Commodore PET 4000 Series Maintenance Guide

Proper maintenance of the Commodore PET 4000 Series is essential to preserve functionality and prevent age-related failures. This guide covers cleaning, keyboard servicing, capacitor replacement, power supply care, and CRT-specific issues unique to these systems.

Regular Cleaning

The PET 4000 Series features a hinged metal chassis for easy internal access. Dust buildup and oxidized contacts are common in these systems.

Internal Cleaning

Disconnect the PET from mains power.
Carefully lift the PET's top panel to access the motherboard and CRT.
Use compressed air and an anti-static brush to clean:
- The motherboard (especially under ROM/RAM sockets)
- Power supply area
- CRT yoke and anode cup region
- Cassette deck area (if present)
Use isopropyl alcohol (>90%) and cotton swabs to clean stubborn areas, but avoid soaking.

External Case and CRT

Wipe the painted metal case with a mild soap solution on a damp microfiber cloth.
CRT screen can be cleaned using a lint-free cloth and glass cleaner sprayed onto the cloth.
Avoid aggressive rubbing of the screen label ("Commodore") which may degrade over time.

Power Supply Maintenance

The PET 4000 Series uses an unregulated linear power supply. While these PSUs are reliable, they require inspection:

Inspection Checklist

Check transformer and rectifier board for heat stress and corrosion.
Examine all RIFA X2 safety capacitors — these are known to fail with age. Replace immediately if original.
Check solder joints around bridge rectifier and voltage regulators (7812, 7805, etc).
Confirm voltages at the edge connector:
- +5V, -5V, and +12V should be within 5% tolerance.

Capacitor Replacement (Recapping)

Like many vintage electronics, the PET contains aging electrolytic capacitors.

Symptoms of Failing Capacitors

Vertical hold issues on CRT
Unstable startup or flickering screen
Noise in cassette audio playback/record
Random freezes or garbled text

Recommended Areas to Recap

Mainboard decoupling capacitors (especially around RAM chips)
CRT analog section (for power filtering)
Power supply (main filter caps, axial types)

Refer to the Commodore PET 4000 Series Capacitor Replacement Guide for a full list.

Keyboard Maintenance

The PET 4000 Series typically features a full-travel keyboard, which may suffer from key bounce, non-responsive keys, and deterioration.

Cleaning Procedure

Remove keyboard from the chassis.
Disassemble keycaps and springs (carefully photograph layout).
Clean contact pads on the PCB using isopropyl alcohol.
Polish carbon contact domes if applicable.
Inspect for lifted traces or oxidized vias.

Repair Tips

Use conductive paint to restore worn-out pads.
Replace deteriorated foam/copper domes with modern equivalents (keyboard repair kits exist).
For permanently dead rows/columns, trace the keyboard matrix and check solder joints or connector oxidation.

CRT Maintenance

The CRT in the PET 4000 Series may experience issues over time.

Common Issues

Cold solder joints on CRT neck board or deflection board.
CRT brightness/focus may drift — use internal trimpots for focus and brightness (accessible on deflection board).
If the CRT fails to light up:
- Check filament voltage
- Verify flyback transformer and HV section

Safety Warning: CRT displays contain high voltage and can retain a charge even when powered off. Proper discharge procedures must be followed before servicing.

Diagnostics and Burn-In

Diagnostic Tools

Use a PET tester ROM (e.g., dead test cartridge or test harness) for RAM and ROM diagnostics.
Use a logic probe or oscilloscope to check clock (phi2) and reset behavior.

Burn-In Tips

Leave PET running on test pattern for several hours to identify marginal components.
Monitor voltage regulators for excessive heat.

Voltage Test Points

Use a multimeter to check:

+5V at 6502 Vcc (Pin 8)
+12V on cassette motor supply
CRT filament voltage (typically ~6.3VAC)
Mains side capacitors should not show leakage or bulging.

Tools You’ll Need

ESD-safe screwdriver set
Chip puller or spudger
Isopropyl alcohol (99%)
Contact cleaner (e.g., DeoxIT)
Soldering iron with fine tip
Capacitor tester (optional)
Logic probe or oscilloscope (advanced)

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