The carriage belt has no effect on platter speed.
But the platter belt can have.
If you replaced the platter belt and the speed is now wrong it's most likely because the belt is too tight (Ebay belt?) and should be replaced with the correct type (Beoparts or me).

BTW did you fit the correct soft triangular belt type for the carriage motor? If not I suggest you get the correct belt for this too. The cheap square
ones sold in large numbers on Ebay are junk. They run rough and are far too dense, causing a transfer of vibrations from the motor to
the subchassis. You can hear an annoying humming background sound from the speakers everytime the carriage motor is active.

Else, the speed setting trimmers are on the main circuitboard. See in the servicemanual.

Martin

Hi,
the Beogram TX2 has 2 potentiometers for adjusting the speeds.

The potentiometers can be reached from below (front left) with a small screwdriver
[you have to go into the auto mechanic pose 🙂to see which potentiometer is responsible for 33 RPM and which is responsible for 45 RPM.]
Now you only need an AC ~~ light bulb and a stroboscopic disk to adjust the speed.

As Martin described, a bad belt should be excluded beforehand.