Could this be the same one, that I acquired recently?
First things were to work out why the symbol bulb was not displaying anything, this wasn't too hard, it was very dead, voltage was present on two of the pins. Removing the bulb & checking it with a Megger confirmed it was dead, no signs of life, not even at higher voltages.
Thankfully I had one of these, bought unknowingly from a TE murderer in Penkridge, fitting this I now had a minus symbol and an AC symbol. Next problem was the minus symbol was stuck on in all modes, even with a positive input.

I've worked out some of the circuit, 3 transistors near the bulb deal with switching the +/- symbols and the AC symbol is switched by another transistor found near the mode switch.
Eventually I traced the +/ toggle back to some Nand gates, one was stuck high on both inputs & outputs.
Replacing that 7400 got the symbols lighting one at a time, but with another fault where + or - was staying on in ohms mode.


I believe two more transistors near the range switch, should turn off the +/- symbols when in ohms mode, this is where I've got to with the repair ATM.
David