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Created page with "<templatestyles src="Template:StyledTable/styles.css" /> The '''Atari STE Troubleshooting Guide''' covers systematic fault diagnosis for the '''Atari 520STE''' and '''Atari 1040STE'''. Both models share the same motherboard, custom chipset, and PSU design. The only hardware difference is factory RAM: 520STE ships with 512 KB (2ร— 256 KB SIMMs), the 1040STE with 1 MB (4ร— 256 KB SIMMs). All troubleshooting procedures apply to both models. For routine preventive..."
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This guide covers fault diagnosis and repair for the '''[[Atari 520STE]]''' and '''[[Atari 1040STE]]''' computers. Both models use the same motherboard (CA401177 or CA4003290) with the only hardware difference being RAM population via SIMMs. The STE uses a different custom chipset from the earlier ST/STF/STFM range, so while many troubleshooting principles are shared, the specific ICs and failure modes differ.


The '''Atari STE Troubleshooting Guide''' covers systematic fault diagnosis for the '''[[Atari 520STE]]''' and '''[[Atari 1040STE]]'''. Both models share the same motherboard, custom chipset, and PSU design. The only hardware difference is factory RAM: 520STE ships with 512 KB (2ร— 256 KB SIMMs), the 1040STE with 1 MB (4ร— 256 KB SIMMs). All troubleshooting procedures apply to both models.
For the earlier Atari ST/STF/STFM range (520ST, 1040ST), see [[Atari ST Troubleshooting Guide]].


For routine preventive maintenance, see [[Atari STE General Maintenance]]. For capacitor replacement details, see [[Atari STE Capacitor Replacement Guide]].
== STE Custom Chipset Overview ==


== Safety ==
The STE replaced the separate GLUE and MMU chips with a single combined IC, and introduced a new Shifter with DMA sound capabilities:
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* '''Disconnect mains power''' before opening the case. Wait at least 5 minutes for PSU capacitors to discharge.
* '''Never operate the PSU unloaded.''' If bench-testing, connect dummy loads: 2.2โ€“4.7 ฮฉ 10 W on +5 V, 22 ฮฉ 10 W on +12 V.
* '''Use an ESD wrist strap''' when handling the motherboard.
* PSU primary-side capacitors carry up to 400 V. Exercise extreme caution.
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== PSU Voltage Rails ==
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The STE internal PSU provides three DC rails. Measure at the motherboard power connector pins with a multimeter (PSU connected to the motherboard and powered on).


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{| class="wikitable styled-table"
! Rail !! Nominal !! Acceptable Range !! Measured At
! IC !! Designation !! Package !! Function
|-
|-
| +5 V || 5.00 V || 4.75โ€“5.25 V || Motherboard power connector +5 V pins; also measurable at pin 7 of any DE-9 joystick port, or across bulk filter cap C103 (4700 ยตF)
| GST MCU || U400 (C300720 / C300589) || 144-pin QFP (surface-mount) || Combined GLUE + MMU: bus decode, address generation, DRAM control, video DMA addressing, DMA sound address counter, enhanced joystick/paddle/lightpen ports, clock generation
|-
|-
| +12 V || 12.00 V || 11.40โ€“12.60 V || Motherboard power connector +12 V pin; also at pin 7 of the 14-pin floppy DIN connector
| GST Shifter || (various part numbers) || DIP-40 or QFP (varies by board revision) || Video pixel serialisation, RGB output, DMA stereo sound output (8-bit PCM at 6258/12517/25033/50066 Hz)
|-
|-
| โˆ’12 V || โˆ’12.00 V || โˆ’10.80 to โˆ’13.20 V || Motherboard power connector โˆ’12 V pin; also at the RS-232 line driver IC (MC1489 pin 14)
| BLiTTER || Atari BLiTTER || PLCC-68 || Hardware block transfer for graphics acceleration
|}
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'''PSU ripple measurement:''' Use an oscilloscope set to AC coupling on the +5 V rail. Acceptable ripple is under 200 mV peak-to-peak after a proper PSU recap. Stock aged PSUs commonly exhibit 500 mV to 1 V+ of ripple, which causes random crashes, video noise, and audio hum. Ripple above 300 mV warrants immediate capacitor replacement.<ref>{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=The LaST Upgrade โ€” Atari PSU Repair |publisher=exxos |access-date=2026-03-27}}</ref>
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=== PSU Troubleshooting ===
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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
|-
| PSU completely dead (no output) || Blown mains fuse; failed switching transistor (BU508/2SC2979); open primary winding || Check mains fuse. Test switching transistor for short between collector and emitter. Check transformer continuity.
| DMA Controller || C025913 || DIP-40 || ACSI/floppy DMA (same as ST range)
|-
|-
| +5 V rail low (< 4.7 V) || Degraded output capacitors (C201โ€“C205 depending on PSU model); heavy current draw from fault || Recap PSU. If rail remains low after recap, check for short circuits on the motherboard +5 V plane.
| WD1772 || Floppy controller || DIP-28 || Same as ST range
|-
|-
| +5 V rail high (> 5.5 V) || Failed voltage feedback loop; damaged opto-coupler; incorrect trim pot setting || Check opto-coupler. Adjust trim pot. If uncontrollable, replace the PSU.
| YM2149F || PSG sound + PIO || DIP-40 || Legacy sound; also controls drive select, printer port, RS-232 handshake lines
|-
|-
| Video dims during floppy access || PSU peak current demand insufficient; degraded capacitors || Recap PSU with low-ESR capacitors. On Mitsumi SR98, modify R201/R202 bias resistors per exxos fix (R202โ†’1K, R201โ†’1.2K).<ref>{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=SR98 peak current update |publisher=exxos |access-date=2026-03-27}}</ref>
| LMC1992 || Sound mixer || SMD || Microwire-controlled stereo mixer for volume, bass, treble, balance adjustment
|-
|-
| Audible ticking from PSU || Normal switching noise under load (some PSU models, e.g. ASP34); can also indicate marginal regulation || If regulation is within spec, ticking is normal for some designs. If accompanied by instability, recap.
| MC68901 || MFP || DIP-48 || Timers, interrupts, RS-232 UART
|-
|-
| Burning smell from PSU || Component failure (capacitor, transistor, rectifier) || Disconnect immediately. Inspect for burnt components. Replace failed parts. Consider full recap.
| MC6850 (ร—2) || ACIAs || DIP-24 || Keyboard/MIDI serial interfaces
|}
|}


== Boot Sequence Faults ==
== Boot Sequence ==


The STE boot sequence proceeds as follows: PSU powers on โ†’ CPU reset released โ†’ TOS ROM executes from $FC0000 โ†’ RAM test โ†’ hardware initialization โ†’ GEM desktop loads. Problems at each stage have different symptoms.
The STE boot sequence is similar to the ST, with some additions:


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# '''Power-on:''' PSU provides +5V and +12V. GST MCU holds the 68000 in reset.
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# '''Reset release:''' CPU begins executing from TOS ROM space.
! Symptom !! Possible Cause !! Recommended Action
# '''TOS 2.06 RAM test:''' If running TOS 2.06, a full RAM test is performed at boot. This takes approximately 3 seconds and can be aborted by pressing any key. The screen remains white during this test. On TOS 1.62, the RAM test is briefer.
|-
# '''GEM desktop or floppy boot:''' Same as ST.
| Completely dead (no video, no drive activity, power LED off) || No PSU output; blown fuse; disconnected power connector || Verify mains power reaching PSU. Check PSU output voltages. Check power connector is seated on motherboard.
|-
| Power LED on but completely dead (no video, no drive seek) || CPU not running; /HALT or /RESET held low; failed GST MCU; failed CPU; no clock || Check reset circuit: pin 4 of 7406N should show clean reset pulse (~200 ms low then high). Check 8 MHz clock at CPU pin 15. If clock is present but CPU does not run, suspect CPU or GST MCU.
|-
| Power LED on, floppy seeks, no video || GST Shifter failure; video output circuit fault; monitor/cable issue || Try different monitor. Check 32 MHz video oscillator for output. Check GST Shifter chip (C300588/C301712). Verify Shifter is receiving 16 MHz from oscillator.
|-
| White/colored screen, no boot (no GEM) || TOS ROM failure; ROM socket contact oxidation; wrong ROM jumper configuration (W102/W103/W104) || Re-seat ROM chips. Clean ROM socket contacts. Verify jumper configuration matches installed ROM type. Try known-good TOS ROMs.
|-
| Boot to desktop with corrupted display ("garbage" or missing elements) || RAM fault; GST Shifter fault || Run RAM diagnostic (e.g. from cartridge). Try reseating or replacing SIMMs. If RAM tests pass, suspect GST Shifter.
|-
| Bombs on boot (row of bomb icons) || Software exception in TOS; hardware fault triggering bus error or address error || 2 bombs = bus error (often bad RAM or address line fault). 3 bombs = address error (CPU or bus issue). 4 bombs = illegal instruction (ROM corruption). See "Bomb Code Reference" below.
|-
| Boot to desktop, then random crashes || Marginal PSU; intermittent RAM fault; overheating CPU or custom IC || Check PSU voltages under load. Run extended RAM test. Check for thermal issues (touch test ICs โ€” none should be too hot to hold finger on for 3 seconds).
|}


=== Bomb Code Reference ===
'''STE-specific boot symptoms:'''
* '''White screen for ~3 seconds then desktop appears:''' Normal behaviour on TOS 2.06 (RAM test). Not a fault.
* '''White screen indefinitely:''' RAM fault, ROM fault, GST MCU failure. The additional ~3 seconds of RAM test on TOS 2.06 means users may mistake a slow boot for a hang โ€” wait at least 30 seconds before diagnosing a fault.
* '''Intermittent boot (sometimes works, sometimes white screen):''' Reset circuit capacitor failure โ€” see [[Atari STE General Maintenance#STE-Specific Reset Issue|STE General Maintenance โ€” Reset Issue]].


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== Power Supply Diagnosis ==
{| class="wikitable styled-table"
! Bombs !! Exception !! Common Cause
|-
| 2 || Bus Error || Bad RAM; faulty address decoding (GST MCU); accessing non-existent hardware
|-
| 3 || Address Error || Word/long access to odd address; CPU or bus fault
|-
| 4 || Illegal Instruction || Corrupted TOS ROM; software incompatibility
|-
| 5 || Zero Divide || Software bug (rare hardware cause)
|-
| 6 || CHK Exception || Software bug
|-
| 7 || TRAPV || Software bug
|-
| 8 || Privilege Violation || Software attempting supervisor instruction in user mode; TOS bug (known issue with 68030 compatibility code in TOS 1.06/1.62)
|}


== Video Troubleshooting ==
The STE uses similar internal PSU models to the 1040ST. Voltage rails, testing procedures, and common symptoms are the same as described in [[Atari ST Troubleshooting Guide#Power Supply Diagnosis|ST Troubleshooting Guide โ€” Power Supply Diagnosis]].


=== RGB Output ===
The PSU types found in the STE are typically the same ASTEC, Mitsumi, and DVE models used in the 1040ST. See [[Atari STE Capacitor Replacement Guide]] for PSU cap lists.


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'''STE-specific PSU note:''' The STE draws slightly more current than the 1040ST due to the blitter, DMA sound hardware, and additional port circuitry. A marginal PSU that works in a 1040ST may still fail in an STE. Recapping the PSU is strongly recommended.
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No video output at all || GST Shifter failure; no 32 MHz clock; failed video output transistors || Check 32 MHz oscillator output. Check GST Shifter supply voltages (+5 V at pin 64). Check video output buffer transistors near the monitor connector.
|-
| Missing color channel (e.g. no red) || Failed video output transistor for that channel; broken trace; monitor cable fault || Test with known-good cable. Trace signal from Shifter output pin through buffer transistor to DIN-13 connector. Check for cold solder joints on monitor connector.
|-
| Wavy/unstable picture || PSU ripple; ground loop with monitor; failing filter capacitors || Check +5 V rail ripple. Recap PSU if ripple exceeds 300 mV. Try different PSU. Check ground continuity.
|-
| Picture rolls vertically || Wrong V-sync frequency (50/60 Hz mismatch with monitor); sync circuit fault || Verify sync register setting at $FF820A (bit 1: 0=60 Hz, 1=50 Hz). Check monitor accepts the frequency. If sync is correct, suspect GLUE portion of GST MCU.
|-
| Jailbar pattern (vertical stripes) || RAM noise coupling into video; marginal RAM timing || Re-seat SIMMs. Try different SIMM brand. Check bulk decoupling cap C103 (4700 ยตF 16 V).
|-
| Only 512 colors instead of 4096 || Software writing only 3 bits per channel (ST software, not STE-aware) || Normal for ST software. STE-specific software or programming is needed to access the extra palette bit.
|}


=== Monochrome Output ===
== Display / Video Troubleshooting ==


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The STE video system uses the GST Shifter, which has some different failure modes from the original ST Shifter.
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No mono video || Mono detect pin 4 not grounded (SM124 cable issue); GST Shifter fault || Check cable grounds pin 4 of DIN-13 to signal ground. Test with known-good SM124 cable. Check mono video output on DIN-13 pin 11.
|-
| Mono display with flicker or shimmer || SM124/SM144 monitor fault; failing GST Shifter || Try different monochrome monitor. If problem persists, suspect Shifter.
|}
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== DMA Sound Troubleshooting ==
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DMA sound issues are among the most common faults on STE machines and involve the GST Shifter, 74F374 DAC latches, LMC1992 mixer, and associated passive components.


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! Symptom !! Possible Cause !! Recommended Action
! Symptom !! Possible Cause !! Recommended Action
|-
|-
| No DMA sound at all (YM2149 PSG works) || SCLK oscillator (8 MHz) not running; GST Shifter DMA section fault; LMC1992 failure; disconnected RCA output wiring || Check 8 MHz SCLK oscillator output. Test with DMA sound test program (fenarinarsa's ste_dmatest). Check LMC1992 is receiving power. Verify RCA jack wiring continuity.
| No display, power LED on || CPU not running; GST MCU failure; clock failure || Check 32 MHz crystal. The GST MCU generates all clocks โ€” if it fails, nothing runs. Check for 8 MHz on CPU CLK pin with oscilloscope.
|-
|-
| DMA sound crackles/pops || 74F374 DAC latch noise (ground bounce from fast switching) || Replace both 74F374 ICs with 74LS374 equivalents. Alternatively, add short ground wires between DAC IC ground pins and nearby ground points. If using 74F374, lift VCC pin and reconnect via 40 ฮฉ resistor.<ref>{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=5682 |title=STE Sound DAC Crackle & Pop Fix |publisher=exxos Forum |access-date=2026-03-27}}</ref>
| White screen (indefinite) || RAM fault (try swapping/replacing SIMMs); TOS ROM fault; GST MCU failure || Swap SIMMs with known-good modules. Try known-good TOS ROMs. If neither helps, GST MCU may be faulty (difficult repair โ€” requires SMD rework).
|-
|-
| High-pitched whistle on DMA audio || Degraded coupling capacitors C202, C204, C205 (4.7 ยตF 35 V NP); PSU noise; missing RF shielding || Replace C202, C204, C205 with fresh non-polarized electrolytics or film caps. Recap PSU. Ensure RF metal shielding is fitted over the motherboard.<ref>{{cite web |url=https://www.exxosforum.co.uk/atari/last/STEDACFIX/index.htm |title=STE DAC Audio Noise Fix |publisher=exxos โ€” The LaST Upgrade |access-date=2026-03-27}}</ref>
| Corrupted display / garbled colours || GST Shifter failure; SIMM contact oxidation; motherboard cap failure near Shifter || Clean SIMM contacts. Reseat GST Shifter (if socketed). Recap motherboard.
|-
|-
| YM sound too loud relative to DMA || LMC1992 mixer wiring error (AUDIO2 path tied to ground) || Apply P.Putnik's LMC mixer fix: add 10 kฮฉ + 47 kฮฉ resistor network to restore the โˆ’12 dB YM attenuation path on LMC1992 pins 4/5/25/26.<ref>{{cite web |url=https://temlib.org/AtariForumWiki/index.php/Atari_STE_and_Mega_STE_sound_fixes |title=Atari STE and Mega STE sound fixes |publisher=AtariForumWiki |access-date=2026-03-27}}</ref>
| Enhanced colour palette not working (only 512 colours instead of 4096) || Software not using STE colour registers; GST Shifter fault || Test with known STE-specific software. If STE colour features never work, GST Shifter or bus fault.
|-
|-
| DMA sound distorted or clipped || RCA output level too high for connected amplifier || Add external voltage divider on RCA outputs: 1 kฮฉ in series followed by 150 ฮฉ to ground (approximately 6ร— attenuation). This can be fitted inside an RCA plug.
| Screen jitter / unstable sync || GST MCU sync generation fault; PSU ripple; failing master clock || Check PSU voltages and ripple. Check 32 MHz crystal. If stable with recapped PSU, suspect GST MCU.
|-
|-
| Microwire volume/tone controls have no effect || LMC1992 not receiving Microwire data; LMC1992 failure || Verify Microwire clock and data signals at $FF8922/$FF8924 with oscilloscope. If signals are present but controls non-functional, replace LMC1992.
| Display corruption only when scrolling hardware is used || GST MCU hardware scroll registers faulty || Test with STE-specific demos that use hardware scrolling. If non-scrolling software works fine, GST MCU has partial failure in scroll logic.
|-
| One stereo channel missing || Failed DAC latch (74F374) for that channel; broken trace; LMC1992 channel fault || Signal-trace from GST Shifter through DAC latch to LMC1992 input on the affected channel. Replace the DAC latch IC. Check coupling capacitors.
|}
|}


== Enhanced Joystick Port Problems ==
== RAM Fault Diagnosis (STE-Specific) ==


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The STE's SIMM-based RAM makes diagnosis and replacement much easier than the soldered RAM on the ST:
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Enhanced port completely dead || GST MCU failure (port logic is integrated); bent/broken pin in connector || Inspect 15-pin connector for bent pins. If physically intact, likely GST MCU fault โ€” no user-serviceable repair (BGA/QFP rework required).
|-
| Paddle input erratic || Dirty connector contacts; noisy analog input; paddle controller fault || Clean connector contacts with DeoxIT. Test with known-good controller. Check for noise on +5 V supply to the port.
|-
| Standard joystick works in enhanced port but no analog || Normal โ€” standard joysticks do not provide analog signals || Use STE-specific analog paddles or controllers.
|}
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== RAM (SIMM) Fault Diagnosis ==
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RAM faults produce a wide range of symptoms depending on which bit(s) or address line(s) are affected.


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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
! Symptom !! Likely Cause !! Diagnostic Steps
|-
|-
| Machine reports less RAM than installed || Failed SIMM; poor SIMM contact; SIMM speed too slow || Re-seat SIMMs. Clean SIMM contacts with eraser + IPA. Try known-good SIMMs. Ensure SIMMs are 80 ns or faster.
| White screen at boot || SIMM not seated; SIMM failure; SIMM socket oxidation || Remove and reseat all SIMMs. Clean contacts with pencil eraser and IPA. Try with minimum RAM (2ร— 256 KB in Bank 0). Try known-good SIMMs.
|-
|-
| Random crashes during operation || Intermittent RAM bit failure; marginal SIMM contact || Run extended RAM diagnostic from cartridge (e.g. Atari diagnostic cartridge). Swap SIMMs between sockets to isolate the faulty module.
| Reduced RAM detected || Failed SIMM; mismatched SIMM sizes; dirty contacts || Check TOS RAM counter at boot (TOS 2.06 displays memory count). Swap SIMMs between banks to isolate. Both SIMMs in a bank must be the same size.
|-
|-
| Consistent crash at specific operations || Stuck bit in RAM || RAM test will identify the address and bit. Replace the faulty SIMM.
| Random crashes after warm-up || Marginal SIMM; thermal sensitivity || Run YAART or similar memory tester. Use freeze spray to identify heat-sensitive SIMM.
|-
|-
| 2-bomb error on boot || Bus error during RAM test โ€” likely a completely dead SIMM or address line fault || Remove all SIMMs and install one pair at a time to identify the faulty module. Check SIMM socket solder joints.
| 2 MB detected when 4 MB installed || Bank 1 SIMMs not recognised; SIMM socket fault || Check Bank 1 socket solder joints. Swap Bank 0 and Bank 1 SIMMs. If Bank 1 position never works regardless of SIMMs, suspect socket or GST MCU bank select.
|-
| Garbled display || Video RAM region corrupted; bit fault in first 32 KB || If only video is affected, the fault is in the RAM region used for the screen buffer (default at top of RAM). Move screen base address to test. Replace SIMMs if fault follows.
|}
|}


'''520STE note:''' With only 2 of 4 SIMM sockets populated, ensure the SIMMs are in the correct pair of sockets (typically the two closest to the edge of the board). Consult the motherboard silkscreen for socket numbering.
== Custom IC Failure Diagnosis (STE-Specific) ==
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'''1040STE note:''' All four sockets populated. When diagnosing, try removing one pair at a time (operating with 512 KB) to isolate the faulty bank.
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== Custom Chip Failures ==


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{| class="wikitable styled-table"
! Chip !! Part Number !! Failure Symptoms !! Notes
! IC !! Failure Symptoms !! Diagnostic Steps !! Notes
|-
| GST MCU || C302183-002 || No boot; no clock outputs; address decoding failures; DMA faults; enhanced joystick ports dead || 144-pin QFP; not user-replaceable without SMD rework station. Common point of failure if machine has suffered power surge.
|-
| GST Shifter || C300588 / C301712-001 || No video; corrupted video; no DMA sound; palette faults || Socketed on many boards (DIP-64); replaceable. Two known versions โ€” some boards have compatibility issues with certain Shifter revisions.
|-
| BLiTTER || C302014 || BLiTTER operations fail (software falls back to CPU); some games crash || PLCC-68; can be removed to test without BLiTTER (TOS falls back to CPU blitting). On later combined MCU boards, integrated and not separately replaceable.
|-
|-
| MC68000 CPU || MC68000FN8 / MC68000P8 || No boot; illegal instruction bombs; random crashes || PLCC-68 (socketed) or DIP-64. Relatively available as replacement parts. Test by substitution.
| '''GST MCU''' (U400) || Machine completely dead; no clocks generated; white screen; RAM not detected; no interrupts; enhanced joystick ports non-functional; DMA sound addressing fails || Check for clock outputs: 8 MHz on CPU CLK, 2 MHz on YM2149, 500 kHz on ACIAs. If all clocks are absent, GST MCU is likely faulty. || This is a 144-pin surface-mount chip. Replacement requires professional SMD rework equipment. A failed GST MCU is often uneconomical to repair โ€” a donor board may be the best source.
|-
|-
| MC68901 MFP || MC68901 || No interrupts; no serial I/O; timer-dependent functions fail; keyboard unresponsive || DIP-48; available as replacement. If keyboard works but RS-232 does not (or vice versa), check the specific MFP I/O port rather than replacing.
| '''GST Shifter''' || No display output; garbled video; DMA sound plays at wrong rate or distorted; mono output works but colour doesn't (or vice versa) || Check video output on monitor port. Check SCLK (8 MHz sound clock from separate oscillator). If SCLK is present but DMA sound fails, Shifter is suspect. || On boards where the Shifter is socketed (DIP-40), it can be replaced. On boards with SMD Shifter, professional rework is required.
|-
|-
| DMA Controller || C025913-38 || No floppy access; no ACSI hard disk access || DIP-40; relatively rare failure.
| '''BLiTTER''' || Software that uses blitter crashes; GEM desktop slow (no blitter acceleration); blitter-specific tests in diagnostic cartridge fail || Remove blitter from PLCC socket and test without it โ€” the ST will function without a blitter (just slower). If removing it resolves crashes, blitter is faulty. || PLCC-68 socket allows easy removal/replacement.
|-
|-
| WD1772 FDC || WD1772-02-02 || No floppy disk access; read/write errors; disk not recognized || DIP-28; test with known-good disk first. Available as replacement.
| '''LMC1992''' (Sound Mixer) || No DMA sound output despite Shifter appearing to work; volume/bass/treble controls have no effect; or sound is distorted/unbalanced || The LMC1992 is controlled via the Microwire bus. Test with software that directly accesses the Microwire registers. If the mixer doesn't respond, it may be faulty or the Microwire bus from GST MCU is broken. || Surface-mounted chip. Can be replaced with SMD soldering skills.
|-
|-
| YM2149F || YM2149F || No PSG sound; printer port non-functional; control line failures || DIP-40; dual-function IC (sound + I/O). If printer is dead but sound works (or vice versa), check associated traces before replacing.
| '''WD1772''' || Same symptoms as ST family || Same diagnostic steps || See [[Atari ST Troubleshooting Guide#Floppy Drive Troubleshooting|ST Troubleshooting Guide โ€” Floppy]].
|-
|-
| MC6850 ACIA (keyboard) || MC6850 || Keyboard completely dead; mouse non-functional || DIP-24; test by substitution (two ACIAs on board โ€” swap keyboard and MIDI ACIAs to isolate).
| '''YM2149F''' || Same symptoms as ST family (no PSG sound, printer not working, floppy drive select not toggling) || Same diagnostic steps || See [[Atari ST Troubleshooting Guide#Custom IC Failure Diagnosis|ST Troubleshooting Guide โ€” Custom IC Diagnosis]].
|-
|-
| MC6850 ACIA (MIDI) || MC6850 || MIDI In/Out non-functional || DIP-24; same IC as keyboard ACIA โ€” swap to confirm.
| '''MC68901 / MC6850 / MC68000''' || Same symptoms as ST family || Same diagnostic steps || See [[Atari ST Troubleshooting Guide]].
|-
| LMC1992 || LMC1992 || No audio output; volume/tone controls non-functional || DIP-28; specific to STE. Source from STE donor boards or specialist Atari parts suppliers.
|}
|}


== Floppy Drive Troubleshooting ==
== DMA Sound Troubleshooting (STE-Specific) ==
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The STE's DMA stereo sound system is handled by the GST Shifter and GST MCU together, with the LMC1992 mixer chip controlling output levels:


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! Symptom !! Possible Cause !! Recommended Action
! Symptom !! Possible Cause !! Recommended Action
|-
|-
| Drive does not spin disks || Drive motor failure; drive belt (if belt-driven) broken/stretched; +12 V rail absent || Check +12 V at drive power connector. Test drive motor separately. Replace belt if applicable.
| No DMA sound, PSG (YM2149) sound works || GST Shifter not outputting DMA audio; LMC1992 mixer fault; 8 MHz SCLK oscillator failure || Check the separate 8 MHz oscillator near the GST Shifter (not the main 32 MHz crystal). If SCLK is dead, DMA sound will not function. Replace oscillator. Check LMC1992.
|-
|-
| Drive seeks but cannot read || Dirty heads; head misalignment; WD1772 fault; bad disk || Clean heads. Try multiple known-good disks. Test WD1772 by substitution.
| DMA sound plays but is distorted or at wrong speed || GST Shifter DMA rate registers incorrect (software issue); SCLK frequency incorrect || Verify with multiple programs that use DMA sound. If all are wrong, check SCLK oscillator frequency with oscilloscope (should be 8.000 MHz).
|-
|-
| Reads only single-sided disks || Drive head 1 (lower) failed; head select signal fault || Check drive head select signal. Try known-good drive mechanism.
| DMA sound only on one channel || LMC1992 balance setting incorrect; failed channel in LMC1992; broken trace on one audio channel || Check software mixer settings. If stereo balance is centred but only one channel works, suspect LMC1992 or trace fault. Test with headphones on the audio output.
|-
|-
| Write errors / cannot format || Write protect detect stuck; write head fault; WD1772 write circuit fault || Check write protect switch on disk. Clean write protect sensor. Try formatting a fresh disk.
| No sound at all (neither PSG nor DMA) || Audio output path fault; failed coupling capacitor in audio circuit; monitor cable fault || Check audio output on pin 1 of 13-pin DIN. If no signal present at all, check output caps (C216 area). If signal present, check cable and monitor/amplifier.
|-
| Drive LED stays on permanently || WD1772 hung; DMA controller fault; GST MCU fault || Reset machine. If persists, check WD1772 INTRQ and DRQ signals. Check DMA controller.
|}
|}


== MIDI Troubleshooting ==
== Enhanced Joystick Port Troubleshooting ==
ย 
The STE adds two enhanced (active) joystick ports (DE-15 connectors) on the left side of the case, compatible with Team Tap multitaps and Jaguar controllers:


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! Symptom !! Possible Cause !! Recommended Action
! Symptom !! Possible Cause !! Recommended Action
|-
|-
| MIDI Out dead || MC6850 ACIA (MIDI) failure; output driver transistor/buffer fault; DIN connector solder joint || Check solder joints on MIDI DIN connectors. Verify ACIA output at the IC pin. Test with loopback (connect OUT to IN and echo test).
| Enhanced ports not responding || GST MCU port logic failure; software not using STE joystick registers || Test with known STE-specific software. If standard joystick ports work but enhanced ports never respond, GST MCU port circuitry may be faulty.
|-
|-
| MIDI In dead || MC6850 ACIA failure; opto-coupler (6N138 or equivalent) failure || Check opto-coupler. Test ACIA by swapping with keyboard ACIA.
| Erratic input on enhanced ports || Bad cable or connector; interference from other peripherals || Clean connector. Try a different controller. Some early STE software has compatibility issues with the enhanced ports โ€” verify with multiple programs.
|-
|-
| Intermittent MIDI data || Cable fault; marginal ACIA; noisy +5 V supply || Try different MIDI cable. Check +5 V rail stability. Re-seat ACIA IC.
| Standard joystick ports still work || This is expected โ€” enhanced and standard ports are separate systems || Not a fault. Standard ports are handled by the IKBD controller via ACIA; enhanced ports are handled directly by the GST MCU.
|-
| MIDI timing unstable || 500 kHz ACIA clock drift (unlikely with crystal); software sequencer timing issue || Verify 500 kHz clock at ACIA clock pin. Usually a software issue rather than hardware.
|}
|}


== Diagnostic Tools ==
== Software Compatibility Issues ==
ย 
The STE has some known software compatibility problems that can be mistaken for hardware faults:
ย 
* '''Some ST games crash or behave incorrectly on the STE.''' This is caused by programs that access hardware registers directly (bypassing TOS) and encountering the different register layout of the GST MCU/Shifter. This is a software issue, not a hardware fault.
* '''Joystick plugged in causes strange behaviour in some applications.''' Notably, the word processor 1st Word Plus can behave erratically with a joystick connected to the enhanced ports. Remove joysticks from enhanced ports when running non-STE software.
* '''TOS 1.62 has known bugs.''' Upgrading to TOS 2.06 resolves many stability issues and adds features including 1.44 MB floppy support and improved hard drive compatibility.
ย 
== Diagnostic Flowchart (STE-Specific) ==


The following tools are useful for STE troubleshooting:
# '''Check PSU:''' Measure +5V and +12V under load. Recap if necessary.
# '''Check SIMMs:''' Remove and reseat all SIMMs. Clean contacts. Try known-good modules.
# '''Reseat all socketed ICs:''' CPU (if DIP), TOS ROMs, WD1772, YM2149, MFP, ACIAs, BLiTTER.
# '''Try diagnostic cartridge:''' Works the same as on the ST. Serial output is available for headless diagnosis.
# '''Check TOS ROMs:''' Swap with known-good set. Upgrade to TOS 2.06 if on TOS 1.62.
# '''Check clocks:''' Verify 32 MHz master crystal and 8 MHz SCLK oscillator.
# '''Check GST MCU/Shifter:''' If all above are good, fault is likely in the surface-mount custom ICs. Inspect solder joints under magnification. Professional reflow may be required.


* '''Atari Diagnostic Cartridge''' โ€” Tests RAM, ROM, chip timing, BLiTTER, display, keyboard, mouse, joystick, floppy, DMA, MIDI, serial, and parallel ports. Requires a cartridge ROM image burned to a 128 KB EPROM.
== Related Pages ==
* '''fenarinarsa's STE DMA Sound Test''' โ€” Specifically designed to diagnose DMA audio crackling and popping issues. Available at https://github.com/fenarinarsa/ste_dmatest/releases/
* '''Multimeter''' โ€” Essential for voltage checks, continuity testing, and capacitor ESR measurement (with appropriate meter).
* '''Oscilloscope''' โ€” Required for PSU ripple measurement, clock signal verification, and signal tracing.
* '''Logic probe''' โ€” Quick check for clock signals, reset lines, and chip select activity.
* '''Loopback plugs''' โ€” For testing serial (RS-232) and MIDI ports in self-test mode.


== See Also ==
* [[Atari 520STE]]
* [[Atari 1040STE]]
* [[Atari 1040STE]]
* [[Atari STE General Maintenance]]
* [[Atari STE General Maintenance]]
* [[Atari STE Capacitor Replacement Guide]]
* [[Atari STE Capacitor Replacement Guide]]
* [[Atari ST Troubleshooting Guide]]


== References ==
== References ==
<references />
<references/>


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{{Navbox-AtariVintageComputers}}


[[Category:Atari Troubleshooting Guides]]
[[Category:Atari Troubleshooting Guides]]
[[Category:Atari Systems]]

Latest revision as of 17:21, 29 March 2026

This guide covers fault diagnosis and repair for the Atari 520STE and Atari 1040STE computers. Both models use the same motherboard (CA401177 or CA4003290) with the only hardware difference being RAM population via SIMMs. The STE uses a different custom chipset from the earlier ST/STF/STFM range, so while many troubleshooting principles are shared, the specific ICs and failure modes differ.

For the earlier Atari ST/STF/STFM range (520ST, 1040ST), see Atari ST Troubleshooting Guide.

STE Custom Chipset Overview

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The STE replaced the separate GLUE and MMU chips with a single combined IC, and introduced a new Shifter with DMA sound capabilities:

IC Designation Package Function
GST MCU U400 (C300720 / C300589) 144-pin QFP (surface-mount) Combined GLUE + MMU: bus decode, address generation, DRAM control, video DMA addressing, DMA sound address counter, enhanced joystick/paddle/lightpen ports, clock generation
GST Shifter (various part numbers) DIP-40 or QFP (varies by board revision) Video pixel serialisation, RGB output, DMA stereo sound output (8-bit PCM at 6258/12517/25033/50066 Hz)
BLiTTER Atari BLiTTER PLCC-68 Hardware block transfer for graphics acceleration
DMA Controller C025913 DIP-40 ACSI/floppy DMA (same as ST range)
WD1772 Floppy controller DIP-28 Same as ST range
YM2149F PSG sound + PIO DIP-40 Legacy sound; also controls drive select, printer port, RS-232 handshake lines
LMC1992 Sound mixer SMD Microwire-controlled stereo mixer for volume, bass, treble, balance adjustment
MC68901 MFP DIP-48 Timers, interrupts, RS-232 UART
MC6850 (ร—2) ACIAs DIP-24 Keyboard/MIDI serial interfaces

Boot Sequence

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The STE boot sequence is similar to the ST, with some additions:

  1. Power-on: PSU provides +5V and +12V. GST MCU holds the 68000 in reset.
  2. Reset release: CPU begins executing from TOS ROM space.
  3. TOS 2.06 RAM test: If running TOS 2.06, a full RAM test is performed at boot. This takes approximately 3 seconds and can be aborted by pressing any key. The screen remains white during this test. On TOS 1.62, the RAM test is briefer.
  4. GEM desktop or floppy boot: Same as ST.

STE-specific boot symptoms:

  • White screen for ~3 seconds then desktop appears: Normal behaviour on TOS 2.06 (RAM test). Not a fault.
  • White screen indefinitely: RAM fault, ROM fault, GST MCU failure. The additional ~3 seconds of RAM test on TOS 2.06 means users may mistake a slow boot for a hang โ€” wait at least 30 seconds before diagnosing a fault.
  • Intermittent boot (sometimes works, sometimes white screen): Reset circuit capacitor failure โ€” see STE General Maintenance โ€” Reset Issue.

Power Supply Diagnosis

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The STE uses similar internal PSU models to the 1040ST. Voltage rails, testing procedures, and common symptoms are the same as described in ST Troubleshooting Guide โ€” Power Supply Diagnosis.

The PSU types found in the STE are typically the same ASTEC, Mitsumi, and DVE models used in the 1040ST. See Atari STE Capacitor Replacement Guide for PSU cap lists.

STE-specific PSU note: The STE draws slightly more current than the 1040ST due to the blitter, DMA sound hardware, and additional port circuitry. A marginal PSU that works in a 1040ST may still fail in an STE. Recapping the PSU is strongly recommended.

Display / Video Troubleshooting

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The STE video system uses the GST Shifter, which has some different failure modes from the original ST Shifter.

Symptom Possible Cause Recommended Action
No display, power LED on CPU not running; GST MCU failure; clock failure Check 32 MHz crystal. The GST MCU generates all clocks โ€” if it fails, nothing runs. Check for 8 MHz on CPU CLK pin with oscilloscope.
White screen (indefinite) RAM fault (try swapping/replacing SIMMs); TOS ROM fault; GST MCU failure Swap SIMMs with known-good modules. Try known-good TOS ROMs. If neither helps, GST MCU may be faulty (difficult repair โ€” requires SMD rework).
Corrupted display / garbled colours GST Shifter failure; SIMM contact oxidation; motherboard cap failure near Shifter Clean SIMM contacts. Reseat GST Shifter (if socketed). Recap motherboard.
Enhanced colour palette not working (only 512 colours instead of 4096) Software not using STE colour registers; GST Shifter fault Test with known STE-specific software. If STE colour features never work, GST Shifter or bus fault.
Screen jitter / unstable sync GST MCU sync generation fault; PSU ripple; failing master clock Check PSU voltages and ripple. Check 32 MHz crystal. If stable with recapped PSU, suspect GST MCU.
Display corruption only when scrolling hardware is used GST MCU hardware scroll registers faulty Test with STE-specific demos that use hardware scrolling. If non-scrolling software works fine, GST MCU has partial failure in scroll logic.

RAM Fault Diagnosis (STE-Specific)

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The STE's SIMM-based RAM makes diagnosis and replacement much easier than the soldered RAM on the ST:

Symptom Likely Cause Diagnostic Steps
White screen at boot SIMM not seated; SIMM failure; SIMM socket oxidation Remove and reseat all SIMMs. Clean contacts with pencil eraser and IPA. Try with minimum RAM (2ร— 256 KB in Bank 0). Try known-good SIMMs.
Reduced RAM detected Failed SIMM; mismatched SIMM sizes; dirty contacts Check TOS RAM counter at boot (TOS 2.06 displays memory count). Swap SIMMs between banks to isolate. Both SIMMs in a bank must be the same size.
Random crashes after warm-up Marginal SIMM; thermal sensitivity Run YAART or similar memory tester. Use freeze spray to identify heat-sensitive SIMM.
2 MB detected when 4 MB installed Bank 1 SIMMs not recognised; SIMM socket fault Check Bank 1 socket solder joints. Swap Bank 0 and Bank 1 SIMMs. If Bank 1 position never works regardless of SIMMs, suspect socket or GST MCU bank select.

Custom IC Failure Diagnosis (STE-Specific)

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IC Failure Symptoms Diagnostic Steps Notes
GST MCU (U400) Machine completely dead; no clocks generated; white screen; RAM not detected; no interrupts; enhanced joystick ports non-functional; DMA sound addressing fails Check for clock outputs: 8 MHz on CPU CLK, 2 MHz on YM2149, 500 kHz on ACIAs. If all clocks are absent, GST MCU is likely faulty. This is a 144-pin surface-mount chip. Replacement requires professional SMD rework equipment. A failed GST MCU is often uneconomical to repair โ€” a donor board may be the best source.
GST Shifter No display output; garbled video; DMA sound plays at wrong rate or distorted; mono output works but colour doesn't (or vice versa) Check video output on monitor port. Check SCLK (8 MHz sound clock from separate oscillator). If SCLK is present but DMA sound fails, Shifter is suspect. On boards where the Shifter is socketed (DIP-40), it can be replaced. On boards with SMD Shifter, professional rework is required.
BLiTTER Software that uses blitter crashes; GEM desktop slow (no blitter acceleration); blitter-specific tests in diagnostic cartridge fail Remove blitter from PLCC socket and test without it โ€” the ST will function without a blitter (just slower). If removing it resolves crashes, blitter is faulty. PLCC-68 socket allows easy removal/replacement.
LMC1992 (Sound Mixer) No DMA sound output despite Shifter appearing to work; volume/bass/treble controls have no effect; or sound is distorted/unbalanced The LMC1992 is controlled via the Microwire bus. Test with software that directly accesses the Microwire registers. If the mixer doesn't respond, it may be faulty or the Microwire bus from GST MCU is broken. Surface-mounted chip. Can be replaced with SMD soldering skills.
WD1772 Same symptoms as ST family Same diagnostic steps See ST Troubleshooting Guide โ€” Floppy.
YM2149F Same symptoms as ST family (no PSG sound, printer not working, floppy drive select not toggling) Same diagnostic steps See ST Troubleshooting Guide โ€” Custom IC Diagnosis.
MC68901 / MC6850 / MC68000 Same symptoms as ST family Same diagnostic steps See Atari ST Troubleshooting Guide.

DMA Sound Troubleshooting (STE-Specific)

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The STE's DMA stereo sound system is handled by the GST Shifter and GST MCU together, with the LMC1992 mixer chip controlling output levels:

Symptom Possible Cause Recommended Action
No DMA sound, PSG (YM2149) sound works GST Shifter not outputting DMA audio; LMC1992 mixer fault; 8 MHz SCLK oscillator failure Check the separate 8 MHz oscillator near the GST Shifter (not the main 32 MHz crystal). If SCLK is dead, DMA sound will not function. Replace oscillator. Check LMC1992.
DMA sound plays but is distorted or at wrong speed GST Shifter DMA rate registers incorrect (software issue); SCLK frequency incorrect Verify with multiple programs that use DMA sound. If all are wrong, check SCLK oscillator frequency with oscilloscope (should be 8.000 MHz).
DMA sound only on one channel LMC1992 balance setting incorrect; failed channel in LMC1992; broken trace on one audio channel Check software mixer settings. If stereo balance is centred but only one channel works, suspect LMC1992 or trace fault. Test with headphones on the audio output.
No sound at all (neither PSG nor DMA) Audio output path fault; failed coupling capacitor in audio circuit; monitor cable fault Check audio output on pin 1 of 13-pin DIN. If no signal present at all, check output caps (C216 area). If signal present, check cable and monitor/amplifier.

Enhanced Joystick Port Troubleshooting

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The STE adds two enhanced (active) joystick ports (DE-15 connectors) on the left side of the case, compatible with Team Tap multitaps and Jaguar controllers:

Symptom Possible Cause Recommended Action
Enhanced ports not responding GST MCU port logic failure; software not using STE joystick registers Test with known STE-specific software. If standard joystick ports work but enhanced ports never respond, GST MCU port circuitry may be faulty.
Erratic input on enhanced ports Bad cable or connector; interference from other peripherals Clean connector. Try a different controller. Some early STE software has compatibility issues with the enhanced ports โ€” verify with multiple programs.
Standard joystick ports still work This is expected โ€” enhanced and standard ports are separate systems Not a fault. Standard ports are handled by the IKBD controller via ACIA; enhanced ports are handled directly by the GST MCU.

Software Compatibility Issues

[edit | edit source]

The STE has some known software compatibility problems that can be mistaken for hardware faults:

  • Some ST games crash or behave incorrectly on the STE. This is caused by programs that access hardware registers directly (bypassing TOS) and encountering the different register layout of the GST MCU/Shifter. This is a software issue, not a hardware fault.
  • Joystick plugged in causes strange behaviour in some applications. Notably, the word processor 1st Word Plus can behave erratically with a joystick connected to the enhanced ports. Remove joysticks from enhanced ports when running non-STE software.
  • TOS 1.62 has known bugs. Upgrading to TOS 2.06 resolves many stability issues and adds features including 1.44 MB floppy support and improved hard drive compatibility.

Diagnostic Flowchart (STE-Specific)

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  1. Check PSU: Measure +5V and +12V under load. Recap if necessary.
  2. Check SIMMs: Remove and reseat all SIMMs. Clean contacts. Try known-good modules.
  3. Reseat all socketed ICs: CPU (if DIP), TOS ROMs, WD1772, YM2149, MFP, ACIAs, BLiTTER.
  4. Try diagnostic cartridge: Works the same as on the ST. Serial output is available for headless diagnosis.
  5. Check TOS ROMs: Swap with known-good set. Upgrade to TOS 2.06 if on TOS 1.62.
  6. Check clocks: Verify 32 MHz master crystal and 8 MHz SCLK oscillator.
  7. Check GST MCU/Shifter: If all above are good, fault is likely in the surface-mount custom ICs. Inspect solder joints under magnification. Professional reflow may be required.
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References

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