AX3500SC R/C input unusable without mod

I hooked up my AX3500 for the first time yesterday. I have it running from two 12V lead-acid batteries in series. I have a Spektrum DX3E transmitter going to the AX3500 for throttle, and going to an Arduino for steering control. The steering system works flawlessly with the RC trasmitter/receiver. Also, the AX3500 works fine with Roborun.

When I hook up the AX3500 input #1 to my RC receiver, the motors just randomly spin and I have no control over them. I used a scope probe at the receiver and the pulse signal looked clean. When I put the scope probe on the right side of R6 on the AX3500 PCB, the noise magically stopped and I could control the motors with my transmitter. As soon as I removed the scope probe, the motors were again uncontrollable.

Is there something wrong with my AX3500 PCB? I put a 0.1nF capacitor between the right side of R6 and ground so I can continue development, but I\'m wondering if I need to send it in for repair?

p.s. I even had nothing else running but the AX3500, and the RC reciever is powered by four AA batteries so there is nothing else that can be inducing noise into the system.

The AX3500 needs to see a pulse that reaches the full 3.3V. Some of the new radios do not get the level quite that high. To correct, insert a 1K resistor between each of the RC input and the 5V output that is on the 15-pin connector. Remove your capacitors.

I have the same exact problem.

@roboteq in my case I was level-shifting my 3.3V signal up to the 5V supplied by the RC harness using optos, so signal level should not have mattered.

Ultimately I gave up on RC and implemented RS-232 because it is far more predictable. RC mode was totally erratic and nearly impossible to get it to do what I expected.