Oh no! I've bought a DX7 on the wrong mode
You are not alone! I bought mine on Mode 2 and then realised that there's no easy way to get it to function on Mode 1 (see diagram above for the differences in the modes). Apparently it requires a return to base firmware modification, at some cost.
After removing the back of the set and moving the ratchet from the left stick to the right, and vice versa with the return spring I found the only way I could implement 'delta' mixing was to create two custom mixers one mixing 'AIL to THR' and the other mixing 'THR to AIL'. Bear in mind that 'THR' in my case is actually my elevator servo.
The problems with this solution were that I lost Dual Rates on elevator - instead the elevator DR switch gave me throttle dual rate! Furthermore, I had some issues with the failsafe not doing what I wanted. Not good eh?
So what next?
All my web searches amounted to nothing so I decided to grab my soldering iron and see what I could do. Fortunately, it turns out that I could put the soldering iron away and just grab a crosspoint screwdriver instead! This is a non-destructive and completely reversible mod!
Disclaimer: I modded my Tx as I cannot be without it while it's sent away for official remoding. For 'proper' modding, why not send your set back to the source. If you wish to (probably) invalidate your warranty and be frustrated by losing tiny invisible screws then read on! Any damage caused to your Tx following these instructions is your doing.
Note: any screws you take out, please put them somewhere safe - and preferably in separate piles with a note to say where they're from.
Let's get started - remove the backplate
OK, flip the Tx over take out the battery and unplug it, remove all the case screws and then remove the back of the Tx.
You'll find that a single circuit board containing the battery socket is attached to the backplate. It's easier if this is removed so you can get the backplate completely clear. Notice that the orange/grey wire attach to the circuit board where it says 'red/black' - irrelevant but made me chuckle. The single crosspoint screw detaches this PCB.
NOTE - from now on, the pictures will show the sticks labeled as they appear from the front of the set! Also note that, as mentioned, I've already changed the ratchet and spring to my mode.
The mission objective
Our mission is to somehow get the output from the right stick (six white wires with a brown one) over to the opposite side of the board where the left stick (six white wires with an orange one) are plugged in. However, they're not plugged in. The white connector is a soldered joint despite appearances and even if we could successfully unsolder all six at once, would it work? Who knows?
An easier solution would be to swap the Y axis potentiometers, i.e. to make the left stick turn the right potentiometer and vice versa. These are fortunately mounted on symmetrical PCBs and so can be directly swapped.
See diagram below - incidentally, in this diagram I've carefully removed the RF board from the Tx by removing the four black screws holding it in place. Handle this with care and, if you unplug the antenna lead please remember to replace it later!