Whole

The platform of this instrument refused to start up after being unloaded. When you turn the timer switch the rpm needle swings wildly, sticks at 400rpm when i turn the timer to 10 min., but it won't start up. I checked the belt and it's fine. Other possibilities are motor and analog speed control. I believe I can remove the motor more easily. I did check the brushes and tho there is a difference is size between sides there is still plenty of brush. I'm thinking, hoping it's the cheaper analog speed control - has anyone experienced this and do you know how to access behind the panel easily to test this?

Equipment: Thermo Scientific MaxQ 4000 shaker model 4320 (SHKA4000)