Editing Atari ST Troubleshooting Guide (section)

== Display / Video Troubleshooting ==

The video subsystem (Shifter, GLUE, and related circuitry) is identical across the 520ST and 1040ST.

=== No Display at All ===

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No display, no power LED || PSU failure || See Power Supply Diagnosis section above.
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| Power LED on, completely blank screen || CPU not running; no clock signal; or monitor cable/connection fault || Verify monitor cable is connected and monitor is set to correct input. Check 32 MHz crystal oscillator with oscilloscope. Reseat CPU, Shifter, GLUE.
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| Power LED on, white screen only (colour monitor) || CPU running but failing during early TOS initialisation. Common causes: RAM fault, TOS ROM fault, MMU failure, GLUE failure. || Reseat all socketed ICs. Try known-good TOS ROMs. Try with diagnostic cartridge — if cartridge menu appears, motherboard is partially functional and fault is likely in RAM or TOS ROMs. If no cartridge output, fault is in ROM address space decoding (GLUE/MMU).<ref name="diagcart"/>
|-
| Power LED on, white screen only (monochrome monitor) || As above, plus: mono detect pin not being grounded (faulty cable/connector) causing Shifter to output colour mode to mono monitor || Check pin 4 on the 13-pin DIN connector. It should be grounded for monochrome mode.
|-
| Display appears briefly then goes blank || PSU voltage droop under full load; thermal fault in Shifter or GLUE || Measure +5V rail under load. If voltage is stable, suspect thermal failure in Shifter — use freeze spray to isolate.
|}

=== Display Corruption ===

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
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| Vertical or horizontal bars/stripes || Shifter failure; bad connection between Shifter and data bus; failing RAM affecting video memory area || Reseat Shifter chip. Check Shifter retaining clips for adequate tension. If problem persists, try known-good Shifter from donor board.
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| Wrong or garbled colours || Shifter output fault; bad solder joint on RGB output path; monitor cable fault || Check 13-pin DIN cable for continuity on R, G, B pins (6, 7, 10). Check monitor connector for bent pins. Reseat Shifter.
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| Screen jitter or rolling || Sync signal fault; HSync or VSync not being generated correctly by GLUE || Check HSync (pin 9) and VSync (pin 12) on monitor port with oscilloscope. If absent, suspect GLUE chip. Check 32 MHz crystal.
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| Ghosting or wavy lines || PSU ripple; poor grounding; RF interference from internal PSU (1040ST) || Recap PSU. Check ground connections.
|-
| "Jail bars" (thin vertical lines on solid colours) || Impedance issues on the video output path; common on STFM boards || Sometimes improved by replacing decoupling capacitors near the Shifter. A known characteristic of the ST video circuit rather than a definitive fault.
|}

=== RF Modulator Issues (FM Models — 520STFM / 1040STFM) ===

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
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| No RF output, RGB works fine || RF modulator failure; modulator not connected || Check modulator solder connections. A composite video mod bypassing the modulator is a common upgrade.
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| RF output is snowy/noisy || Modulator mistuned; weak signal || Adjust tuning slug on modulator. Clean modulator connections.
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| RF output with herringbone pattern || Ground loop or interference || Ensure proper grounding. Consider composite/RGB output instead.
|}