1.  The LED and photodiode that are on the
"far" side of the head (the
 side towards the rear of the TK50).  These detect "stripes" that are in
 the leader. 
    I think I've found this pair...the whole wheel came out when
unscrewed...connected to a plastic strip on the bottom.  Cleaned this pair.  And
managed to get the tape out more or less intact.
  2.  The LED and photodiode that are around the spindle
on the far side
 of the head.  This is a bit more complicated - you have to remove
 several screws, and it's probably best if you pull whatever tape remains
 before trying this.  When you get the cover off you'll see the little
 rotational encoder wheel that the LED's shine through - make sure this
 is clean, too, but don't bend it! 
    Not sure about this one...
  To pull the tape, pull back on the solenoid (right
side of the drive,
 near the front), move the lever, and pull the cartridge out.  Now pull
 all the tape out - preferably right into the garbage can! 
    I take it you think the tape isn't worth feeding back through?
    Another curious thing...after I got the tape out, I tried to power the unit up
again.  Same thing happened...three short jerks on the spool, and then the
flashing red light.  I checked the phototransistor and LED...they seem to be aimed
through the hole in the guide belt.  Is it possible that one is not working?
Cheers,
Chris