X-10 Maxi Controller Strange Behavior

[Pat <pat@xxxxxxxxx>]

I have an old SC503 X-10 Maxi Controller.  Its main use is for me to
press "all off" when going to bed at night.  Recently, it stopped
sending the all-off command.  However, I can still send commands to
individual modules.  For example, pressing "6", "7", "off" works fine,
but pressing "all off" does nothing.  (The bedroom lights happen to be
D6 & D7).  That makes it sound like the problem is the switch itself,
but the indicator LED still operates normally so the switch must still
work.

Any ideas?
Pat