One of the things on my to-do list today was to get a Bang & Olufsen Stereopladespiller Type 42 up and running. Unfortunately, I didn’t have a stroboscopic disc for testing the speed. Since a quick search on the Internet didn’t turn up anything I liked, I decided to make my own.If you’d like to download it, it’s available here as a PDF file for A4 paper, and contains the lines for 50 Hz and 60 Hz mains. You can change the magnification to make it fit on different paper sizes, or to increase or decrease the size of the disc. If your magnification is the same in the X and Y axes, then it won’t change anything.

This meant that I had to do a little math, which goes as follows:

mains_frequency = 50 Hz (this is the rate at which the lights blink)

rpm = 33+1/3

revolutions_per_second = rpm / 60 = 0.555…

revolutions_per_light_blink = revolutions_per_second / mains_frequency = 0.0111…

degrees_rotation_per_blink = 360 * revolutions_per_light_blink = 4º

So, here in Denmark where we have 50 Hz mains, I needed to make a disc with a line every 4º. Since I use a Mac, I used graphic.app to do this, but any decent drawing program will do the trick.

If you want to make your own disc, and you don’t want to do the math, here are the results of the possible mains frequencies and revolution speeds

RPM	50 Hz	60 Hz
16	1.92	1.60
33 1/3	4.00	3.3333…
45	5.3999…	4.50
78	9.36	7.80

For anyone who knows a thing or two about the Type 42… then I’m already ahead of you. I know that the lines are built into the turntable mat itself. However, I was working in pretty bright daylight, and so I needed more contrast on the lines to be able to see the interference from the lighting. And besides, it was fun as a little light recreational math.