I have just bought an ariston rd11 superior that has this problem : when i set the switch on 33rpm the tonearm starts to vibrate vertically. No problem with 45rpm.I wonder if this can be caused by an electrical issue
; i think i have set properly the springs.
Sorry for my english
Not electrical although the motor is involved.
After viewing your video, it appears the tone arm is vibrating in sympathy with the motor, which is not suppose to happen. Kicking the speed up to 45 makes the vibrations less visually noticeable, but I'd bet the arm is still vibrating.
Check for any physical connection between the motor and tone arm/suspension, as in a grounding wire that is too tight, worn bushings between the motor and top plate, or even the tone arm cable coming in contact with the motor. Setting the springs too tightly could also transmit vibration.