I have repaired a large number of Technics and Sony direct-drive turntables with runaway speed like yours.
In many cases, the failure was caused by lack of feedback. If you look closely at the motor, you will see drive coils and feedback elements. Depending on the model, the feedback is acquired either via a tape head type device or via laminated inductors acting as pickup coils.
It was the laminated inductors that caused the problem. Either due to living in an environment with salt air (causing corrosion) or because of a fluid spill, the laminated inductor had opened up, eliminating the feedback.
The motor control IC, not receiving any feedback, dumbly thought, "Gosh, maybe if I go a little faster, I'll get some feedback." After going faster and still not getting any feedback, the IC even more stupidly thought, "Gee, maybe if I go even faster than that, I'll get some feedback."
And so on and so on . . . until the motor was racing at about 120 rpm!
In every case, I found the open connection and jumpered it -- and the problem was fixed.
We accept ship-in repairs -- e.g. Marantz, McIntosh, Pioneer, Yamaha.
Classic Audio Repair