Principles

Modules use a blue 0.22uF/250V (marked 22E4) or 0.68uF/250V (marked .68K) to drop the incoming power  which is then rectified and stabilized to usually -12 or -15V. These caps must be changed to half their value and twice the voltage. So replace a 0.22uF/250 with 0.1uF/400 or 600V. Alternatively, one can connect an identical cap in series with the original - which is electrically equivalent. Use polyester caps.  Never electrolytic caps.

Most modules uses a 10-22 Ohm 1/2W resistor in series with this capacitor in order to reduce the current when turning on power to the module. Change this one to twice the value and twice the power. In most case it will work fine without this modification but it could increase the MTBF (Mean Time Between Failure). Alternatively you could accomplish this by either replacing the resistor or adding a similar one in series.

Some modules like the WS467 have a voltage divider consisting of two capacitors. The value of the second smaller one, going to logical ground, should not be changed. You should instead replace it with the same value and with a 400-600V type. In the case of WS467, this is the 0.1uF. The first and bigger capacitor should be changed to half of the value but 400- 600V. This capacitor is also usually blue.

Almost all modules uses a serial capacitor to insulate the receive/ transmit circuit from the main supply and to adapt the impedance to the electrical power network. It is very difficult to tell what is the actual impedance of your electrical network (your antenna!) at 120 kHz . It will change with whatever you turn on or plug in and even your cable types might affect it. However, statistically and from a practical point of view, consider it to be twice as high on 220V as on 110V if you have the same power (kW) consumption. Therefore, change this receive/ transmit capacitor to 400-600V and just to be able to physically fit it at the same place, half of the value. Even this one is usually also blue.

Most modules use a 330k resistor coming from the incoming supply to two diodes to generate the zero crossing pulse. This value is not very critical. However due to the voltage across this resistor I recommend a physically bigger resistor like a 330k-560k 1/2W.

  Appliance modules and universal modules use a 110V cam relay. To reduce the voltage, connect a 100 Ohm/1W resistor in series with the coil. Universal modules need 33Ohm/2w resistors. Connect the resistor by cutting or desoldering one of the cam wires and then solder the resistor between the cuts.

MOV means Metal Oxide Varistors and are used as over voltage suppressors. Over voltages can be caused by lightning, switching of inductive loads, or even high tension cables falling down on the distribution cables carrying the main supply of 110-240V. The size of a MOV usually indicates it's power dissipation capacity which normally corresponds to it's rating in kA, kilo (1000) Amperes, during a very short period. Remember, Power = Voltage * Current. Use the biggest MOV that will fit the pcb (Printed Circuit Board). For 220V use a 250V RMS MOV. If you have 230V or 240V use a 260 or 275V RMS MOV.If a module has a MOV, change the voltage to 400v (250V RMS)

Use this small Excel calculator to generate a bill of materials for modifications. For each module type, enter # of modules to modify. Calculates total number of components, sorted by type. 

A Tip for testing

credit: Guy Lavoie - lavoieg@netcom.ca from an article in HTI.

A module being tested without the dial contacts is set to code M13. So just connect your module for testing (being extra careful with an exposed line voltage circuit board) and then set your test code transmitter to code M and send 13 ON and 13 OFF commands.