The fact that the tension tests work fine suggests that your motors and basic drive circuits are working fine. Your problem is unusual, so I am just speculating. When you load a new reel of tape on the MTR-10, for the first few turns the computer monitors the rotational speed of the supply...
I recently acquired an Otari MTR-10 recorder. I now find that many of the adjustments and calibrations require an extender card PB-76X. Otari does not have these. Does anyone know where I might find one?
I have been looking on Ebay and elsewhere. There seem to be a lot of MTR-10/12s...