Commodore PET 2001 General Maintenance

Commodore PET 2001 with the top lifted open.

Routine care of the Commodore PET 2001 covers cleaning, the keyboard, the chip sockets, supply checks and the CRT hazards. For fault-finding see Commodore PET 2001 Troubleshooting; for component values see Commodore PET 2001 Capacitor Replacement Guide. Commodore's parts lists and schematics for the machine are hosted at Commodore PET 2001 Service Documentation.

Before you start

The mains transformer is mounted in the case, with the power switch and a 1.8 A fuse on its primary. A 16.2 V AC winding feeds the display assembly and a 15.7 V AC centre-tapped winding feeds the main logic board, with a 23,000 µF reservoir capacitor wired in the case.

• Mains voltage is present at the switch, the fuse and the transformer primary whenever the machine is plugged in. Unplug it at the wall.
• The CRT anode runs at about +10 kV from flyback T2, and the tube holds that charge after power-off. Discharge it before working near it; see CRT Discharge Procedure.
• The display assembly also carries a +85 V boost rail and a −30 V rail.
• C1 on the display assembly (4700 µF) and the 23,000 µF capacitor in the case store real energy. Bleed them through a resistor.

Which board do you have

Commodore built four variants of the original static board. Its note on the schematic records that all component locations are the same across the four and only the fitted devices differ; the board number is printed in the upper left corner.

Original PET 2001 static board assemblies

Assembly	ROMs	RAM
320008	6540	6550
320081	2316B	6550
320132	6540	2114
320137	2316B	2114

The later 2001-N, 3000 and 4000 board is a different, dynamic design; see Commodore PET 4000 Series Maintenance Guide.

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 CRT.
• Use compressed air and an anti-static brush on the logic board, particularly around the ROM and RAM sockets, and on the display assembly and the cassette deck.
• 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, for the screen.

Power supply checks

What generates what

Assembly	Produces	Regulators
Main logic board	+9 V unregulated from CR1 and CR2; four +5 V supplies	VR3, VR4, VR5, VR6
Display assembly	+12 V, +85 V boost, −30 V, about +10 kV anode	VR1, a 7812 (Commodore 901527-01)

Commodore's parts cross-reference for the logic board omits VR3 to VR6; they appear on sheet 2 of the schematic. Each feeds its own part of the board, so a single failed regulator can leave one section dead while the rest works.

What to inspect

• The four regulators and their heatsinks on the logic board, and CR1 and CR2, for heat stress, discoloured board and cracked joints.
• The joints at the display assembly's bridge rectifier CR19-CR22 and at VR1.
• The reservoir capacitor in the case and its wiring.

Neither Commodore parts list names a mains suppression (X or Y class) capacitor. The highest-rated capacitors in the display assembly's list are the 500 V ceramic discs at C18, C20 and C30.

Voltage test points

Where to measure

Rail	Where	Note
+5 V	TP1-1 to TP1-4, the four regulator outputs	Measure all four
+5 V	6502 (UF3) pin 8	Pins 1 and 21 are ground; pin 40 is RESET
+9 V unregulated	Regulator inputs; the cassette-motor circuit	Feeds VR3-VR6 and the motor drivers
+12 V	VR1 output on the display assembly	The display board's only regulated rail
+85 V, −30 V	Display assembly	Present whenever the machine is running

Capacitors

The main logic board has four aluminium electrolytics, C7 to C10 (47 µF 16 V), one on each regulator output. The rest of the C1-C49 range on that board is ceramics, apart from C5 and C11-C13, which are 1 µF 25 V tantalums; replace those with tantalums. The bulk capacitors are the 23,000 µF reservoir in the case and C1 (4700 µF 25 V) on the display assembly.

Full lists with Commodore part numbers are on Commodore PET 2001 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

Keyboard

The 2001-4 and 2001-8 have a calculator-style keyboard with small keys.

• 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 it does not last as well as the original plating.

Sockets

The 6502, the two PIAs, the VIA, the character generator, the seven ROM positions and all eighteen RAM positions are socketed. PETs are known for socketed chips working out of their sockets; reseat each chip in its original position and orientation. The original 6540 ROMs and 6550 RAMs are unreliable parts in their own right.

• Remove chips with a proper puller, clean the pins and the socket, and reseat firmly.
• Replace a failed socket with a turned-pin type.

Cassette deck

• Check the drive belt for perishing or slack.
• Clean the head, pinch roller and capstan.
• Check head azimuth if tapes from other machines will not load.

Diagnostics

• A PET test ROM exercises RAM and ROM faster than swapping chips. Tynemouth Software runs one from a ROM/RAM replacement board, which also takes load off the original 6540 and 6550 parts.
• Check 5 V at the user port and the Φ2 clock first, then the CPU's interrupt and reset lines.
• Run the machine for a few hours to show up marginal parts that pass a cold test, and watch the regulator heatsinks while it runs.

Tools

• ESD-safe driver set, chip puller, plastic spudger
• Digital multimeter; a 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 oscilloscope

See Recommended Tools.

Related pages

• Commodore PET 2001
• Commodore PET 2001 Troubleshooting
• Commodore PET 2001 Capacitor Replacement Guide
• Commodore PET 2001 Service Documentation
• Commodore PET 4000 Series Maintenance Guide
• CRT Discharge Procedure
• Capacitor Failure Symptoms