Commodore PET 2001 Troubleshooting: Difference between revisions

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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.

== 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>

== References ==

[[Category:Commodore Systems]]

[[Category:Troubleshooting Guides]]

@@ 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.
+== 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''' &mdash; 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 &mdash; 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 &mdash; 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>
+== References ==
+<references />
 [[Category:Commodore Systems]]
 [[Category:Troubleshooting Guides]]