Editing Atari Mega STE Troubleshooting Guide

{{Infobox computer
| name = Atari Mega STE Troubleshooting Guide
| image = 
| type = Troubleshooting guide
}}

This article provides a structured troubleshooting guide for the '''[[Atari Mega STE]]''' computer. The Mega STE shares many components with the [[Atari 1040STE]] but introduces unique failure modes related to its 16 MHz CPU, cache subsystem, SCSI interface, and Phihong PSU. Mains voltages inside the PSU can be lethal — observe all safety precautions.

== Diagnostic tools ==

* '''Digital multimeter''' (DMM) — For voltage, continuity, and resistance measurements.
* '''Oscilloscope''' — For checking clock signals, bus activity, and PSU ripple.
* '''Atari Diagnostic Cartridge''' — ROM-based hardware test suite. Note: the Mega STE's RTC and some cache-related features can be tested by the diagnostic cartridge.<ref name="diagcart">{{cite web |url=https://www.atari-wiki.com/index.php?title=Basic_Troubleshooting_-_Diagnostic_Cartridge_basics |title=Basic Troubleshooting — Diagnostic Cartridge basics |publisher=Atari-Wiki |access-date=2026-03-27}}</ref>
* '''GENERAL.CPX control panel''' — For switching CPU speed and cache enable/disable, useful for isolating speed-dependent faults.
* '''Known-good PSU''' — For isolating Phihong PSM-5341 failures.
* '''Logic probe''' — For checking bus signal levels.

== Symptom table ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Symptom !! Possible Cause !! Recommended Action
|-
| Machine completely dead (no power LED) || PSU failure; mains fuse; power cable fault || Check mains cable and internal fuse. Test PSU output with dummy load (2.2Ω on +5V, 10Ω on +12V). Recap or replace PSU.
|-
| Power LED on, black screen || TOS ROM failure; CPU failure; GST MCU fault; +5V out of spec || Check +5V (should be 4.95–5.10V). Reseat TOS ROMs and CPU. Try diagnostic cartridge.
|-
| White screen, no boot || TOS ROM failure; address bus fault || Reseat TOS ROMs. Check for damaged ROM pins. Try known-good TOS 2.06 ROMs.
|-
| Boots to desktop at 8 MHz, crashes at 16 MHz || Cache SRAM failure (U3/U4); cache PAL failure || Replace U3 and U4 SRAM chips. Reseat cache PAL ICs. Test with cache disabled (bit 1 of $FFFF8E21 = 0).<ref name="exxoscache">{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=3120&start=60 |title=MegaSTE Repair — Cache Issues |publisher=exxos Forum |access-date=2026-03-27}}</ref>
|-
| Random crashes at 16 MHz with cache enabled || Cache SRAM degradation; PSU instability at higher power draw || Replace U3 and U4. Recap PSU. Test at 8 MHz to confirm PSU is not the root cause.
|-
| Random crashes at both 8 MHz and 16 MHz || PSU capacitor degradation; parity SIMMs; bad RAM SIMM || Recap PSU. Remove parity chips from SIMMs (or replace with non-parity). Reseat and swap SIMMs to identify failing module.<ref name="mandatory">{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1585 |title=Mandatory Fixes |publisher=exxos Forum |access-date=2026-03-27}}</ref>
|-
| Bombs on boot (row of bomb icons) || 2 bombs = bus error (RAM/bus fault); 3 = address error; 4 = illegal instruction || Note bomb count. Reseat SIMMs. Try minimal RAM config (1 MB). Run diagnostic cartridge.
|-
| Memory test shows less RAM than installed || Faulty SIMM; incorrect SIMM type; dirty socket contacts || Reseat SIMMs. Clean socket contacts with DeoxIT. Try SIMMs individually to identify failure. Use non-parity 30-pin SIMMs only.
|-
| Floppy drive not reading/writing || Dirty heads; drive belt failure; WD1772 fault; PSU voltage sag || Clean heads with IPA. Check drive belt. Verify +5V stability during floppy access (should not dip below 4.8V). Add 1772 pull-up resistors if using non-original drive.
|-
| Internal SCSI hard disk not detected || ACSI/SCSI daughterboard loose; SCSI cable fault; drive failure; no driver in boot sector || Reseat ACSI/SCSI adapter board. Check SCSI ribbon cable. Verify drive spins up (listen for motor). Reinstall boot driver (AHDI or HDDriver) from floppy.
|-
| SCSI hard disk intermittent failures || DMA controller issue; DMA pull-up resistors needed; SCSI termination || Add 10K DMA data bus pull-up resistors. Verify SCSI termination (last device on chain must be terminated). Check DMA chip.<ref name="mandatory"/>
|-
| Video ghosting (faint shadow/echo on bright objects) || Impedance mismatch in STE video output circuit || Apply video ghosting fix (resistor modification on video output).<ref name="videoghost">{{cite web |url=https://www.exxosforum.co.uk/atari/last/VIDEOFIX/index.htm |title=STE/MSTE Video Ghosting Fix |publisher=exxos |access-date=2026-03-27}}</ref>
|-
| Screen dims during floppy access || PSU capacitor degradation; insufficient peak current capacity || Recap PSU. This is the most common symptom of aged PSU capacitors.<ref name="exxospsu">{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=Atari PSU Repair — The LaST Upgrade |publisher=exxos |access-date=2026-03-27}}</ref>
|-
| Video noise, wavy lines, or poor display quality || PSU ripple; video cable fault; monitor fault || Recap PSU (primary suspect). Check video cable. Try different monitor.
|-
| No stereo audio from RCA jacks || LMC1992 mixer fault; DMA audio registers not initialised; software configuration || Verify DMA sound is enabled in software. Check LMC1992 chip. Test with a program that explicitly uses DMA audio (e.g., a tracker or sample player).
|-
| No mono audio from monitor speaker || YM2149F fault; audio trace to video connector broken || Check YM2149 clock (2 MHz). Test audio pin on 13-pin DIN connector with oscilloscope.
|-
| MIDI not working || ACIA 6850 failure; cold solder joints on MIDI DIN connectors || Reflow solder joints on MIDI connectors. Test ACIA with diagnostic software. Verify +5V present on MIDI Out pin 4.
|-
| Keyboard unresponsive or intermittent || Cable fault; membrane trace failure; keyboard controller fault || Check cable continuity. Repair membrane traces with conductive pen. Try known-good keyboard.
|-
| Individual keys not working || Membrane trace break || Open keyboard, inspect membrane for cracks at key position. Repair with conductive pen.
|-
| LocalTalk/LAN port not communicating || Port rarely used; no standard AppleTalk software; hardware fault possible || Verify with RS-422 serial test. Note: Atari never shipped AppleTalk protocol software. Third-party solutions required for LocalTalk networking.
|-
| VME card not detected or unstable || Card not seated properly; incompatible card/driver; bus conflict || Reseat VME card. Clean edge connector. Verify driver software is correct for both card and TOS version.
|-
| Date/time/CPU speed lost on power cycle || NVRAM battery dead || Replace NVRAM battery (CR2032 or Dallas SmartWatch module depending on revision).
|-
| FPU not detected by software || FPU not installed; wrong FPU type; bad socket contact || Verify FPU is physically installed in PLCC socket. Check for correct type (68881 or 68882). Reseat FPU.
|}

== Voltage test points ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Test point !! Expected voltage !! Notes
|-
| PSU +5V output || 4.95–5.10V || Critical rail. All logic and RAM operates on +5V.
|-
| PSU +12V output || 11.8–12.2V || Serial drivers, floppy motor, SCSI drive power.
|-
| PSU -12V output || -11.8 to -12.2V || RS-232 line drivers.
|-
| PSU -5V output || -4.90 to -5.10V || DRAM bias.
|-
| PSU Power Good (PG) || ~+5V || Should be approximately +5V when PSU is stable.
|-
| CPU pin 14 (VCC) || +5V || Verify CPU power.
|-
| CPU pin 15 (CLK) || 8 MHz or 16 MHz || Depends on speed setting. Check with oscilloscope.
|}

== PSU connector pinout ==

The Phihong PSM-5341 connects to the motherboard via a multi-pin connector. The wire colours correspond to the following:

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
|-
! Wire colour !! Voltage
|-
| Red || +5V
|-
| Yellow || +12V
|-
| Blue || -12V
|-
| White || -5V
|-
| Black || Ground (GND)
|-
| Orange || Power Good (PG)
|}

== Recommended preventive modifications ==

# '''PSU recapping''' — Replace all electrolytic capacitors in the Phihong PSM-5341. See [[Atari Mega STE Capacitor Replacement Guide]].
# '''Video ghosting fix''' — Resistor modification to correct STE/MSTE video output impedance mismatch.
# '''DMA bus pull-ups''' — Add 10K pull-up resistor network to DMA data bus.
# '''1772 pull-ups''' — Add pull-up resistors to WD1772 floppy controller outputs if using a non-original replacement drive.
# '''Cache SRAM replacement''' — Proactively replace U3 and U4 if 16 MHz stability issues are observed.

== See also ==

* [[Atari Mega STE]]
* [[Atari Mega STE General Maintenance]]
* [[Atari Mega STE Capacitor Replacement Guide]]

== References ==
<references />

{{Navbox-AtariVintageComputers}}

[[Category:Atari Troubleshooting Guides]]