Editing Atari 400 Troubleshooting Guide (section)

== Display & Chime Diagnostics ==

The Atari 400 should display a solid blue screen (NTSC) or light blue (PAL) with the built-in Memo Pad prompt if no cartridge is inserted.

{| class="wikitable styled-table"
! Symptom!!Likely Cause!!Action
|-
|No video, no sound||No power, blown fuse, or dead voltage regulator||Check PSU, replace fuse, test/replace 7805 regulator
|-
|Black screen, no border||CPU, ANTIC, or RAM failure||See memory/logic section; swap ICs if socketed
|-
|Solid colour screen, no prompt||OS ROM or address decoding fault||Reseat/replace OS ROM; check logic chips
|-
|Rolling, distorted, or B&W video||Bad modulator, wrong TV channel, or faulty GTIA/CTIA||Test on composite output; swap GTIA/CTIA if possible
|-
|Intermittent video loss||Loose RF cable, cracked solder at modulator||Resolder or replace modulator
|}

=== Audio (Power-on Chime) ===
* The Atari 400 does '''not''' produce a startup chime.  
* If you hear static or noise from the TV speaker, the audio circuit is at least partially alive.