this is with another transformer. A 230vAC:30VAC connected to oppose the voltage will work. However you could also replace the transformer since it is only a small (12VA) transformer.
If you are trying to adapt a Japanese standard product to 230VAC you may find that there are primary taps on the transformer that you can use.
Using a resistor is not a good idea generally because the voltage drop will vary with loading.