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?
Best Answer
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.