What Is the Clock Circuit on a Classic Arcade PCB and What Does It Do?

By HighScoreSaves Technical Team

Reviewed and updated September 3, 2026

A classic arcade PCB contains many circuits that need to perform events in the correct order and at the correct time.

The system that provides those repeating timing references is commonly called the clock circuit.

Clock circuitry is fundamental to digital arcade hardware, but it can be easy to overlook because the clock does not contain the game program, save scores, draw artwork, or directly control the joystick.

Instead, it provides the timing that allows other circuitry to do those jobs correctly.

What Is a Clock Signal?

A digital clock signal repeatedly changes between electrical logic states.

Those transitions provide timing events that digital circuits can use to coordinate their operation.

A simplified clock can be imagined as a repeating sequence:

LOW → HIGH → LOW → HIGH → LOW → HIGH

The repetition occurs at a specific frequency.

What Does Frequency Mean?

Frequency describes how many clock cycles occur during a given amount of time.

It is measured in hertz.

• 1 Hz = one cycle per second
• 1 kHz = one thousand cycles per second
• 1 MHz = one million cycles per second

Classic arcade PCB clocks commonly operate in the megahertz range, although the actual frequencies depend entirely on the hardware.

The frequency printed on a crystal is not necessarily the same frequency ultimately reaching every CPU or logic circuit because the original clock may be divided or otherwise processed.

What Is a Crystal on an Arcade PCB?

Many classic arcade boards use a quartz crystal as part of the timing source.

The crystal works with surrounding oscillator circuitry to create a stable repetitive signal.

A metal or ceramic component with a frequency marked on its body may therefore be visible somewhere on the PCB.

However, not every timing source looks the same. Hardware may use:

• quartz crystals;
• crystal oscillator modules;
• discrete oscillator circuitry;
• logic-based oscillators; or
• other game-specific timing arrangements.

Is the Crystal the Entire Clock Circuit?

No.

The crystal may be one part of the oscillator system.

Surrounding circuitry can include:

• logic gates;
• inverters;
• resistors;
• capacitors;
• oscillator ICs;
• counters;
• flip-flops;
• frequency dividers; and
• buffers.

The exact implementation varies considerably between arcade hardware designs.

What Is a Master Clock?

A master clock is a primary timing source from which other timing signals may be derived.

Instead of using a completely independent oscillator for every circuit, an arcade board may begin with one master frequency and divide it into several slower frequencies.

This can help synchronize different parts of the hardware.

What Is a Clock Divider?

A clock divider creates a lower-frequency signal from a faster one.

For example, a counter or flip-flop may divide a clock frequency by two, four, eight, or another value depending on the circuit.

A simplified example might look like:

Master Clock
↓
Divider
↓
CPU Clock
↓
Additional Timing Signals

Actual arcade circuits can be much more complex.

Why Does the CPU Need a Clock?

The CPU executes instructions in a timed sequence.

Its clock provides the timing reference that allows internal processor operations to occur in the proper order.

Without a valid CPU clock, the processor generally cannot execute the game program normally.

For processor fundamentals, read What Does the CPU Do on a Classic Arcade PCB? .

Does the Clock Control Game Speed?

Clock frequency can influence the operating timing of digital circuitry, but arcade game speed is not determined by one simple universal relationship.

The game program, CPU frequency, video timing, interrupt timing, counters, hardware design, and other circuitry can all participate.

This is why substituting an arbitrary crystal or changing clock frequencies is not a proper method of repairing a game.

A replacement timing component should match the requirements of the specific PCB and circuit.

Does Video Hardware Use Clock Signals?

Yes.

Classic arcade video systems require precise timing to determine when information is displayed across the screen.

Clock-derived circuits can help generate:

• pixel timing;
• horizontal timing;
• vertical timing;
• counters;
• memory addressing;
• sync timing; and
• other video-related signals.

This means a timing fault can sometimes produce a video symptom even though the monitor itself is functioning correctly.

See Arcade Video Signals Explained: RGB, Sync, and How the PCB Connects to the Monitor .

Can Clock Problems Cause Garbled Graphics?

Potentially.

Graphics generation depends on coordinated timing, memory access, counters, logic, and output circuitry.

A timing problem can therefore affect the image, although garbled graphics can also result from ROM, RAM, logic, power, connectors, or other video circuitry.

Read Why Are My Arcade Game Graphics Garbled, Missing, or the Wrong Colors? .

Can Sound Hardware Use Clock Signals?

Yes.

Depending on the game, timing sources may be used by:

• sound CPUs;
• sound-generation ICs;
• counters;
• tone generators;
• programmable sound hardware; or
• other audio circuitry.

Some games use clock frequencies that are separate from the main CPU clock, while others derive several signals from shared sources.

For the complete audio path, see How Does Sound Work in a Classic Arcade Game? PCB, Amplifier, and Speaker Explained .

What Happens If the CPU Clock Is Missing?

If a required processor clock is absent, the CPU cannot execute normally.

Possible symptoms can include:

• no boot;
• a static screen;
• watchdog resets;
• repeated startup attempts;
• missing program activity; or
• other game-specific behavior.

Those symptoms do not automatically prove the crystal or oscillator is defective because a missing clock can also result from circuitry between the timing source and the processor.

Can an Unstable Clock Make a Game Freeze?

Yes.

If the timing required by the processor or supporting circuitry becomes unreliable, program execution may become unstable.

A board may:

• freeze;
• reset;
• behave unpredictably;
• produce video errors; or
• fail completely.

For freeze-related troubleshooting, see Why Does a Classic Arcade Game Freeze or Lock Up?

How Does the Clock Relate to a Watchdog Circuit?

A watchdog system is designed to detect when normal program execution is no longer occurring as expected.

If the CPU stops operating because a required clock disappears, the program may stop servicing the watchdog and the board may be reset.

The watchdog is therefore often reacting to the underlying fault rather than creating it.

Read Why Does a Classic Arcade Game Keep Resetting? .

How Is a Clock Circuit Shown on an Arcade Schematic?

The schematic may show:

• the crystal or oscillator;
• oscillator circuitry;
• logic gates or inverters;
• counter ICs;
• divider stages;
• signal names;
• CPU clock pins;
• video timing signals; and
• connections to other sheets.

Signal labels may allow one clock line to continue across several pages without drawing a physical wire across the entire document.

See What Is an Arcade PCB Schematic and How Do You Read One? .

Why Are TTL Logic Chips Often Part of the Clock Circuit?

Classic arcade PCBs frequently use general-purpose digital logic to shape, divide, buffer, count, and distribute timing signals.

Depending on the design, you may find:

• inverters;
• flip-flops;
• binary counters;
• buffers; or
• other 74-series logic devices

between the original oscillator and the circuits that use the resulting clock.

The companion article What Are TTL Logic Chips on a Classic Arcade PCB? explains those devices in more detail.

Can You Identify a Clock Problem Just by Looking at the Board?

Usually not.

A cracked component, damaged trace, obviously broken crystal, corrosion, or poor previous repair may be visible, but many clock faults leave no obvious visual evidence.

Electrical diagnosis normally requires understanding the schematic and checking whether the expected timing signals are present.

What Tools Are Used to Check Clock Signals?

Technicians commonly use tools such as:

• an oscilloscope;
• a logic probe;
• a frequency counter; or
• other appropriate electronic test equipment.

The tool must be appropriate for the frequency and signal being measured.

Incorrect probing can short adjacent IC pins or damage vintage hardware, so powered measurements should only be performed by someone familiar with electronic test procedures and the equipment being used.

Should You Replace the Crystal First?

No.

Replacing a crystal solely because the board does not run is poor troubleshooting.

The clock path may include several stages, and the problem may be somewhere after the original timing source.

A controlled diagnosis follows the clock through the circuit instead of assuming the first timing component is defective.

Does PCB Revision Matter?

Yes.

Different hardware revisions can contain different component locations, part numbers, jumper arrangements, or timing circuitry.

Confirm that the schematic and repair information apply to the actual board being inspected.

Read What Is an Arcade PCB Revision and Why Does It Matter? .

Clock Circuits and Arcade Upgrade Kits

A HighScoreSaves daughterboard operates as part of the supported original arcade hardware system. It does not make the original PCB's required timing circuitry irrelevant.

If an original board already has unstable power, clock, memory, sockets, or logic, installing an upgrade does not repair those unrelated underlying faults.

Examples of game-specific upgrades include:

• Pac-Man Free Play and High Score Save Kit
• Donkey Kong Free Play and High Score Save Kit
• Galaga Free Play and High Score Save Kit

Always verify the original PCB and follow the game-specific installation documentation.

Key Takeaway

The clock circuit is the timing foundation of a classic arcade PCB.

A crystal, oscillator, or other timing source can generate a master clock that is divided, buffered, and distributed to the CPU, video circuitry, sound hardware, counters, and other digital circuits.

A missing or unstable required clock can prevent a game from booting correctly, cause freezes or resets, or affect other timing-dependent functions.

The correct troubleshooting approach is to identify the actual PCB and revision, use the correct schematic, understand the clock path, and verify the signal through the circuit rather than automatically replacing the crystal.

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