I just wanted to share my 2 cents.
I have a Summit which originally came with a 2.4Ghz TX Non-TQ link (TRX 2230), and a 2.4Ghz RX TQ link. I was a bit disappointed that I did not have the TQ Link feature on my TX, because I got a second model, and it would have been a very neat feature.
Soooo, after reading here on the forum about the 2ch to 4ch mod, I went ahead and bought a 2.4Ghz TQ Link 2 channel (TRX 2238).
Now, I opened up both TX's by unscrewing the 5 screw on the back, and looked at the green PCB mounted on top of the battery compartment. I discovered that the green PCB board in the 4ch Non-TQ link is actually EXACTLY the same as the on in the 2ch TQ Link version!!! The only difference is the small square 2.4Ghz module which is placed on top of the green PCB (and has the antenna connected to it).
So, I thought, why de-solder all the wires, transplant the board, and solder all the wires, when I should just be able to switch the 2.4Ghz modules? (they are easy to remove, as then are mounted on removable headers).
Opening your transmitter VOIDS your warranty!
Before you do this, check that the letters/numbers and revision on the PCB's of both TX's are the same! Mine are the same, it might work if they are not, but if you break something or kill it, it's not my fault!
The numbers are in the yellow square, the 2.4 Ghz module is in the red square. (Picture courtesy of danielhr77, from his original topic).
my non-TQ link (original remote) did not have shielding on the 2.4Ghz module, you could just see the chip. The TQ link version I replaced it with did have shielding
Next step: I removed the 2.4Ghz module from the green PCB from the 2ch TQ Link version, and put it into my own 4ch non-TQ link.
I put everything back together, and switched on the TX while holding SET (registration mode). Then I switched on my first model, and held down LINK on the RX.. THEY LINKED! It worked!
So I did the same with model 2: switch off TX, switch off model 1.
Switch on TX holding SET, switch on model 2, holding LINK on RX, and they LINKED.
Now my 4 channel autmatically links between models like the 2 ch did, and ALL channels from my old 4 channel worked!
No soldering required, no modding required!
The good part about this is that after the mod, you actually have:
1x 4ch 2.4Ghz TQ link
1x 2ch 2.4Ghz non-TQ link <- sell this one, or use it as a backup remote (TQ-Link RX's can remember/link with multiple TX's)
It might be nice to know for you that the 2ch TQ Link that I bought did -not- display the extra menu items (only 2 blinks max), that the previous 2ch to 4ch model described, so I though it would not have been possible.
This might indicate that the "4ch" or "2ch" part is actually programmed in the chip on the green PCB. The TQ Link functionality is clearly programmed into the 2.4Ghz module put on top of the green PCB.
This also explains why some people where able to mod the 2Ch to 4ch by just soldering extra buttons to the green PCB, because Traxxas probably programs the green PCB, but the 2.4Ghz module comes from a general supplier?