From the previous post:

 For almost a year I have a vintage turntable from Perpetuum Ebner at home.It's not mine. It belongs to someone that asked me to fix it. And it turned out that fixing the motor would cost too much.I asked the owner to collect the tt. That hasn't happened yet and the machine is collecting dust at my home. So I'll take the freedom to attempt a non-intrusive repair with modern components. I'm also thinking about making it an Enchanted Objects Design Challenge.

In this post I'm measuring up the different gears, pulleys and wheels, and I calculate the speed of the original motor.

To measure the original motor's speed, I need to know the size of the gears that play a role in power transfer.

Because the desired platter speed is known (16 - 33 1/3, 45 and 78 RPM), I can  trace back the original motor's speed when I know the reduction ratios.

Three measures play a role here:

• The motor pulley,
• The step pulley
• The outer rim of the table platter.

The motor speed is transferred to the step pulley via a belt. So the first factor is the size of both pulleys.

The speed of that step pulley is then transferred to the platter. The idler takes care that the spinning of the step pulley is transmitted.

That idler can move up and down to select a particular position of the step pulley.

That is how the right speed is selected: by putting the idler on the part with the correct diameter for the given speed.

So two factors play a role in deducting the speed:

• the fixed ratio between motor pulley and step pulley. It doesn't change when the speed selector is operated, hence a fixed ratio.
• the variable ratio between the steps on the step pulley and the outer rim of the platter (those are the two parts that are connected via the idler wheel)

The size of the belt and the diameter of the idler don't come into play. They just transfer the power between two rotating parts.

That gives me the following measurements and results:

The formula to derive the motor speed from the platter speed is:

[platter speed] * ( [platter diameter] / [stepper speed diameter] ) * ( [stepper outer rim diameter] / [motor pulley diameter] )

examples:

45 RPM: 45 * (177.2 / 14.7) * (29.7 / 5.5) = 2940.3 RPM

33 RPM: 33.3333 * (177.2 / 10.9) * (29.7 / 5.5) = 2934.0 RPM

Platter Speed
(rpm)
Motor Pulley
diameter
(mm)
Outer rim
diameter of
stepped pulley
(mm)
RatioSpeed dependent
diameter of
stepped pulley
(mm)
Platter outer
rim diameter

(mm)
Ratio

Calculated motor

speed

(rpm)

165.529.7

5.4

5.5177.232.22782.1
16 2/35.529.75.45.5177.232.22898
33 1/35.529.75.410.9177.216.32934.0
455.529.75.414.7177.212.12940.3
785.529.75.425.4177.27.02948.4

All 4 calculations should result in the same motor speed number. My measures are not precise, and that shows in the results.

The 3 highest speeds are close. 16rpm is off compared to them. I must have made a mistake when measuring that part on the step pulley - also its bigger ratio makes that a mismeasure effects the calculation most.

The above table and explanation is edited after a comment from RPLaJeunesse. He explained that 16 RPM is not exactly 16 RPM, but the half of 33 1/3 RPM. The 16 RPM figure is now in line with the 3 other speeds.