Williams System 7 Diagnostic Codes: What the CPU LED Is Telling You

On a Williams System 3 to 7 pinball machine (1977-1984), the CPU board has its own small voice. On System 3 to 6 it is a pair of LEDs; on System 7 it is a single 7-segment digit. If your game is dead, stuck, or showing a strange number, that light is the first thing to read.

The short version: at power-on the LEDs (or the System 7 digit) should come on and then go out as soon as the processor starts running. Locked on means the CPU isn't booting, isn't resetting, or has locked up. One flash then back on usually points at the ROMs or their sockets. On System 7, pressing the CPU diagnostic button runs a real memory test: 0 = pass, 1 and 2 = RAM, 3 to 7 = ROMs, 8 or 9 = the 5101 memory chip, memory protect, or simply a closed coin door.

Contents

• What a healthy boot looks like
• LED or digit locked on
• One flash, then on again
• System 7 diagnostic codes, 0 to 9
• "Code 9" and the coin door: the most common false alarm
• It boots, but the game still won't start
• The cheap fixes to try first
• When a replacement board makes sense
• FAQ
• Sources

What a healthy boot looks like

The CPU board (Williams called it the MPU, or microprocessor unit) is the brain in the backbox. It is built around a 6800 or 6802 processor, 6821 PIA chips (peripheral interface adapters, which let the processor talk to switches, lamps and coils), a 6810 working RAM, and a 5101 CMOS RAM that remembers your settings and audits while the batteries keep it alive.

Here is the part that confuses many owners who have worked on Bally machines first: Williams does not run a full self-test every time you switch on. The LEDs start lit and the processor turns them off once it is running. A proper test only happens when you press the diagnostic button on the CPU board.

• System 3 to 6: the LEDs come on at power-up, then go out as the CPU starts. On the bench, with no driver board connected, a working board will typically blink on, go off and then come back on steady.
• System 7: the digit blinks "0" and goes out. On the bench it may come back on steady.

Before you read anything into the light, make sure the machine is actually getting power. If it isn't, start with our step-by-step guide for a pinball machine that won't turn on.

LED or digit locked on

If the light comes on and never goes out, the processor isn't getting going. The usual suspects, in the order most techs check them:

1. The 5 V logic supply

Measure 5 V on the CPU board itself, with the board connected, not just at the power supply. A short somewhere in the board set can pull the supply down. One owner on Pinside isolated a collapsing 5 V by unplugging the playfield, then the sound board, then the CPU in turn until the voltage recovered. It turned out to be a metal mounting tab touching a trace on the back of a board. Note that the regulator shuts down on a short, so a fuse that hasn't blown proves nothing.

2. The 40-pin interconnect

The CPU and driver boards are joined by a 40-pin connector carrying the address and data lines. Pinrepair.com puts it bluntly: if any of those lines drops out for even a millisecond, the whole game locks up. Look for bent, dull or cracked pins and reseat it.

3. Sockets and ROMs

Original Williams boards used Scanbe sockets, which are notorious. The long-standing advice is to replace them on both the CPU and driver boards rather than keep chasing intermittent faults. The flipper ROMs at IC17 and IC20 are also known failure points.

One flash, then on again

If the LEDs go out once and come straight back on, the board booted and then hit a problem. Pinrepair.com lists this as most likely ROM-related (bad sockets or bad ROMs) or a bad PIA. If the lights flash once and then stay off but the displays are wrong, suspect ROM sockets again, or a board that won't service its display interrupt; the guide names the 4020 at IC25 there.

System 7 diagnostic codes, 0 to 9

On System 7, pressing the diagnostic button on the CPU board runs the ROM and RAM test and shows a single digit. This is the list commonly quoted on Pinside from the PinWiki repair guide:

Code	What it points at
0	Test passed
1	IC13 RAM
2	IC16 RAM
3 to 7	One of the ROMs (flipper ROMs IC17 and IC20, game ROMs IC14 and IC26); code 7 is game ROM 0 at IC26
8	IC19 5101 CMOS RAM, or the memory protect circuit
9	Coin door closed, memory protect circuit, or IC19 5101 CMOS RAM

Two honest caveats. First, the test only means something if the board has actually booted; on a locked-up board, experienced techs warn that the digit can be meaningless. Second, a ROM code isn't always a bad chip. One owner got a steady code 7 from home-made replacement game ROMs that simply didn't carry the checksum the test expects, while the game played perfectly.

On System 3 to 6 there is no digit. When you press the diagnostic button on a healthy board, the LEDs give a quick couple of blinks and then the lamps and displays go dark. That is the board passing, not failing.

"Code 9" and the coin door: the most common false alarm

The 5101 memory is write-protected unless the coin door is open. So if you run the test with the door shut, you can get a 9 on a perfectly good board. Open the door, run it again, and see if it changes to 0 before you order any chips.

If the code stays at 8 or 9 with the door open, then look at the 5101 at IC19 and its batteries. When that memory loses its contents, the game can come up in audit mode instead of attract mode. The fix can be as simple as fresh batteries; in one Pinside thread about a game stuck in the audits, the owner cleared everything using the button on the CPU board rather than the procedure in the manual, and the game played normally afterwards.

It boots, but the game still won't start

A board that boots and passes its test can still leave you with a machine that won't play. Look outward from the CPU:

• Batteries. In a well-known Pinside thread about a Williams Flash, the game only reached attract mode after a working battery pack was fitted.
• Fuses. The same owner found a fuse missing from the display circuit. Check every fuse against the backbox label, not just the ones that look blown.
• Connector pins. Pinrepair.com recommends replacing the header pins at connector 3J6 on the power supply, and the terminal pins in its housing, as a standard upgrade.
• The playfield. A stuck switch or a coil locked on can make a game look faulty. Pinrepair also notes that a locked-on driver transistor commonly damages the solenoid PIA.

The cheap fixes to try first

A lot of Williams boot faults are solved with a multimeter and some patience. In rough order of cost:

1. Open the coin door and rerun the System 7 test.
2. Fit fresh batteries, ideally in a remote holder away from the board. If the originals have leaked, read our guide to spotting and stopping pinball battery corrosion.
3. Measure 5 V at the CPU board, and unplug boards one at a time if it's low.
4. Check all fuses against the backbox label.
5. Reseat the 40-pin interconnect and inspect the pins.
6. Replace Scanbe sockets and tired header pins. This needs a soldering iron, but it cures a surprising number of intermittent lock-ups.

A tip before you start: take the solenoid fuses out while you diagnose, as Pinrepair.com advises, so a fault can't lock a coil on while the game is half-working.

When a replacement board makes sense

Replacement becomes sensible when battery damage has eaten traces, when the 6800-era chips you need are hard to source (the MC6875 clock on System 3 and 4 boards is described as obsolete and impossible to find), or when you fix one fault and the next arrives a month later.

If you get there, the WillFA7 replaces both the CPU and driver boards on Williams System 3 to 7 games such as Black Knight. It runs the original ROMs, mounts on the original standoffs and connectors, and each board is set up for one specific game, so check your model first. We test boards before they ship and support them afterwards. Our Williams System 3 to 7 replacement board guide compares the options, and you can see one fitted in our Williams Flash repair.

A new board won't fix a missing fuse or a flat battery pack, though. That is why the light is worth reading first.

FAQ

What should the LED do when a Williams System 7 boots?

The digit blinks "0" and goes out once the processor is running. If it stays on, the CPU isn't booting, resetting or running.

What does code 9 mean on a Williams System 7 CPU board?

Coin door closed, a memory protect fault, or a bad 5101 CMOS RAM at IC19. Open the coin door and rerun the test first.

Does a Williams game run a self-test at power-on?

Not a full one. The real ROM and RAM test runs only when you press the diagnostic button on the CPU board.

Why are the LEDs on my Williams System 6 CPU board stuck on?

The processor isn't starting. Check 5 V at the board, the 40-pin interconnect, the ROMs and their sockets.

Can dead batteries stop a Williams pinball machine from starting?

Yes. With the 5101 memory empty, the game can come up in audit mode or refuse to reach attract mode until good batteries are fitted.

Is a diagnostic code always a bad chip?

No. Sockets, connectors and even non-standard ROM images can trigger codes. Reseat and inspect before replacing parts.

Sources

• Pinrepair.com: Williams System 3-7 repair guide, part 2 (boot-up and CPU board)
• Pinrepair.com: Williams System 3-7 repair guide, part 1 (before turning the game on)
• PinWiki: Williams System 3 - 7
• Pinside: Wms System 7 MPU Error Code
• Pinside: Williams System 7 - Game won't start, Diagnostic Problem
• Pinside: Please Help – I don't understand Williams Diagnostics
• Pinside: Williams Sys.7 Selftest error 7 Faulty game ROM
• Pinside: Need help with Williams System 7 power supply... no 5V
• Pinside: Williams system 4 cpu/mpu

Not sure what your CPU board is telling you? Send us your machine and what the light does. Our technical support is free, before and after purchase, including when the answer is a cheap fix you can do yourself.