YES -- the 3GX WILL work with a 72 MHz system.
I took advantage of Grand RC's Black Friday specials, and bought a couple of 3GX systems at a killer price.
Remembering this thread, I dug out my old JR 8103 and an R790 SPCM receiver. I found a functional NiCad flight pack, and plugged everything together.
AND -- nothing worked as it should, although the servos did move when I moved the 3GX controller, and all the lights lit up, and I could select the different setting modes. This told me the 3GX controller was happy, but something else was going on.
Next, I pulled the three wire plug that goes between the 3GX system BATTERY OUT PORT and the RECEIVER BATTERY IN PORT.
I plugged that cable into an unused channel of the 7-channel RX. My thought was that none of the 3GX cables that plug into the servo ports of the RX have a ground wire, perhaps a ground between the two units was missing (although I had to keep telling myself that the ground pin on the R790 is common to ALL connectors on the RX.
But, with the 3GX battery out port connected to an unused RX servo port, the 3GX system came alive, I could control all servos from the TX and the gyro sensors would correct for my moving the 3GX box around.
Next, I plugged the cable back into where it should go between the 3GX box and the RX battery in port. I then jumpered the servo block ground pins to the battery input port pin. Still, although things lit up and the gyro sensors seemed to work, the system didn't. This confirmed that I wasn't seeing ground problems.
Then, it dawned on me. The third "unused" wire in the cable between the 3GX battery out port and the RX battery in port, ISN'T unused. In my JR receivers, it's the DSC port -- direct servo control. Further, that same pin on the 3GX box is used to put satellite RX's that would be plugged into the 3GX (assuming you're using 2.4 GHz Spektrum/JR satellites) into BINDING MODE.
I used an x-acto knife to lift up the contact retaining tab on the ORANGE, "UNUSED" wire in the 3GX battery out to RX battery IN port, so that the RX ONLY sees two pins connected from the 3GX box -- +Volts, and Ground.
Voila. The 3GX system worked as it should. The appropriate servos followed the appropriate stick movement, and the gyro sensor when moved, made all servos move.
So YES, you CAN connect the 3GX system to a standard 72 MHz receiver. Just use a TWO WIRE HARNESS between the 3GX battery OUT port and the RX battery IN port. Power and ground, disconnect the third "signal" wire in the harness.
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