Atari Mega STE 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
[edit | edit source]- 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.[1]
- 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
[edit | edit source]| 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).[2] |
| 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.[3] |
| 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.[3] |
| Video ghosting (faint shadow/echo on bright objects) | Impedance mismatch in STE video output circuit | Apply video ghosting fix (resistor modification on video output).[4] |
| Screen dims during floppy access | PSU capacitor degradation; insufficient peak current capacity | Recap PSU. This is the most common symptom of aged PSU capacitors.[5] |
| 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
[edit | edit source]| 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
[edit | edit source]The Phihong PSM-5341 connects to the motherboard via a multi-pin connector. The wire colours correspond to the following:
| Wire colour | Voltage |
|---|---|
| Red | +5V |
| Yellow | +12V |
| Blue | -12V |
| White | -5V |
| Black | Ground (GND) |
| Orange | Power Good (PG) |
Recommended preventive modifications
[edit | edit source]- 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
[edit | edit source]References
[edit | edit source]- ↑ Basic Troubleshooting — Diagnostic Cartridge basics, Atari-Wiki—link(accessed 2026-03-27)
- ↑ MegaSTE Repair — Cache Issues, exxos Forum—link(accessed 2026-03-27)
- ↑ 3.0 3.1 Mandatory Fixes, exxos Forum—link(accessed 2026-03-27)
- ↑ STE/MSTE Video Ghosting Fix, exxos—link(accessed 2026-03-27)
- ↑ Atari PSU Repair — The LaST Upgrade, exxos—link(accessed 2026-03-27)