Commodore PET 4000 Series Maintenance Guide

Commodore PET 4032 with the top panel lifted.

Routine care of the Commodore PET 4000 Series covers cleaning, supply checks, the keyboard, the sockets and the CRT hazards. For fault-finding see Commodore PET 4000 Series Troubleshooting Guide.

The electrical figures come from Commodore's PET 4000 Series Technical Manual, P/N 990438, hosted here.

The 4000 series uses the universal dynamic PET board, not the static board of the Commodore PET 2001. The rails, regulators and crystal differ, so figures do not carry between the two.

Before you start

• Mains voltage is present at the switch, fuse and primary of T1 whenever the machine is plugged in.
• C1 in the supply chassis is 23,000 µF. Bleed it through a resistor before working near it.
• The CRT holds its anode charge after power-off. Discharge it before working near the tube; see CRT Discharge Procedure.

Regular cleaning

Internal

• Unplug the machine and discharge the CRT before lifting the top.
• The top lifts on its hinge to expose the logic board and the display.
• Use compressed air and an anti-static brush on the logic board, particularly around the ROM and RAM sockets and the regulator heatsinks, and around the CRT neck board.
• Use isopropyl alcohol (over 90 %) on swabs for stubborn areas. Do not soak the board.

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.

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	+5 V	901528-01
VR2	7812, TO-3	+12 V, 1 A	901528-04
VR1	7905, TO-220	−5 V, 1.5 A	901527-03

The +5 V regulator is an LM323, not a 7805; replace it with an LM323. The −5 V rail from VR1 does not exist on the PET 2001's static board.

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.
• The supply-chassis drawing lists one capacitor, C1, the 23,000 µF filter on the low-voltage side, and no mains suppression capacitor.

Voltage test points

Where to measure

Rail	Where	Note
+5 V	VR4 output, or 6502 (UB14) 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
Unregulated	Across C1 in the supply chassis	23,000 µF. Treat with respect

Commodore's drawings give no tolerance for these rails; work to the regulators' data-sheet figures. A rail well off nominal points at the regulator, its heatsinking or the filtering ahead of it.

Capacitors

Commodore's list for the logic board gives these electrolytics and tantalums.

Universal dynamic PET board, electrolytics and tantalums

Ref	Value	Type	Commodore part no.
C1, C16, C17, C23, C24	10 µF 25 V	Low-leakage electrolytic	900109-03
C2	470 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	900402-13

Commodore lists C1, C16, C17, C23 and C24 as low-leakage parts; fit low-leakage electrolytics there, and tantalums at C6 and C51. The rest of the C1-C97 range on this board is ceramics and mica. The bulk filter, C1 in the supply chassis, is a separate 23,000 µF can.

See Capacitor Failure Symptoms.

Keyboard

The 4000-series machines use a full-travel keyboard.

• 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 points at the connector or a broken trace, not a single key.
• Conductive paint will restore a worn pad but does not last as well as the original plating.

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

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 has PCB waveform drawings for both displays, 321450 for the 12-inch and 321447 for the 9-inch, with the oscilloscope settings printed on each.

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 shows whether the CPU is running. On this board the CPU must program the CRT controller before any picture appears, so a blank screen alone does not mean a dead video circuit. 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