With the Berg 4L in the spare parts bin, we decided to try modifying an already familiar receiver. Before modifying the R168DF, I did a rough range check to verify its performance.
To open the receiver, simply press the tabs in and pull the case apart.
With the receiver open wires will need to be soldered for +5V, GND, and the PPM signal. The +5V and GND can be done in any convenient location. The PPM signal can be tapped off of the resistor, or anywhere along the trace shown.
Potential PPM solder points.
Update 2010-01-16: In my pathetic attempt to solder the PPM wire to the board, I somehow killed it. The PPM signal line now constantly sits at ground. Checking continuity revealed that nothing was bridged on any of the ICs, and that the PPM trace was not somehow shorted to ground. Swapping crystals and transmitters proved to be a dead end. Outputs to the servos sit steady at around 3V. I may try replacing the 682 resistor. In any case we plan to switch over to a PPM encoder board and use 2.4GHz RC gear.
Update 2010-01-20: After spending more time fiddling with the board, I figured that the 682 resistor was destroyed. Measuring the resistance from the PPM trace to the other side of the resistor resulted in around 90kΩ. I removed the resistor and the measurement resulted in the same resistance. I salvaged a 4.7kΩ from another circuit board and soldered that on.
New resistor (472) and the PPM wire.
With the new resistor in place, the receiver came back to life. On the other side of the board, make connections for the +5V and GND. File down the corners of the PCB to easily snake the wires out.
Convenient solder points for the GND and +5V wires.
While the receiver is working, a range test in the parking lot revealed that we still had a system that failed at any decent distance. With this in mind, a transition to PPM encoder boards is almost guaranteed.