The early TM Twins had a hardware fault with the chip that is located directly behind the VFD display, this had to be manually replaced with a new chip from Technomate which was posted out (I remember doing this on my own receiver and one for Andyblack about a week later). It's possible that this chis is the one that has failed on your receivers but I doubt that Technomate still have any stock as I believe they no longer sell this receiver although I could be wrong on that one.
You could try contacting Technomate directly and see what they say about it although they will likely fob you off.