There's an auto shut off on the control board that stops the machine if no pulses from the reel motors is detected (at least, that's how I'm guessing it works). It seems to take a half second or so to engage. If the reels should be turning (as in play, rewind etc) but aren't, it stops and (in play) the pinch rollers and tape lifter will disengage.
I would check all the easy stuff first (cause that's the kind of hair-pin I am) and be sure that nothing is binding or an adjustment hasn't slipped.
Speaking of easy, be sure that the "battery power" switch on the back is in the off position and check the internal fuses.
While you're at it, fool around with the "timer" , speed and pitch controls on the front. Those can be the source of odd behavior since they never get used and their contacts can get pretty oxidized.
Open the back and release the brakes (power off!) and see if both of the motors are turning freely.
While you're around the brakes, make sure that all the connections to the motor circuit board are still attached. One of mine just fell off a long time ago and it looked like just a single strand of the wire was originally soldered to the board. You have to remove the motor to fix it but as long as you don't drop one of the 3 screws into the bowels of the machine, it ain't too tough.
Then I'd pull and clean all the multi pin connectors and reseat them.
Try working the "tension" adjustment pots. These are pretty "cheap" parts (I keep meaning to replace mine) and get progressively harder and harder to adjust correctly. I can easily imagine that one of these might eventually get so bad that it becomes an open circuit.
So, try all that stuff off the top of my head and let us know how it goes.
