It is confusing with the modular type radios like the 9C.

Whatever module you have in the back of the 9C is what is going to be transmitted. If you have a Futaba 2.4Ghz module in the back, it will be transmitting 2.4Ghz FASST which is proprietary to Futaba, so it will not talk to a Hitec 2.4 Ghz receiver like the Optima 7. However, if you have, say a channel 52 - 72 Mhz FM module in it, like I do, then it will transmit both PPM or PCM on 72 Mhz on Channel 52, and since it is Futaba, it will be negative shift. This means you can use the Futaba 72 Mhz receivers provided you have the right crystal, or you could use Hitec's shift selectable, or negative shift Rx's or Berg... But, you probably could not use the JR receivers as they are positive shift.

Going back to 2.4 - There are a lot of modules for these types of transmitters, but they will only allow you to use their receivers. i.e. Corona, Assan, Hitec, Futaba, Frisky, WFly, Graupner - they all make them, but they only control their own Rx's.

I have a 9C and will be converting, but am awaiting the findings of a test on RCModelReviews.com to see which Chinese comapny I want to take my buisness to as paying $60 for a receiver seems to be a little crazy.

Best Regards,
Curtis