Commodore PET 4000 Series Troubleshooting Guide: Difference between revisions
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[[File:Commodore PET 4000 Series (photo).jpg|thumb|right|300px|Commodore PET 4000 Series. Source: Wikimedia Commons.]] | |||
This guide provides a troubleshooting reference for the Commodore PET 4000 Series (models 4016, 4032, etc.). It covers known fault modes specific to the "Fat 40" models with 6545 CRTC, 40-column video, and BASIC 4.0 ROMs. | This guide provides a troubleshooting reference for the Commodore PET 4000 Series (models 4016, 4032, etc.). It covers known fault modes specific to the "Fat 40" models with 6545 CRTC, 40-column video, and BASIC 4.0 ROMs. | ||
For cleaning and chip reseating techniques, see [[Commodore PET 4000 Series Maintenance Guide | For cleaning and chip reseating techniques, see [[Commodore PET 4000 Series Maintenance Guide]]. For recapping procedures, refer to the [[Commodore PET 4000 Series Capacitor Guide]]. | ||
== Power Supply and Basic Signals == | == Power Supply and Basic Signals == | ||
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Boards like PETVet replace ROM/RAM and help isolate multiple faults. With one plug-in, they can eliminate several problem areas at once. | Boards like PETVet replace ROM/RAM and help isolate multiple faults. With one plug-in, they can eliminate several problem areas at once. | ||
== Garbage screen (the classic PET fault) == | |||
The PET's video runs independently of the CPU and continuously displays the first ~1000 bytes of screen RAM. At power-on that RAM is random, so a healthy PET briefly shows garbage and then clears the screen once the CPU runs its start-up code. '''A screen that stays full of static garbage means the CPU is not running''' — almost always a ROM or RAM fault.<ref name="pet">[https://6502.org/users/andre/petindex/repairs.html Commodore PET repair info], André Fachat; [http://blog.tynemouthsoftware.co.uk/2017/07/commodore-pet-2001-repair-garbage.html PET 2001 Repair — Garbage screen], Tynemouth Software; and [http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/ D'Asaro Designs]. Source for the garbage-screen ROM/RAM diagnosis, the 74LS244 bus-driver faults, the socket-contact issues, and the linear PSU (78L05) and Molex-connector video-wobble fault.</ref> Work the ROMs and RAM first: | |||
* Bad RAM is common, but the '''74LS244 bus-driver chips''' fail at least as often — check or replace them first. | |||
* '''Reseat every socketed chip.''' PET sockets are cheap and develop poor contacts, which on its own causes garbage-screen and dead faults. | |||
* Chip types: early 2001 boards use MPS6540 ROMs and MPS6550 RAM; later boards use 2316B ROMs and 2114 RAM. | |||
== Power supply and video wobble == | |||
The PET uses a simple, reliable linear supply: a transformer and filter capacitor feeding four '''78L05''' +5 V regulators on the main board. A '''wobbly, warped or jumping display''' is classically caused by an '''oxidised or cracked Molex power connector''' making the board voltage jump between about 4 and 5 V several times a second — reseat, clean or replace the connector. Screen "noise" on the 2001 is screen-RAM read/write contention.<ref name="pet">[https://6502.org/users/andre/petindex/repairs.html Commodore PET repair info], André Fachat; [http://blog.tynemouthsoftware.co.uk/2017/07/commodore-pet-2001-repair-garbage.html PET 2001 Repair — Garbage screen], Tynemouth Software; and [http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/ D'Asaro Designs]. Source for the garbage-screen ROM/RAM diagnosis, the 74LS244 bus-driver faults, the socket-contact issues, and the linear PSU (78L05) and Molex-connector video-wobble fault.</ref> | |||
== Video timing == | |||
A display where each character line advances the screen address by only 36 instead of 40 (overlapping or wrongly-wrapped characters) points to a '''weak character clock''' in the video circuit rather than to RAM.<ref name="pet">[https://6502.org/users/andre/petindex/repairs.html Commodore PET repair info], André Fachat; [http://blog.tynemouthsoftware.co.uk/2017/07/commodore-pet-2001-repair-garbage.html PET 2001 Repair — Garbage screen], Tynemouth Software; and [http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/ D'Asaro Designs]. Source for the garbage-screen ROM/RAM diagnosis, the 74LS244 bus-driver faults, the socket-contact issues, and the linear PSU (78L05) and Molex-connector video-wobble fault.</ref> | |||
== CRTC (4000/8000 series) == | |||
The 4000/8000-series PETs generate the display with a '''6545 CRT controller''' rather than the 2001's discrete video logic. A rolling, mis-sized or blank raster with the rest of the machine alive points to the 6545, its clock, or the monitor-adjustment section; reseat the 6545 and check its clock before suspecting the CRT.<ref name="pet">[https://6502.org/users/andre/petindex/repairs.html Commodore PET repair info], André Fachat; [http://blog.tynemouthsoftware.co.uk/2017/07/commodore-pet-2001-repair-garbage.html PET 2001 Repair — Garbage screen], Tynemouth Software; and [http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/ D'Asaro Designs]. Source for the garbage-screen ROM/RAM diagnosis, the 74LS244 bus-driver faults, the socket-contact issues, and the linear PSU (78L05) and Molex-connector video-wobble fault.</ref> | |||
== References == | |||
<references /> | |||
[[Category:Commodore Systems]] | [[Category:Commodore Systems]] | ||
[[Category:Troubleshooting Guides]] | [[Category:Troubleshooting Guides]] | ||