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I'm working on an old portable cassette recorder I had lying around and was wondering if it's possible to slow down the motor with an LM317 and a potentiometer. I've read about PWM, but I'm not sure how to do that or if it'll fit inside the case.

edit: it's a Sony TCM-818, the motor is hooked up directly to the motherboard, there's no circuit board/motor controller on the motor itself

Ryan Knutson
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  • Be aware that some cassette motors were speed-regulated with an internal mechanical governor. Applying an external speed-control circuit would have the internal one fighting the external one. – glen_geek Jan 20 '20 at 15:29

2 Answers2

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Probably not.

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Figure 1. Two cassette player motor internals. Source.

Most cassette players were intended to be run from battery supplies and had to run at constant speed with varying battery voltage. To do this they had speed controllers in the back of the motors. In Figure 1 you can see the trim pots used to set the motors' speed. Usually these are adjustable through a small hole in the back of the motor - perhaps covered with a quality control sticker.

I modified my cassette deck about thirty years ago to allow a little up-down adjustment to correct the pitch of songs that had not been recorded in concert pitch so that I could play along. I de-soldered the pot and wired an external one. I also put in a switch to go between fixed (normal) speed and variable. It worked well.

Transistor
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  • @ Transistor Earlier Cassette motors had a mechanical governer that used flying weights that opened contacts that placed extra resistance in the motor circuit .This was when I was very young . – Autistic Jan 20 '20 at 10:26
  • Was that steam powered? d:^) I never saw one like that but suspect that they may have been common on reel-to-reel tape decks. The result would have been the same. Reducing the voltage wouldn't change the speed until you dropped it enough that the governor wouldn't work anymore and then the speed would vary with the take-up tension. – Transistor Jan 20 '20 at 13:34
  • I'll have to check this out. Is that underneath a cover on the motor? – Ryan Knutson Jan 21 '20 at 19:57
  • That is explained in the answer already. – Transistor Jan 21 '20 at 20:04
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LM317 a linear component. In this case, that means any voltage that it drops is dropped as a result of resistance. Drop more voltage at the same current? Dissipate more heat. Allow more current at the same voltage? Dissipate more heat. In this case, the LM317 doesn't gain you anything over using a plain old potentiometer to control the motor speed. Both will dissipate the extra power as heat.

For PWM, you will use (usually) a 555 chip as a timer. That part of the circuit will include a potentiometer to vary the 555's output (the duty cycle or percent time the output is on). The circuit may have a maximum or minimum output, like 95% or 5%. Then you will use that output signal to rapidly switch the motor on or off, usually by switching its power with a MOSFET or BJT (Darlington unless the motor is tiny). Since the power is mostly on or off with very little time in transition (for transition is where the voltage is dissipated in the transistor as heat), the circuit is more efficient. Will the motor like its power being pulsed like that? I'm not sure, but you can post details of the motor and someone more educated than me will tell you.

To choose between a MOSFET and a BJT, look at how much power will be dissipated by each. The MOSFET will present a resistance when it's on (you can see this as a graph in the data sheet), and the BJT will present an almost constant voltage drop.

piojo
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