Editing Atari 800XL Troubleshooting Guide (section)

== Display & Chime Diagnostics ==

The 800XL should display a blue "READY" BASIC screen and a single beep on successful power-up.  
If you see a blank screen, coloured bars, or hear unusual beeps, proceed as follows:

=== No Video Output ===
* Confirm TV/monitor input and cable are correct (RF or composite).
* Test both RF and monitor outputs.
* Inspect the modulator and monitor jack for broken pins or solder cracks.
* Check for +5 V at the video output circuit (Q2, Q3 transistors).

=== Black Screen, No Beep ===
* Indicates CPU, OS ROM, or RAM failure, or missing clock/reset.
* See [[#Memory & ROM Faults]] and [[Atari 800XL Troubleshooting Guide#Component-level Tests (Clock, Reset, Voltages)|#Component-level Tests]] below.

=== Coloured Bars or Garbage Characters ===
* Suggests partial RAM failure, bad character ROM, or bus contention.

{| class="wikitable styled-table"
! Symptom !! Probable Cause !! Action
|-
| Black screen, no beep || Dead CPU (U23), OS ROM (U4), or RAM; missing clock/reset || See memory, ROM and clock test sections
|-
| Coloured vertical bars || RAM fault (U9–U16), address decoder (U17, U18) || Swap/test RAM; check 74LS138/LS158 chips
|-
| Garbage characters, screen flicker || Bad character ROM (U5), bus conflict || Replace U5; check for bent pins/shorts
|-
| No video but normal beep || Video circuit (U19, U20, Q2, Q3), modulator || Test/replace video ICs or modulator
|}