Turntable weight affects speed

Agree with @noromance.

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From the manual:

TD 850
Drive System: Thorens Belt drive
Motor: Fully electronically regulated AC-Synchronius motor

If the belt is clean/new and the bearing is lubed then the problem could be the added load on the AC motor and or regulated power supply caused by the weight.

FWI, Increase the load on an AC motor current goes up to maintain HP, speed. Current adds heat both to the motor winding as well as the "Fully electronically regulated" AC power supply. (The speed of a AC-Synchronius motor is controlled/maintained by the AC frequency.)

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Assume:

Without the added weight the TT platter RPM speed is correct when playing a record.

Add the weight the platter RMP speed slows down when playing, listening, to a record. ("It noticeably slowed it down.") Remove the weight platter RPM speed is correct when playing a record. Therein platter is up to normal RPM running speed.

My guess is the added weight puts more pressure on the spindle and the bearing it rides on. Therein added friction, drag.

I guess if the belt is worn the added friction, drag, caused by the weight there would some slippage around the motor pulley... Slower platter RPM speed.

One way to see if the motor RPM is slower with the added weight would be to check the motor pulley RPM speed playing a record without the weight and then again with the weight. If the motor pulley RPM is "noticeably" slower then it is not belt slippage. My guess then would be the "Fully electronically regulated AC" power supply can not supply the added power, (P = I x E), needed to maintain the power and frequency needed for the correct RPM speed of the AC-Synchronius motor.

At that point I would not use the weight!

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