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Old 19th November 2011, 08:14 AM   #1
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Default External turntable pitch control - Technical problem.

I have a friend who is a musician and he had asked me to build him a box to connect to a turntable which gives him accurate control of pitch and if possible a couple of adjustable presets he can adjust.

My idea is the motor control comes in to the box and passes through an eight way switch, the selections are 33,45,fixed preset 1,2,3,4 (fixed by 0.1% resistor to be a semitone and tone change either up and down) and also two adjustable pots with 10 turn dials so he can have custom settings.

There will also be a 33/45 button, and this is where my brain is ticking over, it has to change the custom potentiometer settings to 136% their value so if he plays a 45 RPM record the presets will still move up and down to their tone settings.

The fixed presets are ok, the button will switch in another resistor, but for the pots I think I would need a special two gang pot with say a 13.6K and 10K track, does this sound right?
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Old 19th November 2011, 08:24 AM   #2
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If need be I can disassemble a dual gang 10K pot and using very very fine sand paper sand one track to 13.6K.
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Old 19th November 2011, 12:07 PM   #3
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OK I looked at a few service manuals and figured out how it is done - a selector switch chooses between 2 voltage dividers, in my case 5.4V is placed across a 10K/5K POT/1K pot with wiper to base of transistor/8.2K ground and for 45RPM the 8.2K and 10K are switched. All I have to do is replace the 5K pots with 1K pots and replace the 1K pitch pot with a switchable set of 5K 10 turn pots.
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Old 19th November 2011, 06:54 PM   #4
Nanook is offline Nanook  Canada
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Default OK...

I was going to ask what kind of table and what kind of motor. As it uses resistors as a voltage divider, and that controls speed, then it must be a DC motor. If AC, then a variable frequency generator of some sort must be used.
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