Extra wires on DC motor

[yelneerg]

I scavenged a dc motor out of an old tape deck but it has four wires instead of two (and no it's not a stepper). The wires are marked "- + H L" and when I apply a voltage to the - and + wires the motor spins fine. What are the H and L wires for? When I measure the resistance between them it various in the few KOhm region as I turn the motor. If I apply a voltage to them nothing happens. If I run the motor at 5V and connect the H and L wires to a voltmeter I measure 4V out. So maybe they are for feedback control?

[kc8apf]

Sounds a lot like a position potentiometer. What was the motor attached to in the tape deck?

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[Cubdriver]

Likely some sort of a tachometer output to allow circuitry in the tape deck to monitor the motor speed.

-Pat

[yelneerg]

Hmm, I don't think the wires could form a pot, the resistance doesn't smoothly change and why would a tap deck need to know the absolute position of the motor anyway?

I tried to trace the wires but they get lost in a bunch of ribbon cables.

[IanB]

A tape deck needs precise speed control for accurate pitch and to avoid wow and flutter. So possibly those wires provide some kind of speed feedback signal.

[yelneerg]

That makes sense. I assume that there should then be a linear relationship between speed and output voltage?

[Paul Moir]

Most likely frequency.  There will be a sprocket shaped thingy on the shaft of the motor and a ring gear shaped thingy with a coil of wire around it on the motor.  Magnetism is available.  When the prongs on the sprocket line up with the teeth on the ring gear a little current is induced in the coil.
Google "magnetic tachometer" or "reluctance tachometer".  I can't seem to find a picture of what you're likely to find in the motor, but you'll get the idea.  If your multimeter has a frequency measurement setting, you'll be able to check this pretty easily.

[Spyke]

These are just constant current speed controllers contained in cheap tape decks. Having played with loads of tape deck guts and built working decks out of broken ones as a kid, the H and L are just the two set speeds of the motor, for high (hi speed dubbing), and L being normal playback. If you look closer at the back of the motor you should see two holes that contain the pots used to set the speed for each, usually stamped H and L as well.

Some tape decks used 3 wires, one being for the speed which was further controlled on the PCB of the tape deck. Very few cheap decks actually had some sort of closed loop speed control feedback to the capstan, if any... that was all territory of the very high-end decks, which used synchronous motors and full capstan speed monitoring, usually by a hall sensor. Some even have those hall controlled pancake capstan motors, more commonly found in modern VCR's.

[Rick Law]

My tape deck has High and Low speed rewind/fastForward.  That H/L could also be for different speed.

[yelneerg]

@Paul I hooked up my multimeter in frequency mode and it would periodically measure a value if I put a load on the motor but nothing consistent. Looking at the output on my scope it is a pretty smooth dc output until I zoom way in.

@Spyke How would they function as speed controllers? What combinations of wires would have a voltage applied to them for the slow or fast?

Here is what happens when I apply 5 volts to the various combinations of wires
+-: runs
+L: runs
+H: nothing
-L: nothing
-H: nothing
LH: nothing