Commodore PET 2001 Troubleshooting: Difference between revisions
No edit summary |
No edit summary |
||
| (2 intermediate revisions by the same user not shown) | |||
| Line 1: | Line 1: | ||
This guide provides a | This guide provides a troubleshooting reference for the Commodore PET 2001 personal computer. It focuses on common faults specific to the PET 2001 (original models with MOS 6502 CPU, 6540 ROMs, 6550 SRAM, etc.) and offers diagnostic steps and solutions. Use this guide alongside general maintenance practices (see [[Commodore PET 2001 General Maintenance]]) and refer to the PET schematics (see [[:Category:Commodore PET 2001 Schematics|Commodore PET 2001 Schematics]]) as needed. | ||
Due to its age and early semiconductor technology, many faults today relate to failing memory chips, ROMs, or power issues. When diagnosing problems, it is important to proceed step-by-step, starting from the most basic elements (power, reset, clock) before advancing to CPU logic, video output, I/O subsystems, and peripherals. | |||
== | == Power Supply and Startup == | ||
A completely dead PET often stems from power-related issues. Begin by checking the fuse and power switch. The PET's main fuse is located near the power input and should be tested for continuity. A blown fuse may indicate a shorted capacitor or rectifier. | A completely dead PET often stems from power-related issues. Begin by checking the fuse and power switch. The PET's main fuse is located near the power input and should be tested for continuity. A blown fuse may indicate a shorted capacitor or rectifier. | ||
| Line 12: | Line 12: | ||
The PET 2001 uses linear voltage regulators, such as 7805 for +5V. Measure their input and output voltages. A working regulator will have a few volts higher on the input than the output. If not, replace the regulator. Always verify voltages at the logic board input before proceeding with deeper diagnostics. | The PET 2001 uses linear voltage regulators, such as 7805 for +5V. Measure their input and output voltages. A working regulator will have a few volts higher on the input than the output. If not, replace the regulator. Always verify voltages at the logic board input before proceeding with deeper diagnostics. | ||
== | == Reset and Clock Signals == | ||
When power is stable, the next step is ensuring that the CPU is properly reset and clocked. The 6502 CPU must receive a brief low pulse on the RES* line at power-on, after which the line should go high. Use a logic probe or oscilloscope to monitor this behavior. | When power is stable, the next step is ensuring that the CPU is properly reset and clocked. The 6502 CPU must receive a brief low pulse on the RES* line at power-on, after which the line should go high. Use a logic probe or oscilloscope to monitor this behavior. | ||
| Line 20: | Line 20: | ||
If both clock and reset lines behave correctly, and the CPU is still unresponsive, move on to probing logic activity. | If both clock and reset lines behave correctly, and the CPU is still unresponsive, move on to probing logic activity. | ||
== | == Video Symptoms and Display Analysis == | ||
[[File:Commodore PET 2001 Random Symbols.png|thumb|Commodore PET 2001 Random Symbols]] | [[File:Commodore PET 2001 Random Symbols.png|thumb|Commodore PET 2001 Random Symbols]] | ||
The PET has an integrated CRT display. Many faults can be interpreted visually based on screen behavior. | The PET has an integrated CRT display. Many faults can be interpreted visually based on screen behavior. | ||
| Line 30: | Line 30: | ||
If the screen displays consistent patterns (e.g., vertical stripes, checkerboards), this may point to stuck address lines, faulty video RAM chips (usually 2114s), or bad data buffers. | If the screen displays consistent patterns (e.g., vertical stripes, checkerboards), this may point to stuck address lines, faulty video RAM chips (usually 2114s), or bad data buffers. | ||
Missing or garbled specific characters often trace to the character generator ROM, which contains the glyph data. Swapping or verifying this ROM is a common step when isolated character corruption occurs. | |||
== | == CPU, ROM and RAM Troubleshooting == | ||
The MOS 6502 is a robust CPU, but it must receive valid reset and clock signals, and must be able to access ROM and RAM. | The MOS 6502 is a robust CPU, but it must receive valid reset and clock signals, and must be able to access ROM and RAM. | ||
| Line 44: | Line 44: | ||
Address decoders such as the 74154 or 74LS138 can also fail, resulting in inaccessible memory regions. If parts of memory are non-functional, examine these chips. | Address decoders such as the 74154 or 74LS138 can also fail, resulting in inaccessible memory regions. If parts of memory are non-functional, examine these chips. | ||
== | == Using Diagnostic ROMs == | ||
Installing a known diagnostic ROM, such as PETTESTER or Tynemouth Software’s diagnostics, can be invaluable. These ROMs replace the EDIT ROM or are run via a plug-in board and generate visual patterns or codes that reveal system status. | Installing a known diagnostic ROM, such as PETTESTER or Tynemouth Software’s diagnostics, can be invaluable. These ROMs replace the EDIT ROM or are run via a plug-in board and generate visual patterns or codes that reveal system status. | ||
| Line 50: | Line 50: | ||
For example, if PETTEST fills the screen with a pattern and it remains unchanged, that implies the CPU is running but unable to write to RAM. If the pattern changes or an error is displayed, follow the diagnostic guide’s meaning for that result. | For example, if PETTEST fills the screen with a pattern and it remains unchanged, that implies the CPU is running but unable to write to RAM. If the pattern changes or an error is displayed, follow the diagnostic guide’s meaning for that result. | ||
== | == Logic Probing and Signal Analysis == | ||
A logic probe or oscilloscope helps immensely in PET repair. Monitor chip select (CS), read/write (R/W), and data/address activity. | A logic probe or oscilloscope helps immensely in PET repair. Monitor chip select (CS), read/write (R/W), and data/address activity. | ||
| Line 68: | Line 68: | ||
Compare signal activity between similar chips to locate anomalies. | Compare signal activity between similar chips to locate anomalies. | ||
== | == Common Failures and Replacements == | ||
6550 SRAMs – often fail due to heat or age. Replace with 2114s or adapter boards. | 6550 SRAMs – often fail due to heat or age. Replace with 2114s or adapter boards. | ||
| Line 82: | Line 82: | ||
Character ROM – if screen is mostly normal but letters are incorrect, replace with a good 2332. | Character ROM – if screen is mostly normal but letters are incorrect, replace with a good 2332. | ||
== | == Cassette and Sound Troubles == | ||
The PET’s built-in datasette often suffers from mechanical wear. If LOAD fails: | The PET’s built-in datasette often suffers from mechanical wear. If LOAD fails: | ||
| Line 94: | Line 94: | ||
For sound, the PET uses a piezo buzzer triggered via a PIA line. Try PRINT CHR$(7) in BASIC to trigger the bell. If no sound, trace the signal from PIA output to the buzzer. | For sound, the PET uses a piezo buzzer triggered via a PIA line. Try PRINT CHR$(7) in BASIC to trigger the bell. If no sound, trace the signal from PIA output to the buzzer. | ||
== | == Keyboard and I/O Issues == | ||
Keyboards may fail due to corroded contacts, bad connections, or PIA faults. If certain keys or rows/columns are dead, inspect the matrix wiring and test lines on the mainboard. | Keyboards may fail due to corroded contacts, bad connections, or PIA faults. If certain keys or rows/columns are dead, inspect the matrix wiring and test lines on the mainboard. | ||
| Line 101: | Line 101: | ||
IEEE-488 issues often trace to the 6520 or 6522 I/O chips or the 75160/75161 transceivers. If the PET cannot detect drives, monitor activity on the bus lines. Swapping out the transceiver chips often restores function. | IEEE-488 issues often trace to the 6520 or 6522 I/O chips or the 75160/75161 transceivers. If the PET cannot detect drives, monitor activity on the bus lines. Swapping out the transceiver chips often restores function. | ||
[[Category:Commodore Systems]] | [[Category:Commodore Systems]] | ||
[[Category: | [[Category:Troubleshooting Guides]] | ||