I now have all the items I need to test out whether a receiver from Hitec will work with my Multiplex Profi Car 403 transmitter. To set the stage, here is my test equipment setup;

Transmitter = Multiplex Profi Car 403
Tx Crystals = Multiplex brand, Channels 67, 70, 80 & 81
Receiver = Hitec HFS-03MC Single Conversion FM surface 75mHz
Rx Crystals = Hitec brand Channels 67, 70, 80 & 81. All receiver crystals are 75mHz FM single conversion receiver crystals. PN-6687567, PN-6687570, PN-6687580, PN-6687581
Servo = HPI SX-1 with Airtronics Z connector (same as JR and Hitec) Basically a 94201 servo with a new label from HPI.

I plug the servo into CH1 on the receiver, insert Rx crystal on channel 70, leave Tx crystal channel 70 in the transmitter, select the menu on the Profi 403 and change to FM modulation, plug in a 4-cell receiver battery pack. Leave the Tx turned on and then turn on the power from the receiver pack.

Results = NOTHING.

Now what?