In this video I replace the old keybed felt which sits on top of the aftertouch sensing strip in my Yamaha DX7 and then calibrate the aftertouch.
The keys hit this felt when they are pressed and then push on the aftertouch sensor strip. As it was worn out some of the keys hit directly on the aftertouch strip without dampening and produced a very clunky sound. I had also noticed that the aftertouch did not work very well. This was probably due to the aftertouch strip having changed characteristics over the years and needed a recalibration.
Unfortunately I could not find specific instructions how to perform the calibration. So what I did was use MIDI Monitor on my MacBook (You can use e.g. MIDIOX if you use Windows) to see how the Channel Pressure messages looked. Before the calibration I could only get up to 15 or so even when I pressed so hard my fingers hurt. The original DX7 should get around 1-100 (they behave a bit oddly as 127 should be the maximum value. This might differ if your DX7 has an upgraded firmware or is using a Grey Matter E! or a SuperMAX expansion card).
As I found no instructions I monitored the output from the DX7 MIDI out and made tiny adjustments of the Offset and Gain trimmers until I got full range output and it worked reliably over the entire keyboard. You’ll have to do this by feel and get it to a point where it works comfortably for you
If you haven’t removed the keys in a DX7 I would suggest looking at this video for reference. It is for a DX11, but the principle is the same: