Editing Atari TT Troubleshooting Guide (section)

== Common Fault Table ==

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Dead — no power LED, no fan || PSU failure; blown fuse; failed mains switch || Check mains fuse in IEC inlet. Test PSU output voltages off-load. Recap PSU (see [[Atari TT Capacitor Replacement Guide]]). Check bridge rectifier.
|-
| Power LED on, no video, no boot || CPU or FPU failure; ROM failure; ST-RAM failure || Reseat CPU/FPU (daughter board on early models). Reseat ROM chips. Test with minimal configuration (no VME cards, no TT-RAM). Check for corroded CPU socket pins.
|-
| Bombs on boot (2 or 3 bombs) || Bus errors; faulty ST-RAM; DMA issues; VME bus timing || Remove all expansion cards (VME, TT-RAM, IDE). Test with ST-RAM only. Check for cold solder joints. Apply VME bus fix if not already done.<ref>{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1000 |title=TT VME Bus Fix |publisher=exxos Forum |access-date=2026-03-27}}</ref>
|-
| Random crashes during operation || Degraded PSU capacitors; TT-RAM failure; overheating || Recap PSU. Run YAART memory test for extended period. Check fan operation and CPU/FPU thermal condition. Verify +5V rail stability under load.
|-
| Floppy drive not recognised || WD-1772 failure; cable fault; drive failure || Test with known-good floppy drive. Replace WD-1772-PH chip. Check ribbon cable continuity. Verify drive-select jumper.
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| SCSI devices not detected || NCR 5380 failure; termination error; cable fault || Check SCSI termination (active or passive terminator on last device). Verify SCSI ID conflicts. Test with single SCSI device. Check NCR 5380 chip.
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| No sound || YM2149 failure; DMA sound circuit fault || Test PSG sound (system beep). Test DMA sound separately. Check YM2149F chip and associated capacitors.
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| NVRAM settings lost / clock reset || Dead backup battery || Replace NVRAM battery (3.6V lithium cell). Check MC146818A RTC chip.
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| Keyboard not responding || Keyboard cable fault; HD6301V1 failure; ACIA fault || Test with alternate keyboard cable. Check HD6301V1 keyboard processor. Check MC6850P ACIA chips.
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| Video problems — wrong colours, distortion || TT Shifter fault; monitor cable; capacitor degradation near video circuit || Check video cable and monitor. Inspect and recap capacitors near TT Shifter. Reseat TT Shifter if socketed.
|-
| VME card not recognised || VME bus timing issues; card compatibility; power || Apply VME bus fix (Atari-issued rework). Check VME slot power pins. Verify card compatibility with TT VME implementation.
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| Intermittent bus errors with VME cards installed || VME bus fix not applied; signal integrity || Apply the official Atari VME bus fix. This is a rework procedure involving trace cuts and jumper wires to correct bus timing on older board revisions.<ref>{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1000 |title=TT VME Bus Fix – Does it really make it faster? |publisher=exxos Forum |access-date=2026-03-27}}</ref>
|-
| TT-RAM errors || Faulty SIMMs; MCU failure; incorrect SIMM type || Test each SIMM individually. Ensure correct SIMM type (30-pin or 72-pin as appropriate for the expansion board). Clean SIMM contacts with IPA. Run YAART for extended test.
|}