RetroTechCollection

Commodore PET 2001 Troubleshooting

Fault-finding on the Commodore PET 2001 starts with which board it has: the original static board, with the 6502 at UF3, 6540 or 2316B ROMs and 6550 or 2114 RAM. For cleaning, socket work and the supply layout see Commodore PET 2001 General Maintenance; Commodore's parts lists and schematics are indexed at Commodore PET 2001 Service Documentation.

A PET 2001 showing random symbols. The video circuitry reads screen RAM independently of the processor, so this display means the video path works; if it never clears, the processor is not running.

Do not carry figures across from the Commodore PET 4000 Series. That is a dynamic board with a CRT controller, a different set of regulators and a −5 V rail for its DRAM.

Which board do you have

edit

Commodore built four variants of the static board with identical component locations and different fitted devices. The assembly number is printed in the upper left corner.[1]

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

Power supply and startup

edit

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 logic board through connector J8, with a 23,000 µF reservoir capacitor wired in the case.[2]

  • Mains voltage is present at the switch, fuse and transformer primary whenever the machine is plugged in.
  • The CRT anode runs at about +10 kV from flyback T2 and holds its charge after power-off; see CRT Discharge Procedure.
  • The display assembly also carries +85 V and −30 V rails.[3]
  • C1 on the display assembly (4700 µF 25 V) and the reservoir in the case store real energy. Bleed them through a resistor.
What generates what
Assembly Produces Regulators
Main logic board +9 V unregulated from rectifiers CR1 and CR2; +5 V for four sections of the board, at test points TP1-1 to TP1-4 VR3, VR4, VR5, VR6[1]
Display assembly +12 V, +85 V, −30 V, about +10 kV anode VR1, a 7812, Commodore 901527-01[4][3]

Commodore's parts cross-reference for the logic board does not list VR3 to VR6; they are on sheet 2 of the schematic.[5][1]

Dead machine, in order

edit
  1. Fuse continuity, then the power switch. A fuse that blows again points at a short on a secondary: a rectifier or a reservoir capacitor.
  2. AC at the two transformer secondaries.
  3. The +9 V unregulated rail at the regulator inputs, then +5 V at each of TP1-1 to TP1-4. One dead test point with the others good is that regulator or a short in its section.[1]
  4. +5 V at 6502 pin 8 (UF3). Pins 1 and 21 are ground; pin 40 is RESET, not a supply pin.[2]
  5. For a picture fault, the display assembly's rectifiers CR19-CR22, reservoir C1 and VR1's input and output.[3]

Reset and clock

edit

The crystal Y1 is 8 MHz.[1] The 6502 must see a brief low on RES (pin 40) at power-on, then a steady high. The processor clock runs at about 1 MHz: pin 37 is Φ0 in, pin 39 is Φ2 out and pin 3 is Φ1 out.[2]

A voltmeter is enough for a first pass. A running Φ2 reads about 2 to 3 V because the meter averages its high and low halves; a steady 5 V or 0 V means the clock is dead.[6]

If clock and reset are both correct and the machine is still unresponsive, move to bus activity.

The garbage screen

edit

On this machine the video circuitry reads screen RAM continuously and independently of the processor. At power-on that RAM holds random data, so a working PET 2001 briefly shows garbage and then clears the screen once the processor runs its start-up code.[7]

  • A screen full of static garbage that never clears means the processor is not running, usually because of a ROM or RAM fault or a chip out of its socket.[7][6]
  • The same screen shows that the video path from the screen RAM onwards works.[6]

Work in this order:

  1. Reseat every socketed chip. PETs are known for socketed chips working out of their sockets.[6]
  2. Suspect the original 6540 ROMs and 6550 RAMs, which are unreliable.[7]
  3. Then the address decoding and the bus buffers.

Processor, ROM and RAM

edit

Check for address and data bus activity with a logic probe. Pulsing address lines mean the processor is fetching; static lines mean it is halted or held in reset.

  • A 6550 with faulty enable lines can be enabled over more of the address space than it should be, corrupting memory, yet still work in the video RAM positions (UC3, UC4), where it is enabled all the time. Tynemouth Software restored a board to the full 8 KB by swapping such a chip into the video RAM position.[7]
  • Address decoding is done by a 74LS154 at UG2. A stuck output leaves the matching ROM or RAM region unselected. If part of memory is dead while the rest works, probe the decoder outputs before swapping memory.[5]
  • A ROM/RAM replacement board running a PET test ROM finds bad RAM faster than chip swapping, and takes the load off the original 6540 and 6550 parts.[7]

Holding the diagnostic sense line, user-port pin 5, low during reset sends the machine into the machine-language monitor instead of completing a normal reset. Commodore also made a diagnostic clip: a 40-pin clip over the processor with a ROM and a reset switch, which forced the address bus to $9xxx so that the machine started from the clip's ROM.[6]

Video faults

edit
  • Running the board with the video RAM removed gives a chequerboard, because the floating data lines read as $FF. A chequerboard with the RAM fitted points at the video RAM or its data path.[7]
  • Snow on the picture is the screen RAM being written while it is read, which the 2001 design allows. In Tynemouth's case it was constant because a 74LS157 multiplexer had its output stuck in read mode while writes were also enabled.[7]
  • Characters changing shape all the time point at the character generator (UA2). Constant vertical lines in the same position in every character row are a bad character generator or the shift register that takes its data.[6][5]
  • No raster, no neck glow and no EHT whine point at the display assembly, not the logic board.

Keyboard and I/O

edit

The 2001-4 and 2001-8 have a calculator-style keyboard.[8]

  • A whole dead row or column points at the connector, a cracked trace or a corroded via rather than a single switch. Clean the pads with isopropyl alcohol and check continuity.
  • No keys at all, or phantom keypresses, points at the keyboard PIA or the 74LS145 decoder at UG9.[5]
  • The 6520 PIAs are at UB8 and UG8 and the 6522 VIA at UA5. The two PIAs are the same part and can be swapped to confirm a fault.[5]

IEEE-488: watch the bus during a LOAD or SAVE. On the PET 2001 the bus transceivers are three MC3446s at UA7, UA8 and UA9.[5]

Cassette

edit

The two cassette-motor drivers run from the +9 V unregulated rail, each stabilised by a 7.2 V zener (VR1, VR2), with TIP29, 2N4401 and 2N3904 transistors.[5][1]

  • Motor does not run when PLAY is pressed: check the +9 V rail at the driver, then the driver transistors, then that the port line toggles during LOAD.
  • Check that the deck's sense switch closes when a key is pressed. PET-Shack's tape drawing shows it closed by any button except RECORD and STOP.[2]
  • Read failures: clean the head, check alignment and the belt, then the read amplifier. Write failures: check the write amplifier.

The PET 2001 has no sound hardware.[5]

Logic probing

edit

Compare activity between similar chips to find the odd one out:

  • 6502: SYNC for each instruction fetch, Φ2, RES.
  • ROMs and RAMs: chip-select lines should pulse as the processor accesses them.
  • 74LS154 at UG2: a stuck output leaves its ROM or RAM permanently unselected.
  • 74LS244 buffers (UB3, UB4, UG5, UG6, UH8, UH9): check for stuck outputs.[5]
  • 6520 PIAs and the 6522 VIA: a shorted one can hold the bus and stop the machine.
edit

References

edit
  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Commodore Business Machines, PET Computer Main Logic schematic, drawings 320130 and 320026, hosted at File:Commodore PET 2001 Main Logic Schematic (320008).pdf. Sheet 1: cassette-motor drivers on the +9 V unregulated rail with 7.2 V zeners VR1 and VR2. Sheet 2: J8, rectifiers CR1-CR3 and regulators VR3-VR6 with outputs at TP1-1 to TP1-4. Sheet 3: the 8 MHz crystal Y1 and the note that component locations are common to assemblies 320008, 320081, 320132 and 320137.
  2. ↑ 2.0 2.1 2.2 2.3 PET-Shack (P.S. Software House), PET 2001 schematic and data-sheet bundle, c. 1978, hosted at File:PET-Shack PET 2001 Bundle.pdf. Third-party. Console wiring drawing (power switch, 1.8 A fuse, 16.2 V AC and 15.7 V AC centre-tapped windings, 23,000 µF 15 V capacitor), the tape deck switch drawing, and manufacturers' data sheets including the 6502 pin assignments.
  3. ↑ 3.0 3.1 3.2 Commodore Business Machines, Schematic, Service, Display Circuit for the PET 2001, hosted at File:Commodore PET 2001 Display Circuit Schematic.pdf. CR19-CR22 rectifiers, reservoir C1, regulator VR1 producing +12 V, the +85 V boost and -30 V rails, flyback T2 and the +10 kV anode supply.
  4. ↑ Commodore Business Machines, Display Assembly Component Cross Reference for the PET 2001, hosted at File:Commodore PET 2001 Display Assembly Parts List.pdf. Includes C1 at 4700 µF 25 V and regulator VR1, Commodore part 901527-01.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Commodore Business Machines, 8K Main Logic Assembly Parts Cross Reference for the PET 2001, hosted at File:Commodore PET 2001 Main Logic Assembly Parts List.pdf. Components of the system board by reference designator and Commodore part number: 6502 at UF3, 6520 PIAs at UB8 and UG8, 6522 VIA at UA5, character generator at UA2, MC3446 at UA7-UA9, 74LS154 at UG2, 74LS145 at UG9, 74LS244 at UB3, UB4, UG5, UG6, UH8 and UH9; cassette-motor transistors TIP29, 2N4401 and 2N3904. It does not list the +5 V regulators.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 André Fachat, PET repairs—link(accessed 2026-10-01) Community repair reference. The voltmeter check on Phi2 (2-3 V average), socketed chips moving out of their sockets, non-CRTC PETs displaying screen memory without the CPU, flickering characters and vertical lines pointing at the character ROM or its shift register, the diagnostic sense line on user-port pin 5 (quoting Olaf Seibert) and Commodore's 40-pin diagnostic clip (quoting William Levak).
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Dave Curran, Commodore PET 2001 Repair - Garbage screen with noise, July 2017—link(accessed 2026-10-01) Community repair write-up: the video circuitry runs independently of the CPU; a static garbage screen is usually a ROM or RAM fault; snow from simultaneous screen-RAM read and write, made constant by a stuck 74LS157; the chequerboard with no video RAM fitted; 6550s with faulty enable lines; the PET test ROM on a ROM/RAM board.
  8. ↑ Dan Fylstra, "User's Report: The PET 2001", BYTE vol. 3 no. 3, March 1978, pp. 114-127.
Retrieved from "https://wiki.retrotechcollection.com/index.php?title=Commodore_PET_2001_Troubleshooting&oldid=17421"