Hi!
Regrettably I don't have one of these meters myself I can offer a dump for, but it's very unlikely you've got a total CPU failure - the Siliconix LD120/LD130 series of A/D converter devices are notorious for developing internal faults in their I/P stages causing a permanent overflow, which blanks out the display or causes other silly tricks!
Referring to Schematic no. 400 for the A/D board, begin by linking pins 3 & 6 of IC403 (TL081) with a piece of wire or a 1k resistor - this should cause the LD120 to read a zero input which should be shown on the meter's display - if there is still no display or nonsense, you need to check the "Integrator" output of the (pin 11) LD120 is ramping up and ramping down - this is typically over 32 clock cycles, with the U/D pin 4 going high to produce a rising ramp and then low to produce a falling ramp - a good dual-beam oscilloscope should be able to synchronise to these.
If either the integrator ramp or the U/D control is missing, this could be either a micro or an LD120 fault, the way to find out which is to lift pin 31 (P1.4) of the micro temporarily (I'm assuming the micro is socketed), then apply a TTL square-wave of about 100kHz to pin 4 (U/D) of the LD120 via a 1k limiting resistor, whilst looking at it's integrator O/P pin 11 - if there is no triangular up/down ramp, when a separate TTL input is applied to "exercise" the LD120, the LD120 is suspect and you'll need to source another one!
If there IS a triangular integrator signal on pin 11 and it appears to match the U/D signal on Pin 4, then the A/D converter is at least partly functioning and you need to check the Digit and segment drives from the 8035 microcontroller, and also the /INT signal coming in on Pin 42 of the 300 controller PCB - the micro needs the /INT signal to allow a measurement to begin!
Try these tests if possible and I'll get back later!
Chris Williams
PS!
I've assumed you've checked all the PSUs, reference voltages and all the function-switch contacts - this is basic diagram-work!