Cybernetics Data Products scrolling LED sign

[Charles]

I'm trying to repair a partially functioning Cybernetics Data Products 
ADT-2002 scrolling LED sign from the early 80's. Previous owner said 
there was a power failure or lightning strike, don't remember which, and 
gave it to me. The brief version of the instructions are taped to the 
keyboard, and that is the only manual I can find.

Power supply is functional and delivering 5 volts where it should.

On boot it's supposed to say "UNIT OPERATIONAL" on the display before 
plugging in the keyboard ribbon cable. There is garbage displayed instead.

When the keyboard is plugged in, the display responds to start/stop 
scroll (green and red keys), and messages can be input. The memory 
(yellow key) and control (blue key) functions don't seem to work.

There actually is a microprocessor (8048 or 8051), but inside the 
keyboard only.

Another observation: The display only scrolls a couple of characters, 
jumps back to the beginning and repeats forever.

But the actual display board has a 24-pin Harris PROM (fuse link), seven 
2102's 1K memory for the 7 rows of LEDs and many 7400 TTL including  
4-bit counters, comparators and adders. No CPU at all, so it must be a 
state machine, and a fairly complex one at that unless some of the 
"smarts" are actually in the keyboard microprocessor. The cable from the 
keyboard connects directly to the PROM without any buffering.

I have found that only the lowest 4 bits of the memory address lines 
have activity on them, which explains why two characters scroll before 
it repeats (16 columns). I can't find anything strange looking (e.g. 
non-TTL levels) on the middle and upper nibbles except that they don't move.

There does not seem to be any information on this unit online. I'd like 
to find a repair manual (probably too long a long shot), but even a 
schematic would save a lot of hair pulling!

Thanks for any help.