Michell Gyro SE running fast

I recently noticed the pitch of my Gyro SE was high. Checked the speed with a strobe sheet and confirmed it is. Adjusted the pot in the Power Supply, this did not affect the speed.. Has any else experienced this or aware of a solution? Tested an old and new belt along with groove placement. No impact.

Thanks in Advance for any feedback.
I think they give you more than one belt groove on the plinth. If that doesn't help, you'll probably need to get a speed controller. If you don't have a good dealer to work with, Try Hollywood Sound in FL. All he does is TT's and sells a lot of Michell products. He can definitely tell you what you need to do.
z response is what I have seen most often with speed too fast. other possibility, if the belt rides high where the belt is riding on the top of the pulley, it can cause it to be faster.
Thank you: I've tried belt / groove position. I found a post about an adjustment pot in the power supply. Single small pot. I slowly adjusted the screw counter clockwise and that did the trick. From what I can tell, I'm back to 331/3.
Thank you for the comments.