Commodore PET 4000 Series Maintenance Guide

Commodore PET 4032 with the top panel lifted.

This page covers routine care of the Commodore PET 4000 Series: cleaning, supply checks, keyboard servicing, socket work and the CRT hazards. For fault-finding see Commodore PET 4000 Series Troubleshooting Guide.

Every electrical figure here is read off Commodore's own drawings in the PET 4000 Series Technical Manual, P/N 990438, hosted on this wiki. Where Commodore gives no figure, none is given here.

This is the "universal dynamic PET" board, not the static board of the Commodore PET 2001. The rails differ, the regulators differ and the crystal differs. Do not carry figures between the two pages in either direction.

Before you start

• Mains voltage is on the primary side of T1 whenever the machine is plugged in.
• C1 in the supply chassis is 23,000 µF. That is a large amount of stored energy for a low-voltage rail. Bleed it through a resistor; do not short it with a screwdriver.
• The CRT holds its anode charge long after power-off. Discharge it before working near the tube — see CRT Discharge Procedure.

Regular cleaning

Internal

• Disconnect from the mains and discharge the CRT before the top goes up.
• The hinged chassis lifts to expose the logic board and the display.
• Compressed air and an anti-static brush on the logic board, particularly under and around the ROM and RAM sockets, on the regulator heatsinks and around the CRT neck board.
• Isopropyl alcohol (>90 %) on swabs for stubborn areas. Do not soak.

External

• Mild soap on a damp microfibre cloth for the painted steel case.
• Glass cleaner sprayed onto a lint-free cloth, never onto the tube.
• Do not rub the screen-surround legend; the print is fragile.

Power supply

What is where

Assembly	Contents	Regulators
Supply chassis (drawing 8032051)	Transformer T1, 23,000 µF filter C1, fuse F1 (1.6 A slow-blow, 125 V), rocker switch SW1	None
Logic board (drawing 8032090)	Bridges CR2/CR4, diodes CR1/CR5/CR7, zeners VR5/VR6	VR1, VR2, VR4

Regulators and rails

Ref	Device	Rail	Commodore part no.
VR4	LM323, TO-3	+5 V	901528-01
VR2	7812	+12 V, 1 A	901528-04
VR1	7905	−5 V, 1.5 A	901527-03

The +5 V regulator is an LM323, not a 7805. It is a TO-3 5 A part, the same one Commodore used on the early Commodore VIC-20 board, and a 1 A TO-220 device is not a substitute for it.

There is a −5 V rail on this machine, from VR1. The PET 2001 has none, so rail lists do not transfer between the two.

What to inspect

• Transformer and the rectifier area for heat stress, discoloured board and corrosion.
• Solder joints at VR1, VR2 and VR4 and at bridges CR2 and CR4. These are thermally cycled, heavy-current joints and a cracked one produces an intermittent that mimics a failing chip.
• The regulator heatsinks and the thermal compound under them. Commodore fits heatsinks and thermal compound to this board because these parts run hot.
• Commodore's supply-chassis drawing names no mains-rated X-class or Y-class suppression capacitor — the chassis holds one capacitor, C1, and it is the 23,000 µF filter on the low-voltage side. The "replace the RIFAs on sight" advice that applies to many European machines of this era is not supported by Commodore's documentation for this one. If your machine has such a part fitted, deal with it on its merits; do not go hunting for one on the strength of a generic warning.

Voltage test points

Where to measure

Rail	Where	Note
+5 V	VR4 output, or 6502 pin 8	Pins 1 and 21 are ground; pin 40 is RESET, not a supply pin
+12 V	VR2 output	1 A device
−5 V	VR1 output	1.5 A device. Dynamic RAM of this generation needs it
Unregulated	Across C1 in the supply chassis	23,000 µF. Treat with respect

A rail more than a few per cent off its nominal means the regulator, its heatsinking or the filtering ahead of it. Commodore publishes no tolerance band of its own for these rails, so none is quoted here; work to the regulators' own datasheet figures.

Capacitors

Commodore's list for the logic board. This is the whole electrolytic complement.

Universal dynamic PET board — electrolytics and tantalums

Ref	Value	Type	Commodore part no.
C1, C16, C17, C23, C24	10 µF 25 V	Electrolytic, low-leakage	900109-03
C2	4700 µF 16 V	Electrolytic	900100-44
C5	4700 µF 25 V	Electrolytic	900100-45
C7, C11	47 µF 16 V	Electrolytic	900101-37
C6, C51	1 µF 35 V	Tantalum — fit a tantalum	900402-13

Three things worth noting before ordering:

• C1, C16, C17, C23 and C24 are specified low-leakage parts, not ordinary electrolytics.
• C2 and C5 are the same capacitance at different voltage ratings and are not interchangeable in the safe direction. C5 is the 25 V part.
• Everything else in the C1–C97 range on this board is a ceramic, a mica or a tantalum, and ceramics do not age the way aluminium electrolytics do.

The bulk filter, C1 in the supply chassis, is a separate 23,000 µF can and is the part under the most ripple stress in the machine.

Symptoms that justify pulling the boards:

• Vertical hold or geometry drifting on the CRT
• Unstable start-up, flickering or rolling picture
• Random freezes or garbled text
• Any brown crust or verdigris around a capacitor bung

See Capacitor Failure Symptoms.

Keyboard

The 4000-series machines use a full-travel keyboard. Bounce, dead keys and whole dead rows are the usual complaints.

• Remove the keyboard from the chassis and photograph the keycap layout before taking anything apart.
• Clean the contact pads on the PCB with isopropyl alcohol.
• Inspect for lifted traces and corroded vias, and check the connector at both ends — a whole dead row or column is far more often a connector or a broken trace than it is a bad key.
• Conductive paint will restore a worn pad but does not last as well as the original plating; treat it as a last resort.

Sockets

Commodore's board drawing lists 24-pin and 40-pin low-profile IC sockets and a 14-pin socket, so most of the large devices are socketed. Sockets of this age lose contact pressure: remove suspect devices with a proper puller, clean the pins and the socket, and reseat firmly. Replace failed sockets with turned-pin types.

CRT

• Cold solder joints on the neck board and the deflection board are common.
• Brightness and focus drift; both have trimpots on the display board.
• If the tube does not light at all, the fault is in the heater supply or the EHT section, not the logic board.

The technical manual carries the full drawing set for both the 9-inch and the 12-inch display, starting at drawing 321450 for the 12-inch, including the PCB waveform photographs with the oscilloscope settings printed on the drawings.

The CRT retains a charge with the machine unplugged. Follow CRT Discharge Procedure before working near the tube.

Diagnostics

• A PET diagnostic ROM or test harness will exercise RAM and ROM far faster than swapping chips.
• A logic probe or scope on the ϕ2 clock and the reset line separates "the CPU is not running" from "the CPU is running and the video is wrong" in seconds. Y1 on this board is 16 MHz.
• Running the machine for a few hours on a test pattern will surface marginal parts that pass a cold test. Watch the three regulators while it does.

Tools

• ESD-safe driver set, chip puller, plastic spudger
• Digital multimeter, CRT discharge tool
• Isopropyl alcohol (99 %), contact cleaner, anti-static brush
• Fine-tip temperature-controlled soldering iron, braid or pump
• ESR meter, logic probe or scope

See Recommended Tools.

Related pages

• Commodore PET 4000 Series
• Commodore PET 4000 Series Troubleshooting Guide
• Commodore PET 4000 Series Technical Manual
• Commodore PET 2001 General Maintenance
• CRT Discharge Procedure
• Capacitor Failure Symptoms