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After replacing the motor in a cassette player with a matching 6V DC motor, the minimum speed of the motor is too fast for desired playback speed (even when the internal potentiometer is at the minimum.

What can I add to the circuit to decrease the speed floor of my motor?

More details / what I've tried already.

I'm repairing a boombox cassette player. The motor had seized, and I wasn't able to repair that, so I found a motor with the same dimensions that was rated at 6V. I chose a 6V motor because that was the DC Voltage I measured across the existing motor when "play" was engaged.

After installing the new motor, I found that it ran significantly faster than I desired it to, even when adjusted with it's internal potentiometer to the lowest setting. What I would like to do is slow the motor down so that this lowest speed is below the intended speed, and I can calibrate the speed more delicately with the internal potentiometer.

The DC voltage is provided by a full bridge rectifier after a small power transformer.

I understand that motors typically adjust their speed based upon input voltage. I attempted adding a low value resistor in series with the motor drop, as I understand voltage drops across resistors - the motor did not run at all. I tried adding a low value diode - in this case the voltage dropped from 6V to 3.8V, but the motor did not decrease in speed significantly?

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    \$\begingroup\$ After writing this, I had a brainwave, and measured the AC Voltage at this point, which rates at 12.53V. Possibly this is an AC motor, and my motor is half the value I needed? \$\endgroup\$ – idbentley Jul 18 '18 at 23:19
  • \$\begingroup\$ Only a universal motor can run off of AC or DC. Their is a chance that some speed control is built into the motor. If so it will consume more current at lower voltages to maintain speed. Check the motors current consumption with different voltages. \$\endgroup\$ – Sparky256 Jul 19 '18 at 0:23
  • \$\begingroup\$ IIRC, some old tape players had a centrifugal switch inside their DC motors, a crude speed-regulator, so the RPM wouldn't change as the batteries ran down. Normally the motor would only be powered for much less than 100% of time, so the percent could increase as the volts and torque went down. \$\endgroup\$ – wbeaty Jul 19 '18 at 0:47
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    \$\begingroup\$ agreeing with everyone here, some speed control is certainly happening, although without a picture it's hard to say how it's being implemented. Selecting a replacement motor based on only the voltage rating is definitely not sufficient \$\endgroup\$ – Ocanath Jul 19 '18 at 1:21
  • \$\begingroup\$ You need a low impedance variable DC voltage to adjust speed. not a diode drop on a current source emulated by a high resistance from a voltage. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 19 '18 at 2:04
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Cassette motors used a speed control circuit with feedback from an RPM sensor. I think most speed control circuits used PWM, not a variable DC voltage.

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  • \$\begingroup\$ In case someone later comes along. Adding PWM control is actually pretty straightforward and there is a good explaination here: hotchk155.blogspot.com/2017/03/… \$\endgroup\$ – idbentley Jul 20 '18 at 21:22

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