The electronic version of the trim display uses an integrated circuit that
triggers the diodes depending on what voltage is received. Within the MAC
servo unit itself is a potentiometer that changes resistance in relation to
the position of the servo. The particular IC, an LM3914, has been around for
years and it, or it's sister IC the LM3915, are commonly used to provide
volume displays on stereo equipment.
Take a multimeter and measure across the two wires that go from the
potentiometer to the display (sorry I don't have their colours handy but
they are the wires that don't go to ground and +12v) while moving the
servo. You should see the voltage swing from zero to +12 (ish). If it
doesn't then the pot has failed or got some dirt in somehow. You can easily
open up the servo itself and check this.
If it does change then check the wiring into the display and if that is OK
then the IC within the display block has probably given up the ghost.
As this is a surface mount IC it is not the sort of thing you can repair
yourself. You may be able to find someone who can do surface mount work and
replace the LM3914 IC - they are really cheap and available from Radio
Spares.
Tony
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