Service info on mac tools et955 multimeter

I have an old Mac Tools ET955 multimeter. It is also identical to a unit provided by Matco. The circuit board has the inscription "Actron 400-1486" on the solder side. The problem is it constantly displays the count of "23" or "2.3" or ".23" with nothing connected, depending on the range selector. A 9-volt battery displays "+14v" or "-24v" depending on polarity. Adjusting the pots has no effect. I removed the MM5601BN quad 2-input NOR IC and the 7555 timer and the display reverted to all zeroes. So I assume either the 7555 timer chip or the 5601 chip is bad - not sure.

Anyway, does anyone have any suggestions on what could be the problem and where I could get a schematic and repair manual for the unit. I assume it is American made.

Thanks.