david,
Eric has basically retired. At his age and his personal issues he has endured over the last few years, he is entitled.
Now, as for your wanting a coil for your Goldscan 5C, you can use a TDI coil just fine but you may need to change the connector. I don't remember if the 5C uses the same connector as the TDI or not. I know Eric changed to the 5 pin connector but I don't know just when. So, if your detector has the 5 pin mic type connector, then the TDI coils should work. Actually, so should ML PI compatible as well as third party TDI coils.
Now, if your GS 5 doesn't have the 5 pin connector, then you can replace the connector on your GS 5 and on your coils you presently have and then you can use all the coils that work on the TDI.
Reg
PS, I forgot to mention, you can even add the single tone feature like what is on the TDI if you so desire. My basic design does act just a little different than that found on the TDI but it is the original design that was installed before the days of the TDI. I am sure Mr. Bill could do the mod. If he is interested I will send him the instructions.
Reg