Editing Atari 130XE Troubleshooting Guide (section)

== Display & Boot Diagnostics ==

The 130XE should display a blue '''READY''' prompt and produce a brief key-click sound on successful boot (if BASIC is enabled). If OPTION is held at power-on, BASIC is disabled and the machine enters the Self-Test or awaits a cartridge.

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{| class="wikitable styled-table" style="width:100%; text-align:center;"
! Symptom !! Likely Cause !! Diagnostic Action
|-
| No video, no sound, power LED off || Dead PSU, blown fuse, bad power switch, open bridge rectifier || Test PSU output; check rectifier diodes; clean/replace switch
|-
| No video, no sound, power LED on || CPU, ROM, FREDDIE, or MMU failure; total RAM failure || Check +5V at CPU; see Black Screen flowchart below
|-
| Black screen, brief click at power-on || CPU running but ANTIC/GTIA not producing video || Swap/test ANTIC, GTIA; check clock signals
|-
| Solid colour screen (no text) || ROM corruption or RAM fault preventing boot || Reseat/replace OS ROM; test base RAM
|-
| Garbage characters on screen || RAM fault (partial failure), bus contention, or bad ROM || Run RAM test cartridge; swap ROM; check bus signals
|-
| Rolling/tearing/no sync video || Bad GTIA, ANTIC, or colour clock crystal || Swap GTIA/ANTIC; verify crystal frequency on scope
|-
| Correct video but wrong or missing colours || GTIA fault (common on Chinese-made XEs) || Replace GTIA
|-
| No key-click at power-on (video OK) || POKEY fault or speaker disconnected || Test/replace POKEY; check speaker wiring
|-
| Self-test runs but BASIC fails || BASIC ROM (CO24947A) fault || Replace BASIC ROM
|-
| Boots with cartridge but not to BASIC || BASIC ROM bad or OPTION key stuck || Check OPTION key; replace BASIC ROM
|}

=== "Black Screen" (No Boot) Flowchart ===
# Confirm +5V at mainboard and at multiple IC Vcc pins.
# Check for a brief click or beep at power-on (indicates CPU is executing code).
# Verify clock signal — probe 6502C pin 39 (Φ0 output) with oscilloscope; expect ~1.79 MHz (NTSC) or ~1.77 MHz (PAL).
# Verify RESET line — 6502C pin 40 should pulse LOW at power-on then go HIGH (+5V). If stuck LOW, check reset circuit (R/C network).
# If clock present but CPU not running: swap/test FREDDIE (CO61991) — it generates the master clock to the CPU.
# Reseat or substitute OS ROM (CO61598B). A blank or corrupted ROM will prevent any boot.
# Test base 64K RAM (first bank) — see RAM Faults section.
# Check MMU (CO61618) — address decoding failures prevent all chip selects.