I have 3 Spirit units, on 3 different sized helis.

The Oxy3 and Gaui X3 can work with a single 4650T without worry I believe. Using the information here (http://manual.spirit-system.com/index.p ... 2C_SPM9747) and either your purchased adaptor cable or a home-made version seem straightforward.

Is there some significant difference between the 4651T and 4650T, or is the case the only change?

On a 600 size heli where I would be more comfortable with a second signal receiver source how do I wire something like this up if the SAT ports on the Spirit Pro don't work any longer? Do I need to purchase a separate RX so that I can plug the 9747 in, or can I wire up 2 of the 3 SRXL2 type units somehow via the Spirit? It would be great if I could just use 2 of the 4650T units, but I'm guessing the 9747 is required as the second receiver signal source.

Thanks for reading my post.

The SPM 4650t is fly-by telementry only were as the spm4651t is full range. If you want more than one rx, on a big heli i think the best way would be to have a AR6610t rx with a SRLX2 sat plugged in to that. I would use an SPM4651t as i already have 2 of them...You could use a SPM9747 if you wish..

What does fly by telemetry mean?

Short range, only receive on your tx when you fly close...