MPU LED Flash 1.

      The first MPU LED flash indicates the reset, clock and zero cross circuits are working properely. If there is no first flash, see the section above for explainations of repairing the reset, clock, or zero cross circuits. Also be sure to check the MPU board is getting 4.9 to 5.2 volts (TP1), 12 volts (TP2), and 24 volts (J1 pin 3), and ground (TP7).

MPU LED Flash 2.

      The second MPU LED flash indicates the zero cross over pulse is OK. If no second MPU LED flash, look for 25 volts DC at MPU connector J1 pin 3. A bad U10 (Z80CTC) can cause no second LED flash. Also check chips U3 (74LS04) and U2 (LM339). Last check for a failed U11 (Z80), U6/U7 (6551), U12/U13/U26 (Game ROMs), or U17 (8255 PIA).