Editing Atari 1200XL Troubleshooting Guide (section)

== Display Faults ==
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|+ '''Display Fault Diagnosis'''
|-
! Symptom !! Possible Cause !! Component(s) !! Recommended Action
|-
| Snowy/noisy screen || RF cable damage, switch box fault, or RF modulator misalignment || RF cable, switch box, A3 (RF modulator) || Check cable and switch box. Adjust RF modulator (61.25 MHz on Ch. 3).
|-
| Black or grey screen (no display) || +5V rail failure, ANTIC failure, clock failure || A1/A2, L4–L8, ANTIC (U20), CR8, Y1 || Verify +5V rail. Check clock at CPU pin 37. Replace suspected component.
|-
| Green/yellow screen || RAM or ROM fault, address decoding failure || U27, U1–U9, U12, U13 || Run RAM and ROM self-test. Replace faulty chip per diagnostic results.
|-
| Solid blue screen || GTIA fault, oscillator fault, video circuit failure || U24 (POKEY), Q3, Y1, Q2, Q4, U19 (GTIA) || Check crystal Y1. Verify GTIA (U19) oscillator output at pin 28. Replace as needed.
|-
| No colour or incorrect colour bars || Colour frequency misadjustment or GTIA fault || VR1, U20, U19 (GTIA), Q3, Y1, RF modulator || Adjust VR1 for correct colour bar alignment. Run SALT Color Bar test.
|-
| Upside-down characters or inverted player field || ANTIC or GTIA data path fault || U20 (ANTIC), U19 (GTIA), Q3 || Replace ANTIC or GTIA as indicated by further testing.
|}

=== Colour Adjustment (VR1) ===
# Insert the SALT 2.05 cartridge and run the '''Color Bar Test''' (command: C).
# A 15-colour rainbow displays above a reference bar, with a single colour bar below.
# Adjust '''VR1''' (accessible through the top or bottom cover) until the colour bars directly above and below the reference bar are identical (goldenrod).
# Leave the test running for at least 60 seconds to detect intermittent colour faults.

=== Grey Bar Test ===
Run the SALT 2.05 '''Gray Bar Test''' (command: G) to verify GTIA luminance outputs (Lum 0, 1, 2):
* Eight horizontal bars should display, progressing evenly from black (top) to white (bottom).
* No colour should appear anywhere on screen.
* If colour appears or bars are uneven, suspect GTIA (U19).