Since my last reply, I have downloaded the service manual.
The first step is to use a scope, if you have one, or a DVM otherwise and check all the power supply voltages for their indicated voltages and especially for noise and ripple. These points are clearly shown on the schematic.
If you find that they check ok, it might be some problem, even with the 8051 MCU related circuit, but there are also I see, many other more likely possibilities, many other components that are very much more likely to fail, such as optoisolators or discrete transistors to check.
First check the power supply voltages and let me know if any of the +15. +1.2 +5. -15. -1.2 power supply voltages are out of spec.