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Mains frequency varies and only averages to 60.0 Hz (or 50.0 Hz) over time. Calibrate your turntable with the highly accurate handheld strobe and free strobe discs. Or, replace your deck's old strobe with the mountable module version.
Easily and inexpensively check the rotational speed of your turntable. Just press the power button, and shine the pulsating LED light on strobe disc or platter timing marks and adjust the turntable's speed until the marks are stationary.
If your turntable's platter has markings for 60Hz, use the 60Hz module. If your turntable has no markings you can use a 60Hz module and the 60Hz strobe disc: 60HzStrobeDisc.pdf.
If your turntable's platter has 50Hz timing marks, you can use the 50Hz module if your turntable platter has 50Hz timing marks, or you can use it with the 50HzStrobeDisc.pdf. Note that exactly calibrating 45rpm with a 50Hz strobe isn't possible. Why? Because you'd need 133-1/3 marks on the disc, an irrational number. Instead, discs are printed with 133 marks, or 45.113 rpm, which is pretty close.
If your turntable has marks for an incandescent bulb operating at mains frequency, then the marks are actually timed for 2X the mains frequency because incandescent lights are illuminated 2 times per mains power cycle. The Turntable Strobe emulates this behavior and flashes the LED at 120Hz (or 100Hz) making your timing marks appear sharp and clear. The strobe discs are also calibrated for 120Hz (or 100Hz) operation, too.
Some later model turntables used LED strobes (like my Technics SL-QD33) and have timing marks calibrated for 60Hz (or 50Hz). The TurntableStrobe works on these, but the marks won't be quite as sharp. You can use a strobe disc instead.
The quartz crystals selected for use on the Turntable strobe modules have a frequency tolerance of ±30 ppm at 25 ºC (±0.003%). Temperature stability is ±30 ppm over (-20ºC - +70ºC) but of course we can expect a much narrower range of frequencies for normal indoor use.
Measurements of ~200 devices demonstrate that frequencies for these boards fall within -0.0015% to -0.0035% of the target frequency. Testing is conducted indoors at ~20°C (datasheet is specified at 25°C). I use a Saleae Logic analyzer with 42ns accuracy. A plot of the results are below:
The bottom line is that TurntableStrobe is plenty accurate enough for its purpose.
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Hi, my name is Mike Shimniok, author of the Bot Thoughts robotics blog (www.bot-thoughts.com). I've been building electronic circuits since 1985 and robotics since 2007. I hold a BS in Computer Engineering, from University of Arizona and an MS in Systems Engineering, from The George Washington University.