RCA combo unit

[TV Engineer]

Originally Posted by Reece

You can also entirely remove the rubber "tire" and find a big and thick O ring that will fit on the wheel. Diameter of the idler is not important as all it does is transfer the rpm of the motor capstan to the turntable rim.

This is completely incorrect. The diameter of the idler is critical to correct rotational speed.

Notice that there are four diameters of the motor shaft, representing the 4 available speeds. What difference does it make whether you change the diameter of the motor shaft, or the diameter of the idler wheel, or even the diameter of the driven surface of the platter? Changing any of the three will affect the speed of the turntable.

There were many changers built in the 1950s that actually used different idler wheels for different speeds. Webcor, RCA, and VM are just a few that come to mind.

[old_coot88]

Originally Posted by TV Engineer

This is completely incorrect. The diameter of the idler is critical to correct rotational speed.

This is true only in the case of a compound idler, ie., two different-sized wheels affixed to a common shaft. The diameter of a non-compound (single wheel) idler does not affect rotational speed, since the rim of the wheel is only a motion-transfer agent between the driving and driven surfaces (analogous to a fan belt). The length of the belt does not affect the speed relationship between the driving and driven pulleys.

[old_tv_nut]

Originally Posted by TV Engineer

This is completely incorrect. The diameter of the idler is critical to correct rotational speed.

Notice that there are four diameters of the motor shaft, representing the 4 available speeds. What difference does it make whether you change the diameter of the motor shaft, or the diameter of the idler wheel, or even the diameter of the driven surface of the platter? Changing any of the three will affect the speed of the turntable.

There were many changers built in the 1950s that actually used different idler wheels for different speeds. Webcor, RCA, and VM are just a few that come to mind.

No, the ratio of the capstain to the idler is reversed by the ratio of the idler to the platter; therefore the idler size does not matter - it only changes the idler RPM.