how to add 78 RPM and/or variable speed pot to DD turntable motor?? - diyAudio
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Old 8th February 2010, 11:19 PM   #1
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Question how to add 78 RPM and/or variable speed pot to DD turntable motor??

I'm making a dreamachine and need a 78RPM turntable....don't have one and would like to add 78 RPM to my old DD sony that I tore apart (33 and 45 RPM). I'd love to make it spin at 78RPM and if possible make it adjustable with a pot up to maybe 90 or 120 RPM

When I hit the speed select button I'm sure it switches between two different circuits. Is there just a resistor I can change or something and add a pot in there somewhere? ....I bet I would be able to figure it out on my own if it was belt drive.
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Old 9th February 2010, 07:28 AM   #2
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Usually the speed switch changes the input frequency of a syncronous motor, thus changing the speed. Most have a feedback loop from a photo-tach and a fine adjust pot.

Lacking a circuit diagram, look over the speed switch circuit and fine adjust to see if it is indeed the resistance that sets the speed. If the speed switch changes fixed resistors, based on those 2 values you can calculate an approximate value to produce 78 rpm. Another option would be making a new tach wheel (usually opto-interupptor setup under the platter) for 78 and switching between opto-interupptors with a 4pdt switch.

All this assumes it'll go to 78 (motor controls/motor limitations) - experimentation will tell.

Why would you want to go to 120rpm? You'd need a laser to track at that speed. Disk brakes. And asprin. Lots of asprin.
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Old 9th February 2010, 02:05 PM   #3
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thanks. I'm making a dreamachine. it doesn't play records... it plays light!
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