I have now installed a TCS M4 decoder in the second U25B (SP 6702). I did the installation nearly in the same way as described for SP 6700 in the previous post. The difference being that this time I did not solder the gray and orange wires directly to the motor contacts. Instead I reused the motor contacts pads on the original light board, and soldered all wires to that board.
Here are a few pictures showing what I mean. The first one shows the light board with the part holding the rear LED cut off/removed (to the right in the picture). You can also see that the resistor and the diode for the front LED (to the left) has been removed, and that all internal board leads has been cut/filed off.
The next picture shows the same thing, but from the underside of the board.
If you look carefully you might also see two soldering joints - one on each of the two motor contact pads (the large rectangular shapes). Those solder joints are brass wires I added. They go right through the board. If you look carefully you may actually see them in the first picture as well. The idea was that the motor contacts should still touch the pads (as when the engine was delivered), and that the decoder wires should make contact with the pads throgh the new wires.
After having prepared the board as shown above I reinstalled it, but since I had cut all the leads on the board, nothing was in contact with anything else.
Now I could solder the decoder wires to the board. I started by soldering the red and the black wires (track power) to the right and left frame contact pads, and the orange and gray wires (motor) to the new brass wires coming up through the board. Before I soldered the wires in place I cut them to length.
The only thing remaining was to solder the blue and white wires to the front LED, with a 1 kOhm resistor in series, as shown in the last picture below.
The unused wires (yellow, green and violet) are tucked away under the decoder. And yes, the heat shrink tube around the white wire (near the decoder) is because I happened to cut it by mistake ;-)