I have 20 TDK SA/SA-X and one SONY Metal Master. Don't really want to open those as they are extremely hard to find nowdays.

I used TDK D/B/EF and TEAC cdx to try the dolby c recording and all got same result. Is it likely the machine is out of "fine tunning"? But as far as I know the original owner never opened the machine, and it only recieved 40 hours of usage in total and was taken care very well.

What model do you have and use, could you please try your model with those tapes + Dolby C and see what happens?

Cheers