I forget the part number for the critical IC, but all I did was Google it, and I found many sources for it, in Asia. Most are on Alibaba (I hope that's the correct name of that website). I just chose a vendor who seemed to understand at least a little English. The cost is cheap, once you find a vendor, because who else wants one, besides the few dozen people in the world who may be fixing TT101s? Albeit, this was 3-4 years ago when I made my purchase. One thing to keep in mind is that the chips are around the same age as the TT101, so I suppose it's possible that they can go bad just from sitting on the shelf under god knows what storage conditions, for 30 years. One of mine was used to repair a TT101, and it worked, is all I know.