Don't fully understand what you're saying, but it seems like you're describing a mode select issue in the mechanism (mechanism won't go into the correct position, doesn't obey the right commands). If your pinch rollers are shot or your belt is out of spec you'll also face issues. This version of the EX series won't release the tape head (and allow you top open the door) unless it's in Full Stop mode in the cam. (In Stop mode, pushing the door lock should release the head)
For logic mechanism issues:
Check solenoid plunger, then the mode select cam. You'll need to desolder the motor and solenoid connections to the board. I'm assuming you have the correct size belt, and your pinch rollers and tape stock are in good condition.
The motor points are marked UWVC, the solenoid is near your BATT - point (above that yellow DC jack box). You should also disconnect the head flex cable, and there seems to be a tape type switch near the solenoid and DC Jack as well.
You may need to replace the dried grease on the cam (which moves the heads up and down, pinch rollers, puts the unit in stop mode) and/or replace the solenoid.