How to adjust motor to rotate at proper speed

How to adjust motor to rotate at proper speed

Vinyl records are designed to be played at 33,3 rotations per minute (rpm). It is plate's duty to rotate at that speed in order to reproduce the music within the correct pitch and tonality. However the motor, connected to the plate with rubber band, does not always come from the factory with a perfect 33,3 rotation speed. It often requires a bit of adjustment to make it perfect.

To find out how fast turntable actually rotates one can apply various measurements. In our first measurement we decided to compare song length of a vinyl ripped example with a correct version of a song.

Measurement and adjustment

In first rough measurement we notice that compared to various dubiously correct song versions, our turntable is often a little too fast.We estimate that it rotates at 33,71 rpm.

(right click picture and click View to enlarge it)

Now we turn the turntable upside down...








 


... and adjust the speed with the screwdriver.

With much trial and error, the LP should now play at desired 33,3 rpm.

Another way to measure the result

Another way to compare a song from our turntable to correct song is by comparing two notes of each song, say a piano note that you know is correct, or in this case an accord from guitar for example.

The way do do this is by first reading out the frequency or the pitch of mentioned note.










The note in question in this case is 370 Hz.

For example, the note "A" on a violin string vibrates at 440 Hz. It is a modern convention that notes on most instruments are tuned in to such frequencies.








To compare how off is our note from a desired note we enter both observations into table.

This shows that the motor is rotating at 33,03 rpm.

The final measurement

The results of above adjustments have calibrated our turntable closer to the desired speed, still we had to assume that either the fine tuning was fine enough, that we have done it with a precise screwdriver and a steady hand, or that instruments were perfectly tuned, so that what we heard sounded alright. In final measurement we are not going to rely on our hand and ear, we are going to use our eye.

To see how many rotations per minute LP makes now, we film it and watch it in slow motion, count how many times it rotates in exact minute. It should be 33 times and a third.

And it is.