OK?..manual is complet and helpfull... for repairing such a old and rare boombox like JVC PCM it is necessary...
Text excerpt from page 13 (click to view)
PREPARATION FOR SERVICING
How to Enter the Service Mode
About Optical Sensors Caution: An optical sensor system is used for the Tape Start and End Sensors on this equipment. Carefully read and follow the instructions below. Otherwise the unit may operate erratically. What to do for preparation Insert a tape into the Deck Mechanism Assembly and press the [PLAY] button. The tape will be loaded into the Deck Mechanism Assembly. Make sure the power is on, connect TP502 (S-INH) to GND. This will stop the function of Tape Start Sensor, Tape End Sensor and Reel Sensors. (If these TPs are connected before plugging in the unit, the function of the sensors will stay valid.) See Fig. 1.
Q503 Q504 TP502 S-INH
Fig. 1
Note: Because the Tape End Sensors are inactive, do not run a tape all the way to the start or the end of the tape to avoid tape damage.
