I’m starting to see a trend of post frequency being highest at the start of a project and decreasing as each project matures. I suppose this makes sense as there is usually a large learning curve at the start, while the rest of the project takes more experimentation.
Until yesterday I had assumed that controlling the speed of the motor would be easy. I assumed it was a normal DC motor and by putting it in series with a variable resistor, I could vary the speed. This is necessary so that I can change the time it takes for a note to get from one tape head to the other.
I thought I ought to test this assumption, so I tried putting different resistances in series with the motor. But what I found was that for a resistance of less than 40 ohms, there was very little change in speed, and for a resistance of greater than 40 ohms, the motor did not turn at all.
Combine this with the fact that there is a hole in the motor allowing you to turn a potentiometer inside it, it became pretty obvious that this was a so-called “capstan” motor – ie it has electronics inside it to regulate the speed.
Initial attempts to open the motor up to have a look were unsuccessful and I soon gave up, resolving to buy a replacement DC motor online last night. However, this morning, following the advice of the guys on EEVBlog, I gave it a second shot and, using a drill, managed to open the motor.
I was right! Now all I had to do was modify the board to bypass this control circuitry and turn it into a normal DC motor. By removing the circuit board, I could find where the motor brushes were soldered on.
Using a Stanley knife, I scratched away the traces to isolate the input connectors as well as the brushes and then proceeded to short the input connectors to the brushes.
Plugging it back into the tape deck, it works and operates at a much higher speed. However, now I can slow it down with a resistor.