Editing Atari STE Troubleshooting Guide (section)

== Display / Video Troubleshooting ==

The STE video system uses the GST Shifter, which has some different failure modes from the original ST Shifter.

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! Symptom !! Possible Cause !! Recommended Action
|-
| No display, power LED on || CPU not running; GST MCU failure; clock failure || Check 32 MHz crystal. The GST MCU generates all clocks — if it fails, nothing runs. Check for 8 MHz on CPU CLK pin with oscilloscope.
|-
| White screen (indefinite) || RAM fault (try swapping/replacing SIMMs); TOS ROM fault; GST MCU failure || Swap SIMMs with known-good modules. Try known-good TOS ROMs. If neither helps, GST MCU may be faulty (difficult repair — requires SMD rework).
|-
| Corrupted display / garbled colours || GST Shifter failure; SIMM contact oxidation; motherboard cap failure near Shifter || Clean SIMM contacts. Reseat GST Shifter (if socketed). Recap motherboard.
|-
| Enhanced colour palette not working (only 512 colours instead of 4096) || Software not using STE colour registers; GST Shifter fault || Test with known STE-specific software. If STE colour features never work, GST Shifter or bus fault.
|-
| Screen jitter / unstable sync || GST MCU sync generation fault; PSU ripple; failing master clock || Check PSU voltages and ripple. Check 32 MHz crystal. If stable with recapped PSU, suspect GST MCU.
|-
| Display corruption only when scrolling hardware is used || GST MCU hardware scroll registers faulty || Test with STE-specific demos that use hardware scrolling. If non-scrolling software works fine, GST MCU has partial failure in scroll logic.
|}