I bought one that was inoperable for $20. I tore it down and rebuilt the sensor system. The unit is what they call a "floating control with feedback" setup which means you have to go off your designated course by a fixed (5-10 degrees) angle before it'll make any correction. Modern systems (RAYMARINE, SIMRAD
etc) are digital Proportional-Integral systems with adjustable settings, these make small corrections for small errors and large corrections for large errors continuously. Depending on how closely you want to follow your course the TM will give you a snake effect in smooth conditions while the others will hold a steadier course.
The unit I bought was an older one that didn't have adjustable power for the LED (it actually had a small incandescent bulb). The bulb was paired to a photo
sensor and used a plastic disk mounted like a compass
needle. When the opening in the disk allowed the light to shine through the unit turned off (on-course condition) when the compass rotated and light turned off (off-course condition) the motor
ran to move the tiller and put you back where you wanted to go. Later versions might have a little more sophistication. The best part was when I went to put the compass sensor system back into the aluminum
container and found the Pepsi logo on the bottom. Truly a field repairable unit! Feedback was accomplished by string wrapped around a potentiometer which indicated the rod position to the electronic board. All in all it worked reliably in smooth water
but wasn't suitable for heavy sea states.
Modern units are much more controllable and have NMEA
interfaces for both autopilot control and heading information. I use SIMRAD
TP22 and TP32 units on my Islander 30.