I'm hacking the speed control on the TCM-20DV sony walkman. Service manual with schematic here: http://pdfstream.manualsonline.com/d/d92cbef9-a37f-4e2b-92e5-521fab68df58.pdf
Out of the box it has a small-range speed control with two speed tiers (normal and half-speed). I'd like to replace this with a single knob to cover the entire range of speeds. I've tried a few approaches, but I'm very new to electronics and was unable to produce satisfactory results.
The motor has red and black terminals. The red is always ~3.0 volts from the two AA batteries. The motor seems to be controlled by varying the voltage drop across the motor, using some sort of "governor" circuit in the walkman's IC. If the motor takes all 3.0 V, it goes as fast as possible. Conversely, the slowest setting is a drop of ~1.0 V. I found this out by measuring the voltage from the black terminal to ground. Ideally I could just hijack the terminals and make a circuit which lets me adjust the voltage drop across the motor.
My first idea was to use a voltage divider and a potentiometer in parallel.
Oddly when I wired this up, the whole voltage drop happened on R4 and the motor did not do anything. First question: why would the first resistor take the whole drop?
My second idea was just putting a potentiometer in series with the motor, to control the voltage drop across the motor because it seems to have some resistance itself. This sort of worked but the motor oddly wouldn't move without a nudge at higher (but still within range) resistance values. Second question: why would it need physical nudging to get started in this case?
And third question: what is the appropriate approach to this problem? I'm a bit lost, since there are many many techniques I am completely unaware of. And any reference material you think is relevant would be much appreciated!