Editing Atari Mega ST Troubleshooting (section)

== Video and Display Issues ==

=== Monitor Detection ===

The Mega ST determines mono/colour mode at boot via pin 4 (Mono Detect) on the 13-pin DIN video connector. The SM124/SM125 grounds this pin; colour monitors leave it floating.

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Video Fault Table'''
|-
! Symptom !! Possible Cause !! Recommended Action
|-
| Flickering/rolling on SM124 mono monitor || V-sync line (pin 12) held low or missing || Check continuity from GLUE chip to pin 12. Inspect diode divider D7/D18. Verify no shorts between video socket pins<ref name="atari-forum-repair"/>
|-
| No video on colour monitor || Mono detect pin shorted to ground || Verify pin 4 is floating when colour monitor connected
|-
| Vertical colour bars or corruption || Shifter video chip failure || Check Shifter outputs on the data bus. Replace if confirmed faulty
|-
| Poor image quality, ghosting || Excessive PSU ripple on +5 V rail || Recap PSU; check 5 V regulation (target < 200 mV ripple)
|-
| Dim/washed-out display || Video output level too low || Check video DAC resistors and Shifter output
|}

=== DB9 Video Mod Issues ===

Some Mega STs have been modified with a DB9 video output replacing the 13-pin DIN. Common problems:
* Audio output lost (DB9 has no audio pin)
* Incorrect wiring — verify pin-to-pin mapping against the original 13-pin DIN connector
* Damaged through-hole pads from the modification — may need bodge wires to restore connections<ref name="atari-forum-repair"/>