howdy! firstly, if you dont have the schematics and manuals, grab these off the net:
http://www.robotron-2084.co.uk/manualsdefender.html
the first manual, "Upright Instruction Manual
(Early Series Boards)", contains a section on how to interpret the led lights on the board. post a little more info about what lights are showing up.
to do this you will need to open the rear door to access the boardset, and, you will need to pull out the interlock switch so you can still run the game with the door open. make sure you dont go near or touch any live mains wiring. around the power supply or interlock switches or the monitor.
there is also a section in one of the manuals about setting up the game using the switches on the coin door. these are the switches referred to in the first manual.
having interpreted the leds to see which rams are referred to as possibly faulty, next step will be to run through a series of checking & cleaning operations.
do you have multimeter ? if so, you should check the dc output voltages from your power supply - the 5v rail, 12v rail, & -5v rails in particular.
next step, measure the voltages at each voltage input on some of the rams which will tell you if the rams are etting the right voltages or not.
if the voltages are low you may have some problems with the power supply so that is a good place to start checking (having turned off the system first of course)
- each power supply pcb wiring header connector on the power supply pcb for signs of overheating, corrosion, cold or cracked solder joints and dirty header pins, or dirty wiring plug socket connectors.
- reflow solder on the header pins, clean the pins with a brillo pad (the green scourers) and then some metho or electrical contact cleaning spray (radio shack ?)
- you could carefully remove each ram - use a fine flat screwy to lever up evenly being very careful not to bend any legs, carefully clean the legs with the same above and reinstall each ram ensuring the legs go in straight and dont fold under or bend.
post up your results!