Some non-Futaba receivers will not operate with the Futaba 9Z transmitter when the 9Z synthesized module is used. Those receivers don't have a problem with the fixed-frequency modules.

The 9C synthesized module you're using in your 8U transmitter has a slightly-different coding that those non-Futaba receivers can accept.

Futaba does guarantee that all Futaba receivers will respond to Futaba transmitters as long as the frequency and coding are correct.