From the "old Clay page":
No Flashes: the LED is Permanently On.
If you turn your Bally game on, and the LED stays on continually, this is one of the hardest problem to fix on these boards. A stuck on LED can be caused by any (or all!) of the following:
The reset circuit is damaged (after all, it's in the battery corrosion area!) This includes transistors Q1 and Q2 (both 2N3904), Q5 (2N4403), and diodes CR5 (1N4148 or 1N914) and VR1 (1N959B or 1N4738A).
The program ROM (read only memory) at U6 is bad.
The ROM jumpers are incorrectly set for the U6/U2 ROMs installed.
The U9 CPU microprocessor is bad.
The U11 PIA (peripheral interface adapter) is bad.
ANY of the sockets for chips U6, U9, U11 are bad.
The ground trace to U11, U6 or U9 are bad. U11 is especially important to check the ground trace, as it's right in the corrosion zone. And corrosion just loves to travel up the ground traces. Use a DMM set to continuity to check ground at U11 pin 1 (and U11 pin 20 for +5). Also check U6 pin 12 for ground (and U6 pin 24 for +5). U9 pin 1,21 (gnd) is less of a problem as it's well outside the corrosion area.
The first and last two points are probably the most common problems.
Go "Bare Bones" (What to Remove for the first "Flicker").
To get the initial flicker out of the MPU board's LED, you only need three chips on the MPU board. These three chips are U9 (the CPU), U11 (PIA) and U6 ROM (except on some Stern games, which also require the U2 and/or U5/U6 ROMs). Remove all other socketed chips! Once you get the initial flicker out of the MPU board, then you can re-install the other removed chips.
The Reset Circuit.
If the reset circuit is damaged, the MPU board will never start, and the LED will stay on at power up. Since the reset circuit is in the "corrosion zone", it is often damaged from battery corrosion. The reset circuit holds pin 40 (reset) of the U9 CPU low until +5 volts is "stable", and then sets pin 40 high (telling the CPU it can start the boot up process). While the reset section is holding pin 40 of the CPU low, the LED will be on. This is one part of the initial "flicker" seen when a working Bally board is first powered on.
Here's what to try first:
With the MPU board OFF, Put a logic probe or DMM set to DC volts on pin 40 of the U9 CPU. The logic probe will be "low" (or the DMM at zero volts).
Power the MPU board on.
In just an instant, the U9 CPU Pin 40 should go "high" on the logic probe, or about 4.75 to 5.5 volts on the DMM. Repeat this for U9 pin 2, which also should also go high.
If the above happens, the reset circuit is probably working fine. If pin 40 of the CPU is low and never goes high, there is a problem with the reset circuit. Since pin 40 never goes high, the CPU will never start the boot process, and the LED will stay on.
Here's some other stuff to try:
With the MPU board powered on and pin 40 of the U9 CPU high (4.75 to 5.5 volts), short the junction of resistors R1 and R3 to ground. To do this, find the junction where resistors R1 and R3 connect together (the right side of R1 is the junction of R1 & R3). With the MPU board on, use a wire and short this junction to ground (TP4).
U9 CPU pin 40 should now go low to zero to .5 volts while the R1/R3 junction is grounded.
Another short cut is to take a small screwdriver and short pins 40 and 39 of the U9 CPU chip together for a second. This again simulates the reset circuit. If the LED starts flashing, the reset circuit is damaged.
The above procedure is simulating what the reset circuit is meant to do. If shorting the junction of R1 and R3 does not pull U9 Pin 40 from high to low, then there is a problem with the reset circuit. Most likely there is a problem with Q1 (2N3904), Q2 (2N3904) and/or Q5 (2N4403). Replace them all (they are cheap!) and repeat the above procedure. Also check/replace diode CR5 (1N4148 or 1N914). Finally check/replace diode VR1, which is a 1N959B (1/2 watt) or 1N4738A (1 watt), 8.2 volts (note VR1 is mis-labeled in the manual as "1N9598"). If you still can't get U9 CPU pin 40 to go low, there is some other problem in the reset circuit (corroded traces?).
Here are some other points to check. Turn the MPU board on and test:
U9 pin 3 = 2.4 volts
U9 pin 36 = 2.6 volts
U9 pin 40 = 4.75 to 5.5 volts
U9 pin 5 = 2.8 volts
If the above voltages check out, then the reset section is probably working correctly. This means your problem probably lies with chips U6, U9 or U11 (or their sockets), or incorrect MPU board jumpers.