NEC PF-68T31 Service Manual page 14

VOLUME LIMIT
Maximum Volume control value by customer would be limited with this figure.
AV LOCK
If this item were set to "ON", after exit from this LOCK mode;
i) Only with AV input are available to use.
ii) RF (Tuner) input function will be disabled.
PANNEL LOCK
If this item were set to "ON";
iii) All front panel keys will be disabled.
iv) Only by remote control, TV can be operated.
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