It seems that this thread is starting to stir up some interest in vintage Kraft radios and how they worked. I have a functional 4 channel system which will allow me to take some pictures of the Rx plug-in setup. It isn't much different than how things work today except that nowadays we can power up Rx's by plugging power into any channel jack. In the '70's the Rx's jacks were specific so the battery had its own jack and the other channels had different male/female connectors to insure one didn't plug in the battery to a channel servo jack and vice-versa.

It seems that it might also be a good time to shed some light on 2.4 decks. Below is a 2.4 deck which is referred to as a "hack" module which basically means it is designed to go into "any" Tx which has a PPM output. There is no module case per se which are typically designed for Futaba/Hitec or JR/Spektrum systems. 2.4 systems work with three leads to the RF deck - 1) Ground (black), 2) Battery voltage (red) and 3) PPM signal (yellow or white or some such). The PPM signal is the voltage modulated signal and is altered from a reference voltage to instruct the RF deck which channel to modulate (send signal to) over the air. The Rx detects this deviation from the reference voltage and knows which channel (servo) it needs to send a corresponding signal to.

2.4 systems need to have the RF deck and Rx lock on to each other. This is what the binding (or marrying) routine is for. The deck below has a separate PCB with the binding button and informational LED which mounts to the Tx case for binding the Tx to a specific model Rx. Other than that, the module just needs an antenna signal lead to go to a ~2 dBi (5 dBi or stronger is sometimes used on telemetric system or for military purposes but 2 dBi provides us with an ample one cubic mile of range or so) 2.4 GHz antenna.

From what I have read and research done by others (including RF engineers), the FrSky (pronounced Freesky) 2.4 modules are among the best of the lower cost 2.4 RF alternatives. Of course they work in concert with Rx's of the same brand as each brand uses its own design variation of mapping operating frequencies to the 2.4 GHz spectrum (e.g., DMS2, FASSST, ACCST, etc.). Shown are two Rx's that work with this module - a 7 channel and an 8 channel. A 4 channel is also available. More info on the ACCST 2.4 system and FrSky can be found on their web site:

http://www.frsky-rc.com

David