Step 1- Make sure that the computer is booting up. It should have ONLY (1) LED LIT either "Normal Operation" OR "Default Pricing".. If you have anything else here, then you may have a computer problem OR a power supply problem.
NOTICE - The computer can act strange if the battery is dead.
Important - There is a 3.6v NiCd battery located in the computer. It WILL eventually cause corrosion of the circuit board, causing permanent damage. I recomend that you remove the battery from the board and install any 3.6v cordless phone battery here. Make sure you get a NiCd and NOT A LITHIUM ION. Solder the (new) battery wires to the board making sure the soldered connection goes through to both sides of the board. Use double sided (foam) tape to secure the battery to the board. (REMEMBER POLARITY IS VERY IMPORTANT)
If you're booting OK go to step 2.
Step 2 - Make sure that the Opto board's LED is flashing while the mech is scanning. If not then you have a problem there. The opto board may need adjusted or cleaned. Make sure there is not a ball of fuzz blocking the sensor window. This board is located on the right side of the record basket. You will have to look UNDER the board while the mech is scanning to see if it is flashing. If it's flashing, go to step 3.
Step 3- Replace the (2) 22uf tantalum capacitors on the mechanism control board. It is located below the turn table. The yellow ones (caps) often fail. You can use a regular electrolytic caps. Be aware of the polarity when replacing these (Very Important). As you can see one was already replaced here.
The mechanism control board. is located be beneath the turntable (see below)
This last pic may help you if you need to confirm the polarity of these caps.
If the problem is still present, you may have additional problems with either this board, the power supply, or perhaps the computer has too much corrosion. At this point you would need to to have schematics and then see if you have the 5 volts on the regulator transistor. (really a chip). I have found one of the little blue 0.1uf capacitors (at the transistor) SHORTED,causing the regulator to do nothing. Check these caps before you order a new regulator. If you confirm all of the voltages are correct. You will need to re-visit the the mechanism control board, saving the computer for last.
If at any point you feel "LOST", you should consult a technician.