Sorry I am confused as to the need for a 680 ohm resistor across. Not sure of the voltage input of the LCRX. Only if it is AC do you need the diode, If it is DC you do not even need the diode, just a proper resistor. If the diode is in proper polarity it wont hurt anything though.
To replace the bulb with an LED use ohms law. Voltage drop divided by amperage. So for a typical LED (20ma., 3.2 volts) 10-3.2/ .02 amps equals 340 ohms,minimum resistance required.. Next standard resistor value is 360 ohms. So that or anything more than that should work. Use a higher ohm value if you want to dim the LED. Wattage used by the resistor is 6.8 X .02 or .136 so a quarter watt resistor is fine. There are resistor calculators on the web that will do this for you.