Open up the power supply box. There's like a screw in there (or two) that you can adjust the speed. I'll take a look at mine when I get home to give you more specifics.
This happened to my Orbe SE (with the DC motor) maybe 5 years ago. It was a pretty dry summer here in AZ and I recall that a couple of healthy static discharges from my finger to the turntable occurred. It seemed to take out the Tacho speed control in the motor and cause the turntable to run fast like yours does.
Because of that I started going barefoot on the rug when handling the turntable or tonearm and that has stopped the static discharge.
Unfortunately, as I recall, I had to send the motor unit and power supply back to Michell in England for repair. I can't recall what they charged but they converted the power supply to the "Never-connected" model and fixed the tacho speed control on the motor assembly. It's been working fine since then.
This might help. You need a strobe and record with timing lines. It helps to have another hand to run the strobe while you adjust the speed screw.
Speed Adjustment Instructions - DC Motor with Standard PSU
Applies to TecnoDec and GyroDec or Gyro SE DC motors with standard (not VC or HR) power supplies.
Under certain circumstances it may be necessary to adjust the output voltage supplied to the DC motor,
so the turntable rotates at the correct speed.
First the speed must be verified before going any further. Place a strobe disc on the platter to verify that
the turntable rotates at correct speed when the belt is on the 33.3 RPM groove of the motor pulley (top
groove). If the speed is too fast or slow, adjust as follows. Note: A neon or fluorescent light source makes
it much easier to read the strobe disc than with an incandescent light. Natural light will not give any
reading at all. The use of a dedicated electronic strobe gives the most accurate results, as it does not
depend on the electrical utility company for the accuracy of the 60 Hz reference frequency.
1. Unplug the DC power supply from the mains and from the motor.
2. Remove the 4 screws on the underside using a #1 Philips screwdriver.
3. Remove the top cover, exposing the printed circuit board (PCB).
4. Using the photo below, locate the blue rectangular multi-turn adjustment potentiometer, with the brass
5. Plug the DC power cord back into the motor and AC line cord into the mains. BE EXTREMELY
CAREFULL NOT TO TOUCH ANY LIVE PARTS WHEN PERFORMING THE ADJUSTMENT.
6. Turn on the turntable. While observing the strobe, adjust the speed by turning the brass adjustment
screw on the PCB in either direction until the correct speed is indicated. The speed should be set ever so
slightly fast to compensate for stylus drag while a record is playing.
7. Unplug the AC cord from the mains, and reinstall the cover on the power supply.
PS...important. Adjust the speed while playing a record. I have the Cardas frequency sweep and burn in record. One side has timing marks for different speeds (33.33, 45, 78, etc.) Believe it or not just the drag of the diamond in the groove is enough to slow down a record....amazing. Perfect speed or a tad fast is OK. Slow is not OK. The human ear will catch the pitch change every time.