The model I have is the 79, probably of a similar vintage. I'm familiar
with the low battery fading too, and that will affect all the segments.
But when those contact need cleaning, it first shows as one or two
segments getting a little dimmer than the others so now I go ahead and
clean it. I say every two years, it was probably more like three or four.

It is fairly simple, open the case, remove a couple of screws to get the
PCB loose and four screws from the back of that hold the display on.

The first time it happened, a guy on a newsgroup told me how to fix it.
That worked, and I was tickled pink.

I was a little tempted to put some dielectic paste on that but I was not
sure if that would be a good thing for whatever the contact strip is
made of. I'm retired, I can spare the time to clean that occasionally.

I love my Fluke, it is perfect for the kind of things I do with it and
generally up to rougher service use.

Jack

Probably not the problem with the AP control head, BUT...........

Advice is cheap here, but this tip (?) has saved me a lot of cash and
strained eyesight. When you clean that rubber-like connector piece on one
edge (that faces the circuit board) remember that the other edge (facing the
LCD's glass back itself also needs cleaning). So in most (all?) instances
when you've gone deep enough to clean the circuit board end or edge of the
rubber-like connector strip, pull it out and with the same q-tip slightly
moistened with the same alcohol, clean the other edge AND the LCD glass
connections as well. The LCD connections are practically invisible except
when holding it just right in a good light. It's some oxide (which I cannot
recall) that is transparent and a conductor.
Almost guaranteed to bring it back to life for a good number of years
(if your alcohol is clean, and you don't leave more scum than you started
with..... Jack Daniels is not a suitable solvent)
Old Chief Lynn