...och U8 är utbytt.

Power-On Test
The three diagnostic LED's will indicate the status of the CPU [/url]as it boots.  The top LED (D19) indicates the status of the blanking circuit and should come on immediately when the CPU is powered up and then go off after about 3 seconds, and stay off  The middle LED (D20) should start flashing when D19 goes out and indicates the CPU is running.  The bottom LED (D21) should be on all of the time and indicates that 5 volts is present.
See Image 7 for the location of the diagnostic LED's.
The diag LED's are also used to indicate some board failures.  This information is provided in the front of the game manual and excerpted below.  All tests presume the supply voltages (5 volts and 12 volts) are good.  Also note that when you turn off power to the board all 3 LED's will glow dimly indicating that the LED's are good.
D20 blinks once -- U6 game ROM bad (G11 on WPC-95).
D20 blinks twice -- U8 CMOS RAM bad.
D20 blinks three times -- U9 ASIC chip bad (WPC-89) or PIC security chip bad (U22 on WPC-S or G10 on WPC-95).
D20 never blinks --  Check D19 status: If it's on the CPU will not boot.  If it's off check the game ROM.
Note: You can get a flashing D20 even if 12 volts is missing at the board, but the board won't work properly.  It is also possible to get a flashing D20 when the board has data or address line problems.  I just worked on a board where most of the address lines were 3 volts instead of 5, but the LED indicated the board had booted.

Blanking Problems
If D19 doesn't turn off the CPU will not boot.  First check pin 3 of U21 (blanking signal from the ASIC), which should be high. If it is high and D19 doesn't turn off then there is likely a problem with the amplifier chips, either U21 or U5.  If it is low then the signal from the ASIC chip is bad indicating a problem with the ASIC, the 6809 processor or one of the clock circuits.
You can test the 8MHz and 32Khz clock signals (see Image at pins 34 and 35 on the CPU.  These signals come from pins 81 and 82 on the ASIC.  If one or the other is missing check the clock inputs to the ASIC at pins 55 and 70.  If either input is missing check crystal X1 (32Khz) or crystal X2 (8Mhz), as appropriate, and their associated circuitry.

Dead CPU
It doesn't happen often on WPC games, but sometimes nothing happens when you power up the CPU.  You have 5 volts and 12 volts at the board, the D21 LED is on (5 volts), but D19 (blanking) and D20 don't even blink.
Do a thorough visual inspection.
Check all previous board work.
Look for cracked or broken traces, pads and through-holes.
Check any questionable areas with a DMM.
Look for battery damage.
Check the ROM jumpers.
Reseat the ASIC.
Check the pins on the ASIC while it's removed.
Reseat the game ROM and CPU.
You can check the checksum of the game ROM with a ROM programmer.
Check reset and IRQ.
Pin 37 of the CPU (U4) is the reset and should be low when the board is powered on and then go high (5 volts).
If it never goes high the probable culprit is U10.
Pin 3 of the CPU is the IRQ and is normally high with fast low pulses.
Check 8MHz and 32Khz clock signals.
Check pins 34 and 35 on the CPU for the clock signals (the ASIC provides these clocks).
Although it's rare, a bad ASIC or CPU socket can keep the game from booting.
The CPU will typically not boot if any address or data lines are missing.
See the following section for more information.