Thanks for everyone who tried to help. I have found the problem. On the suggestion from firmtronics I fitted an ossilloscope to find the refresh rate of the signal from the receivers. The ppm has a speed of 20ms where as the pcm has a speed of 14ms. The device looks for around 20ms and anything else is seen as noise and the device shuts down. They have said they will send me a new one free of charge with updated software. Thanks Firmtronics.