Commodore PET 4000 Series Troubleshooting Guide

Commodore PET 4000 Series. Source: Wikimedia Commons.

This guide provides a troubleshooting reference for the Commodore PET 4000 Series (models 4016, 4032, etc.). It covers known fault modes specific to the "Fat 40" models with 6545 CRTC, 40-column video, and BASIC 4.0 ROMs.

For cleaning and chip reseating techniques, see Commodore PET 4000 Series Maintenance Guide. For recapping procedures, refer to the Commodore PET 4000 Series Capacitor Guide.

Power Supply and Basic Signals

Start by checking the power supply. Ensure the +5V and +12V rails are present, along with the unregulated ~9VAC for cassette power. If the PET is completely dead — no chirp, no screen — suspect blown fuses or failed regulators.

Examine the voltage regulators, especially the two main ones on the board, which often serve different sections like DRAM. Look for charred or cracked solder joints and visually inspect capacitors and resistors, particularly around the CRT section. A frequent fault involves R752/R753 (56Ω) resistors, which may burn out due to aging capacitors (C752/C753).

With power confirmed, verify logic activity. Use a logic probe or oscilloscope to ensure the 6502’s RESET line goes high after power-on and that its PHI2 clock (pin 37) pulses cleanly at 1 MHz. A silent startup with no chirp or screen activity often points to clock, reset, or CPU failure.

CRT and Display Behavior

If the CRT appears lifeless, but the neck filament glows, attention turns to the CRT board. Check the 56Ω resistors and their companion capacitors. Replace any open components.

A single white dot on screen means no horizontal or vertical deflection — a common sign of deflection failure. A horizontal line suggests vertical collapse, while rolling or unstable images point to failed electrolytic caps. In either case, a full recap is advised.

For strange video inversion or flipped images, confirm the VIDEO, H SYNC, and V SYNC jumpers are correctly set on the PET mainboard. Mismatched settings will invert video or scramble display geometry.

6545 CRTC and Video RAM

A blank raster but no characters implies a fault in the video generation logic. Check the 6545 CRTC for clock input and sync output. No HSYNC/VSYNC pulses? Swap in a known-good 6545 or compatible chip like the HD46505.

Garbled symbols or flickering text point to faulty 2114 video RAM. Repeated incorrect characters usually indicate a stuck bit. Probe RAM data and address lines. Piggyback a good 2114 if needed. Also reseat the character ROM — it's often forgotten but crucial.

CPU, ROM, and Main Memory

A blank screen with no cursor or chirp typically signals failure in ROM or DRAM. The PET 4000 uses 4116 DRAMs and several ROMs. ROMs may become corrupt with age — try swapping in EPROM replacements or using a known good ROM set.

Use a logic probe to check 6502 address/data activity. If stuck, suspect clock/reset issues or a dead CPU. If active but no boot, examine chip select lines — a failed PLA or decoder (74LS138/139) may be at fault.

4116 DRAM needs +5V, +12V, and -5V. A missing supply line can make all RAM look bad. You can sometimes remove half of the RAM bank (if socketed) to test minimal boot configurations.

Keyboard and I/O

If the PET displays "KEYBOARD JAM" or ignores keystrokes, start by reseating the keyboard connector. The matrix is scanned by two 6520 PIAs — try swapping them. Often, one port fails while the other is still usable.

If certain keys fail in patterns (entire rows or columns), suspect cracked traces or corroded key switches. Clean with contact cleaner and verify continuity with a multimeter.

IEEE-488 Interface

If disk drives or printers don’t respond, test the IEEE port. The PET uses bus transceivers (MC3446 or SN75160/161) and I/O chips (6520 or 6522). Watch for signal activity during LOAD or SAVE — if no ATN line toggling, suspect a bad PIA or transceiver.

Replace any bus driver ICs that show no activity or feel suspiciously hot. Use BASIC commands or a simple program to send bus signals and watch reactions.

Cassette Interface

For cassette faults, inspect the datasette connector and control transistor. If the motor doesn’t spin after pressing PLAY, check for 6–9V at the motor pin and ensure the PIA toggles it during LOAD.

Clean the read head, verify the amplifier circuit, and test write output during SAVE. Also check the sense switch — a failed switch will prevent LOAD from initiating.

Sound Issues

No chirp at power-up? Confirm the piezo buzzer is present and connected. It’s driven via the 6522 VIA’s CB2 line. If CB2 is inactive, replace the VIA.

A stuck tone or distorted beep might mean the CB2 line is held high — another sign of a failing VIA.

Diagnostics and Test Tools

Use a logic probe to examine address/data bus activity. No movement = halted CPU. Tools like PETTEST2 (which initializes the 6545) can be invaluable for diagnosing RAM and ROM faults.

Boards like PETVet replace ROM/RAM and help isolate multiple faults. With one plug-in, they can eliminate several problem areas at once.

Garbage screen (the classic PET fault)

The PET's video runs independently of the CPU and continuously displays the first ~1000 bytes of screen RAM. At power-on that RAM is random, so a healthy PET briefly shows garbage and then clears the screen once the CPU runs its start-up code. A screen that stays full of static garbage means the CPU is not running — almost always a ROM or RAM fault. Work the ROMs and RAM first:

• Bad RAM is common, but the 74LS244 bus-driver chips fail at least as often — check or replace them first.
• Reseat every socketed chip. PET sockets are cheap and develop poor contacts, which on its own causes garbage-screen and dead faults.
• Chip types: early 2001 boards use MPS6540 ROMs and MPS6550 RAM; later boards use 2316B ROMs and 2114 RAM.

Power supply and video wobble

The PET uses a simple, reliable linear supply: a transformer and filter capacitor feeding four 78L05 +5 V regulators on the main board. A wobbly, warped or jumping display is classically caused by an oxidised or cracked Molex power connector making the board voltage jump between about 4 and 5 V several times a second — reseat, clean or replace the connector. Screen "noise" on the 2001 is screen-RAM read/write contention.

Video timing

A display where each character line advances the screen address by only 36 instead of 40 (overlapping or wrongly-wrapped characters) points to a weak character clock in the video circuit rather than to RAM.

CRTC (4000/8000 series)

The 4000/8000-series PETs generate the display with a 6545 CRT controller rather than the 2001's discrete video logic. A rolling, mis-sized or blank raster with the rest of the machine alive points to the 6545, its clock, or the monitor-adjustment section; reseat the 6545 and check its clock before suspecting the CRT.