Commodore PET 2001 Troubleshooting

This guide provides a comprehensive troubleshooting reference for the Commodore PET 2001 personal computer. It focuses on common faults specific to the PET 2001 (original models with MOS 6502 CPU, 6540 ROMs, 6550 SRAM, etc.) and offers diagnostic steps and solutions. Use this guide alongside general maintenance practices (see Commodore PET 2001 General Maintenance) and refer to the PET schematics (see ) as needed.

The PET 2001 was one of the earliest all-in-one personal computers. Due to its age and early semiconductor technology, many faults today relate to failing memory chips, ROMs, or power issues. When diagnosing problems, it is important to proceed step-by-step, starting from the most basic elements (power, reset, clock) before advancing to CPU logic, video output, I/O subsystems, and peripherals.

⚡ Power Supply and Startup

A completely dead PET often stems from power-related issues. Begin by checking the fuse and power switch. The PET's main fuse is located near the power input and should be tested for continuity. A blown fuse may indicate a shorted capacitor or rectifier.

If the fuse is intact, proceed to verify AC voltage output from the transformer. The PET transformer has several windings that supply different sections of the machine. Measure these at the output terminals. If no voltage is present, the transformer or switch may be faulty.

After confirming AC input, check the bridge rectifiers and filter capacitors. These generate the PET’s +5V logic rail (and other voltages in some versions). Use a multimeter to confirm presence of +5V on the main filter capacitor. Excess ripple or low voltage may indicate failing electrolytic capacitors.

The PET 2001 uses linear voltage regulators, such as 7805 for +5V. Measure their input and output voltages. A working regulator will have a few volts higher on the input than the output. If not, replace the regulator. Always verify voltages at the logic board input before proceeding with deeper diagnostics.

🔄 Reset and Clock Signals

When power is stable, the next step is ensuring that the CPU is properly reset and clocked. The 6502 CPU must receive a brief low pulse on the RES* line at power-on, after which the line should go high. Use a logic probe or oscilloscope to monitor this behavior.

Equally critical is the system clock. The PET operates around 1 MHz. The clock signal should be present on the 6502's PHI2 pin. A missing clock or one stuck high/low will prevent code execution.

If both clock and reset lines behave correctly, and the CPU is still unresponsive, move on to probing logic activity.

🖥️ Video Symptoms and Display Analysis

The PET has an integrated CRT display. Many faults can be interpreted visually based on screen behavior.

A completely black screen (no raster) indicates a CRT or analog power issue, rather than a logic problem. Check for neck glow and listen for high-voltage whine. If present, the CRT circuits are likely functioning.

A raster with no text suggests a problem in video RAM or character generation. The PET’s screen will show random characters if the CPU is not executing properly, since video RAM will not be cleared.

If the screen displays consistent patterns (e.g., vertical stripes, checkerboards), this may point to stuck address lines, faulty video RAM chips (usually 2114s), or bad data buffers.

In contrast, missing or garbled specific characters often trace to the character generator ROM, which contains the glyph data. Swapping or verifying this ROM is a common step when isolated character corruption occurs.

🧠 CPU, ROM and RAM Troubleshooting

The MOS 6502 is a robust CPU, but it must receive valid reset and clock signals, and must be able to access ROM and RAM.

Check for address and data bus activity using a logic probe. Pulsing address lines suggest the CPU is running. If stuck, the CPU may be halted or held in reset.

System ROMs in early PETs are 6540 chips, which are prone to failure. Replace suspect ROMs with tested ones or modern adapters that use 2716 or 2732 EPROMs.

RAM faults are very common, especially in systems using 6550 SRAM chips. One bad chip can cause a crash. Use a diagnostic ROM such as PETTEST to help locate the faulty RAM. Piggybacking a known-good chip or using a logic probe on chip enable lines can also isolate the failure.

Address decoders such as the 74154 or 74LS138 can also fail, resulting in inaccessible memory regions. If parts of memory are non-functional, examine these chips.

🧪 Using Diagnostic ROMs

Installing a known diagnostic ROM, such as PETTESTER or Tynemouth Software’s diagnostics, can be invaluable. These ROMs replace the EDIT ROM or are run via a plug-in board and generate visual patterns or codes that reveal system status.

For example, if PETTEST fills the screen with a pattern and it remains unchanged, that implies the CPU is running but unable to write to RAM. If the pattern changes or an error is displayed, follow the diagnostic guide’s meaning for that result.

🔍 Logic Probing and Signal Analysis

A logic probe or oscilloscope helps immensely in PET repair. Monitor chip select (CS), read/write (R/W), and data/address activity.

Focus on the following ICs:

6502 CPU – SYNC line indicates instruction fetch. If absent, the CPU is not running.

ROMs and RAMs – CS lines should pulse as CPU accesses memory.

74154 – controls chip select lines. If one output is stuck, corresponding RAM or ROM won’t be accessed.

74LS244 and 74LS245 buffers – check for stuck outputs.

PIAs (6520) and VIAs (6522) – interface with I/O. If missing or shorted, may lock up the system.

Compare signal activity between similar chips to locate anomalies.

💥 Common Failures and Replacements

6550 SRAMs – often fail due to heat or age. Replace with 2114s or adapter boards.

6540 ROMs – unreliable and rare. Use modern EPROM replacements.

74154 – address decoding failures cause boot faults. Replace if memory mapping fails.

6520 PIA / 6522 VIA – failure affects keyboard, IEEE-488, tape. Swap with known good.

74-series TTL chips – tri-state buffers and counters frequently fail. Identify via probing and replace.

Character ROM – if screen is mostly normal but letters are incorrect, replace with a good 2332.

📼 Cassette and Sound Troubles

The PET’s built-in datasette often suffers from mechanical wear. If LOAD fails:

Check motor control: the PET switches +6V to the motor via a transistor. If no movement, inspect this transistor or the PIA port that controls it.

The sense switch inside the datasette must close when PLAY is pressed. If it doesn’t, the PET won’t respond. Clean contacts or test continuity.

Read/write issues may be head-related: clean the read head and inspect alignment. Replace belts or motors if required. If writing fails, check the write amplifier circuit.

For sound, the PET uses a piezo buzzer triggered via a PIA line. Try PRINT CHR$(7) in BASIC to trigger the bell. If no sound, trace the signal from PIA output to the buzzer.

⌨️ Keyboard and I/O Issues

Keyboards may fail due to corroded contacts, bad connections, or PIA faults. If certain keys or rows/columns are dead, inspect the matrix wiring and test lines on the mainboard.

The PIA or decoder chip (e.g., 74145) may be at fault if no keys respond or if phantom keypresses occur.

IEEE-488 issues often trace to the 6520 or 6522 I/O chips or the 75160/75161 transceivers. If the PET cannot detect drives, monitor activity on the bus lines. Swapping out the transceiver chips often restores function.

📎 Final Advice

Troubleshooting the PET 2001 requires patience, methodical observation, and basic tools like a multimeter, logic probe, and spare ICs. Reseating chips, cleaning sockets, and inspecting for physical damage solves many issues.

When in doubt, consult the PET 2001 Schematics Overview and PET 2001 General Maintenance pages, and consider using modern test tools like diagnostic ROMs and RAM/ROM replacement boards.