Atari 800XL Troubleshooting Guide: Difference between revisions
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[[File:Atari 800XL (photo).jpg|thumb|right|300px|Atari 800XL. Source: Wikimedia Commons.]] | |||
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Careful diagnosis can restore most 800XLs to full working order and prevent damage from recurring issues. | Careful diagnosis can restore most 800XLs to full working order and prevent damage from recurring issues. | ||
== | == Preliminary & Power-up Checks == | ||
Begin by confirming the '''external power supply''' delivers correct voltages: | Begin by confirming the '''external power supply''' delivers correct voltages: | ||
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== | == Display & Chime Diagnostics == | ||
The 800XL should display a blue "READY" BASIC screen and a single beep on successful power-up. | The 800XL should display a blue "READY" BASIC screen and a single beep on successful power-up. | ||
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=== Black Screen, No Beep === | === Black Screen, No Beep === | ||
* Indicates CPU, OS ROM, or RAM failure, or missing clock/reset. | * Indicates CPU, OS ROM, or RAM failure, or missing clock/reset. | ||
* See [[# | * See [[#Memory & ROM Faults]] and [[Atari 800XL Troubleshooting Guide#Component-level Tests (Clock, Reset, Voltages)|#Component-level Tests]] below. | ||
=== Coloured Bars or Garbage Characters === | === Coloured Bars or Garbage Characters === | ||
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== | == Memory & ROM Faults == | ||
The 800XL uses eight 4164 DRAM ICs (U9–U16, each 64 Kbit) and two ROMs: OS ROM (U4, 16 KB) and character ROM (U5, 2 KB). | The 800XL uses eight 4164 DRAM ICs (U9–U16, each 64 Kbit) and two ROMs: OS ROM (U4, 16 KB) and character ROM (U5, 2 KB). | ||
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== | == Connector & Socket Issues == | ||
Many 800XL faults are caused by poor connections rather than failed chips. | Many 800XL faults are caused by poor connections rather than failed chips. | ||
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* Check for cracked solder joints at the power, video, and SIO connectors. | * Check for cracked solder joints at the power, video, and SIO connectors. | ||
== | == Component-level Tests (Clock, Reset, Voltages) == | ||
=== Voltage Test Points === | === Voltage Test Points === | ||
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# If clock or reset is missing, replace suspect 74LS chips (U19, U20), Q1 transistor, or associated capacitors. | # If clock or reset is missing, replace suspect 74LS chips (U19, U20), Q1 transistor, or associated capacitors. | ||
== | == Audio & I/O Failures == | ||
The 800XL uses the '''POKEY''' chip (U24) for sound and keyboard scanning. | The 800XL uses the '''POKEY''' chip (U24) for sound and keyboard scanning. | ||
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== | == Cartridge & Peripheral Failures == | ||
* If the 800XL boots to a cartridge but not to BASIC, suspect the OS or BASIC ROM. | * If the 800XL boots to a cartridge but not to BASIC, suspect the OS or BASIC ROM. | ||
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* Fit sockets for all replaced chips and recap ageing boards for long-term reliability. | * Fit sockets for all replaced chips and recap ageing boards for long-term reliability. | ||
== | == Diagnostic order and the Star Raiders test == | ||
On the Atari 8-bit machines the components fail, statistically, in this order: '''RAM → OS ROM → POKEY → GTIA/ANTIC → CPU'''. Work through them in that order. The '''Star Raiders''' cartridge is a quick diagnostic: it exercises ANTIC, the CPU and GTIA/CTIA, so if it runs those three are good and the fault is narrowed to the PIA, the OS ROM, the MMU or the RAM. Before swapping chips, leave the machine powered for about ten minutes and feel for an overheating RAM chip, and make sure the RAM, OS ROM, ANTIC, MMU, PIA and CPU are all fully seated (reseating cures many garbage-screen / no-boot faults).<ref name="atari8">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit pictorial fault guide], 8bithardware; [https://atari-owner.com/club/articles/black-screen-problems-xl-xe-fix-checklist.25/ Black Screen [XL/XE] Fix Checklist], Atari Owners Club; and AtariAge repair threads. Source for the RAM→ROM→POKEY→GTIA/ANTIC→CPU failure order, the Star Raiders diagnostic cartridge, the seated-socket checks, the PSU 470 uF capacitor and switch/connector faults, and the SALLY 1.79 MHz clock/reset checks.</ref> | |||
== Power and clock == | |||
* '''Power''': the external "ingot" supply, an oxidised power switch and a cracked DC power connector all cause dead or intermittent operation. Check the switch and connector, and test/replace the '''470 uF 16 V''' radial electrolytic in the left-rear corner of the board.<ref name="atari8">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit pictorial fault guide], 8bithardware; [https://atari-owner.com/club/articles/black-screen-problems-xl-xe-fix-checklist.25/ Black Screen [XL/XE] Fix Checklist], Atari Owners Club; and AtariAge repair threads. Source for the RAM→ROM→POKEY→GTIA/ANTIC→CPU failure order, the Star Raiders diagnostic cartridge, the seated-socket checks, the PSU 470 uF capacitor and switch/connector faults, and the SALLY 1.79 MHz clock/reset checks.</ref> | |||
* '''CPU clock and reset''': check for a clean '''1.79 MHz''' clock at the 6502/SALLY (pin 37), and that RESET pulses low then high at power-up. A RESET stuck low points to the reset circuit.<ref name="atari8">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit pictorial fault guide], 8bithardware; [https://atari-owner.com/club/articles/black-screen-problems-xl-xe-fix-checklist.25/ Black Screen [XL/XE] Fix Checklist], Atari Owners Club; and AtariAge repair threads. Source for the RAM→ROM→POKEY→GTIA/ANTIC→CPU failure order, the Star Raiders diagnostic cartridge, the seated-socket checks, the PSU 470 uF capacitor and switch/connector faults, and the SALLY 1.79 MHz clock/reset checks.</ref> | |||
== Keyboard and POKEY == | |||
POKEY scans the keyboard and generates the sound and the SIO serial signals. '''A dead keyboard while sound still works''' is a classic POKEY (or keyboard-membrane) fault; SIO/peripheral problems also point to POKEY.<ref name="atari8">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit pictorial fault guide], 8bithardware; [https://atari-owner.com/club/articles/black-screen-problems-xl-xe-fix-checklist.25/ Black Screen [XL/XE] Fix Checklist], Atari Owners Club; and AtariAge repair threads. Source for the RAM→ROM→POKEY→GTIA/ANTIC→CPU failure order, the Star Raiders diagnostic cartridge, the seated-socket checks, the PSU 470 uF capacitor and switch/connector faults, and the SALLY 1.79 MHz clock/reset checks.</ref> | |||
== Internal BASIC (XL/XE) == | |||
The XL/XE machines have BASIC built in. A bad internal BASIC ROM can stop the machine booting; '''hold OPTION at power-on to disable internal BASIC''' as a test (and to run software that needs BASIC off). If the machine boots with OPTION held but not without, suspect the BASIC ROM.<ref name="atari8">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit pictorial fault guide], 8bithardware; [https://atari-owner.com/club/articles/black-screen-problems-xl-xe-fix-checklist.25/ Black Screen [XL/XE] Fix Checklist], Atari Owners Club; and AtariAge repair threads. Source for the RAM→ROM→POKEY→GTIA/ANTIC→CPU failure order, the Star Raiders diagnostic cartridge, the seated-socket checks, the PSU 470 uF capacitor and switch/connector faults, and the SALLY 1.79 MHz clock/reset checks.</ref> | |||
== Component-level faults (deep dive) == | |||
=== Chip map === | |||
{| class="wikitable styled-table" | |||
! Chip !! Role | |||
|- | |||
| '''SALLY''' (6502C) || CPU (6502 with HALT for DMA) | |||
|- | |||
| '''ANTIC''' || Display-list / DMA processor | |||
|- | |||
| '''GTIA''' (CTIA on early boards) || Colour, playfield and player/missile graphics | |||
|- | |||
| '''POKEY''' || Keyboard scan, SIO serial, sound, paddles | |||
|- | |||
| '''PIA''' (6520) || Joystick ports and control lines | |||
|- | |||
| '''MMU''' || Memory management (OS / BASIC / RAM banking) | |||
|- | |||
| '''FREDDIE''' || DRAM controller (later 800XL / 65XE) | |||
|} | |||
=== Star Raiders diagnostic === | |||
The Star Raiders cartridge exercises '''ANTIC, the CPU and GTIA/CTIA''' (and needs the first 8 KB of RAM good). If it runs cleanly, those three chips and the low RAM are fine, which narrows a remaining fault to the '''PIA, OS ROM, MMU or RAM'''.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref> | |||
=== POKEY fault signatures === | |||
POKEY fails in tell-tale ways: the keyboard is dead but sound still works; or keyboard and sound work but '''SIO''' does not — sometimes SIO fails only at divisor 0 (the fastest rate) yet works at divisor 4/6/8. '''GTIA and ANTIC are largely binary''' (working or dead); SALLY can show subtler faults.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref> | |||
=== Diagnostic order === | |||
Reseat all socketed ICs first; statistically '''RAM and socket faults are the most common, then ROM, then POKEY'''. After POKEY and the PIA, check the 74158 multiplexers, the 08/137/375, the delay line and the MMU. Use piggy-back or substitution for RAM and logic, fit sockets for any replaced chip, and recap an ageing board. The built-in self-test (RAM, audio-visual and keyboard tests) is a quick first check on a machine that boots far enough to reach it.<ref name="a8flt">[https://8bithardware.wixsite.com/website/post/atari-8-bit-400-800-xl-xl-pictorial-fault-guide Atari 8-bit (400/800/XL/XE) pictorial fault guide], 8bit-Hardware; [https://atariprojects.org/2025/01/01/label-the-6502-cpu-antic-gtia-pokey-and-pia-chips-for-easier-troubleshooting-using-a-multimeter-or-oscilloscope-10-15-mins/ Label the 6502 CPU, ANTIC, GTIA, POKEY and PIA Chips], Atari Projects; and [https://www.atarimania.com/pgefaq_chapitre.awp?id=14 What are SALLY, ANTIC, GTIA, POKEY and FREDDIE?], Atarimania. Source for the Star Raiders diagnostic coverage (ANTIC/CPU/GTIA), the POKEY fault signatures (keyboard vs SIO), the GTIA/ANTIC binary behaviour, and the reseat-first / RAM-and-socket-most-common guidance.</ref> | |||
== References == | |||
<references /> | |||
== Related Pages == | |||
* [[Atari XL/XE Power Supply Guide]] | * [[Atari XL/XE Power Supply Guide]] | ||
* [[Atari XL/XE RAM Replacement]] | * [[Atari XL/XE RAM Replacement]] | ||