X-10 Mods - TM751 RF Transceiver

Here we explain how to modify a USA 115V TM751 Transceiver module to operate at 230V.

Introduction

I have successfully modified three of these units with some erratic results and I accept no responsibility for the effects of anyone making these modifications.   You follow these instructions at your own risk.

The TM751 Transceiver is useful on two counts: first, it is smaller than the RR501 and secondly, it responds to all unit codes for its house code setting.

Please do read the comments at the end of this page as it seems that the design of this device has changed.

As this is a USA unit, it will operate at the USA RF frequency of 310 MHz.

Modification Steps

1. First remove the two screws at the back of the unit.
2. Separate the two halves of the case.
3. Unsolder the black wire going to the copper strip by the ariel.
4. Remove the PC assembly from the case.
5. Remove the large blue 2.2uf 250 V capacitor.  Ideally, you should replace it with a 1uF 400V capacitor, but my 1uF capacitors will not physically fit.  I have therefore used 470nF 400 V capacitors instead.
6. Replace the 200nF 250 V capacitor with a 100nF 400V capacitor.
7. Replace R22, the 220k 0.5W resistor, with a 470k 0.5W resistor.
8. Replace the blue MOV with a 275V MOV.
9. Unsolder one of the wires going to the CAM relay and solder a 100-ohm 2W resistor in series with one of the wires.
10. Make sure there is no stray solder on the circuit board and check your work carefully.
11. Reassemble the unit and test.

If you attempt these modifications, please let me know your results, and read the warning on the X-10 and 115V & 230V page.

My Comments

Having performed these modifications, I have found that the CAM relay does not always operate as expected.

On one module pressing unit code 1 "ON" the relay switches "ON", but pressing unit code 1 "OFF", the relay does not switch off.  Instead, the next press of unit code 1 "ON" switches it off.

The other two modules operate correctly, with unit code 1 "ON" switching the relay "ON" and unit code 1 "OFF" switching the relay off.

Any comments guys?

Comments received at Redoak

• Dec 2002 - Daniel Rubino has commented:
  The problem that I saw, was that the module wasn't getting enough volts. It should be 19 volts before the 78L05 power regulator and 5 volts after it... But I only got 2 volts instead.
• Nov 2005 - Matthew Nazmi  reported
  The design of the module has changed and that these instructions do not work.  Please, therefore, be careful if you attempt any modifications to this module.