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The only way I know to solve a loop problem is to break the loop and measure each part.
So, I think the first thing is to measure the fork pulse going into the chip (with a scope) and see what the heck it's doing when you move it. IF it is good, moving it closer shouldn't hurt, should just make the pulse sharper (steeper sides), as it clips at zero and 12 volts.
Another part is the motor control - disconnect it from the chip output and ground that lead, and the motor should stop or slow way down; connect it to 12 volts and the motor should race. If you can connect that lead to the wiper of a pot (say 10 k), with the ends of the pot at ground and +12, you should be able to manually control the speed - and the right speed should be with the wiper approximately centered. If not, your motor control circuit is not centered and it may be impossible for the circuit to reliably control it. If it's running too fast, you need to either put in a pulley system or perhaps put a high power resistor in series with the motor. I can't tell you what the value and wattage should be without knowing how much current your motor draws.
The third thing to check is the chip operation. You can look at the output with a scope or a voltmeter. Onc eyuou know the fork pulses and sync pusles are OK, reconnect the whole circuit. When the disc is too slow (like the fork is blocked), you should see a high-duty cycle waveform on the chip output that has a DC average (voltmeter reading) close to 12 volts. When the disc is running away at high speed, you should see a low-duty cycle waveform that has a DC average close to zero volts.
When everything is centered on its nominal range and the whole circuit is connected and working, the chip output should average about 1/2 of the power suypply, or 6 volts.
I'm suspicious that your motor is just too uncontrollably fast and needs to be slowed down - If you are running the speed setting close to the "slow" end to achieve this, you may need some other fix to make the adjustment more centered. But you need to check the operation of all the parts, especially first why are you getting such strange results when you move the fork.
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