Does anyone know how to adjust the speed of a Teac TN300 beyond the built in adjustments

I have a Teac TN300 in my home office. I upgraded the stylus to the AT-VM95SH and the sound improvement was tremendous. The original stylus was actually a bit bent. I bought the unit used so I guess the previous owner was a bit rough. Since I was realigning things, I downloaded an RPM app on my phone and checked the speed. It was running fast. I tried to adjust the speed with the ports in the DC motor. One is for 45RPM and one for 33RPM. I only bothered with the 33RPM adjuster. In the end I managed to get the speed down from 34.6 to 34.1, but that is as slow as I can get it. I checked the power supply hoping it was running hot and I could put a lower voltage unit in, but it measured 12.15V without a load. I could us a regulated 12V supply, but if the change is linear that will only bring me to 33.7RPM. That also assumes the current supply does not drop voltage when under load, which is unlikely.

As I googles things, I confirmed that a DC motor's speed is directly proportional to the input voltage. The higher the input voltage, the faster the output speed. The lower the input voltage, the slower the output speed.

Are there any suggestions to get the speed down? I am considering:

1. Going through my power supply inventory to find something with 11.X volt output. I don't use the pre-amp, so I don't mind under powering it and risking damage.

2. Measuring the voltage across the motor when operating as well as the current flow and calculate the right resistor to reduce the voltage applied to the motor to attain a slower speed.

Has anyone ever tried either of these to wrangle a fast turntable to a slower speed? If what I describe sounds overly risky, I'd rather have slightly fast payback than a dead turntable. I could delay this experiment until I buy a new one and this unit moves to the garage. 

A better turntable is on my shopping list, but it will have to wait till the end of the year.