Josh at 16:13, 29 March 2026

2026-03-29T16:13:09Z

← Older revision		Revision as of 17:13, 29 March 2026
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	This guide covers fault diagnosis and repair for the '''Atari 520ST''', '''520STM''', '''520STF''', '''520STFM''', '''[[Atari 1040ST\|1040ST]]''', '''1040STF''', and '''1040STFM''' computers. These machines share common motherboard architectures (including the C070789-001 used in both 520STFM and 1040STFM) and the same custom IC set, so troubleshooting procedures are largely identical. Differences between the 520ST and 1040ST (principally RAM population and PSU type) are noted where relevant.		This guide covers fault diagnosis and repair for the '''Atari [[Atari 520ST\|520ST]]''', '''[[Atari 520STM\|520STM]]''', '''[[Atari 520STF\|520STF]]''', '''[[Atari 520STFM\|520STFM]]''', '''[[Atari 1040ST\|1040ST]]''', '''[[Atari 1040STF\|1040STF]]''', and '''[[Atari 1040STFM\|1040STFM]]''' computers. These machines share common motherboard architectures (including the C070789-001 used in both 520STFM and 1040STFM) and the same custom IC set, so troubleshooting procedures are largely identical. Differences between the 520ST and 1040ST (principally RAM population and PSU type) are noted where relevant.

	For the Atari STE range (520STE / 1040STE), see [[Atari STE Troubleshooting Guide]].		For the Atari STE range ([[Atari 520STE\|520STE]] / [[Atari 1040STE\|1040STE]]), see [[Atari STE Troubleshooting Guide]].

	== Prerequisites ==		== Prerequisites ==

Josh: Remove infobox from supplementary page

2026-03-27T19:46:07Z

Remove infobox from supplementary page

← Older revision		Revision as of 20:46, 27 March 2026
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	~~{{Infobox computer~~
	~~\| name = Atari ST Troubleshooting Guide~~
	~~\| image =~~
	~~\| manufacturer = Atari Corporation~~
	~~\| type = Troubleshooting guide~~
	}}

	This guide covers fault diagnosis and repair for the '''Atari 520ST''', '''520STM''', '''520STF''', '''520STFM''', '''[[Atari 1040ST\|1040ST]]''', '''1040STF''', and '''1040STFM''' computers. These machines share common motherboard architectures (including the C070789-001 used in both 520STFM and 1040STFM) and the same custom IC set, so troubleshooting procedures are largely identical. Differences between the 520ST and 1040ST (principally RAM population and PSU type) are noted where relevant.		This guide covers fault diagnosis and repair for the '''Atari 520ST''', '''520STM''', '''520STF''', '''520STFM''', '''[[Atari 1040ST\|1040ST]]''', '''1040STF''', and '''1040STFM''' computers. These machines share common motherboard architectures (including the C070789-001 used in both 520STFM and 1040STFM) and the same custom IC set, so troubleshooting procedures are largely identical. Differences between the 520ST and 1040ST (principally RAM population and PSU type) are noted where relevant.

Josh: Created shared troubleshooting guide covering 520ST and 1040ST

2026-03-27T19:31:44Z

Created shared troubleshooting guide covering 520ST and 1040ST

New page

{{Infobox guide
|title=Atari ST Troubleshooting Guide
|image=
|applies_to=[[Atari 520ST]], [[Atari 1040ST]]
|skill_level=Intermediate to Advanced
|tools_required=Multimeter, oscilloscope (recommended), logic probe, soldering equipment, spare ICs for swap-testing
}}

This guide covers fault diagnosis and repair for the '''[[Atari 520ST]]''' and '''[[Atari 1040ST]]''' computers, including the STF and STFM variants. The 520ST and 1040ST share the same custom chip set (GLUE, MMU, Shifter, DMA), the same support ICs (MC68000, MC68901, MC6850 ACIAs, WD1772, YM2149F), and in many cases the same PCB (the C070789-001 board was used for both 520STFM and 1040STFM). Troubleshooting procedures are therefore identical for both models, with the exception of RAM faults, where the 520ST has eight DRAM chips and the 1040ST has sixteen.

For general preventive maintenance procedures, see [[Atari ST General Maintenance]].

== Before You Begin ==

# '''Check the PSU first.''' The majority of Atari ST faults trace back to a failing or out-of-spec power supply. See [[Atari ST General Maintenance#Power Supply Maintenance]] for voltage testing procedures. Do not proceed with motherboard diagnosis until you have confirmed +5 V is within 4.75–5.25 V under load.
# '''Re-seat all socketed ICs''' — CPU, GLUE, MMU, Shifter, DMA, ROMs, and DRAM (if socketed). Oxidised contacts in chip sockets are an extremely common cause of apparent hardware failure on 35+ year-old machines.
# '''Inspect for obvious damage''' — cracked traces, burned components, electrolyte leakage from capacitors, corrosion around ICs.

== Bomb Error Codes ==

When the Motorola 68000 encounters a hardware exception that TOS cannot handle, it displays a row of small bomb icons on screen. The number of bombs corresponds to the 68000 exception vector number and indicates the type of fault:<ref>{{cite web |url=https://www.atari-forum.com/viewtopic.php?t=15500 |title=TOS ERROR # and Bombs: What they mean |publisher=Atari-Forum |access-date=2026-03-27}}</ref>

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Bombs !! Exception !! Typical Hardware Cause
|-
| 2 || Bus Error || Failed or missing hardware on the bus. RAM fault, ROM fault, bad custom chip (GLUE/MMU not decoding addresses correctly), or failed peripheral IC. Also triggered if attempting to access unmapped address space.
|-
| 3 || Address Error || 68000 attempted a word or long-word access at an odd address. Usually a software bug, but can indicate RAM corruption (bit flip changing a pointer value) or a failing MMU generating incorrect addresses.
|-
| 4 || Illegal Instruction || CPU fetched an invalid opcode. ROM corruption, RAM fault in program code area, or a failing CPU. Can also occur with incorrect TOS ROM installed.
|-
| 5 || Zero Divide || Division by zero. Almost always a software issue, but can be triggered by RAM corruption changing a divisor value.
|-
| 6 || CHK Instruction || CHK instruction range check failed. Typically software.
|-
| 7 || TRAPV || TRAPV overflow trap. Typically software.
|-
| 8 || Privilege Violation || User-mode code attempted a supervisor-only instruction. Software issue or RAM corruption.
|-
| 9 || Trace || Trace exception (single-step). Should not occur during normal operation.
|-
| 10 || Line A Emulator || Unimplemented Line A opcode. Software or ROM issue.
|-
| 11 || Line F Emulator || Unimplemented Line F opcode. Software or ROM issue.
|}

'''Key point:''' 2 bombs (Bus Error) and 4 bombs (Illegal Instruction) on boot are the most common indicators of hardware faults. If bombs appear immediately at power-on, the fault is almost certainly hardware — ROM, RAM, or a custom chip.

== Power Supply Faults ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Machine completely dead, no power LED || Blown mains fuse in power cable or IEC socket; broken power switch; failed internal PSU fuse; dead PSU || Check mains fuse. Test continuity through power switch (on/off positions). Check internal PSU fuse. If all fuses are good and switch has continuity, the PSU main switching circuit has likely failed — recap or replace the PSU.
|-
| Power LED on but no video, no drive activity || +5 V rail absent or severely low; +12 V present (powers LED circuit on some models) || Measure +5 V at the motherboard power connector. If absent or below 4.0 V, the PSU is faulty. Recap or replace.
|-
| Machine boots but crashes randomly || +5 V rail sagging under load, excessive ripple || Measure +5 V with an oscilloscope if possible. Ripple above 200 mV peak-to-peak indicates failing PSU capacitors. Recap the PSU. See [[Atari ST Capacitor Replacement Guide]].
|-
| +5 V reads above 5.5 V || PSU over-voltage, typically from failed feedback components or dried-out capacitors || '''Do not connect to motherboard.''' Recap the PSU. If voltage remains high after recapping, check the feedback optocoupler and voltage divider resistors in the PSU. Some PSUs have an adjustment pot — adjust with a dummy load only.
|-
| +12 V reads very low (<9 V) or very high (>15 V) || Transformer winding ratio issues or failed series diode || On some PSUs a series diode on the +12 V line drops ~1 V. Check for a failed diode (open or shorted). If the transformer secondary is damaged, the PSU needs replacement.
|}

== Boot Sequence Failures ==

The normal 520ST/1040ST boot sequence is:

# Power-on reset (GLUE generates /RESET signal)
# CPU begins executing from ROM (vector table at $000000 points to TOS entry point in ROM space $FC0000–$FEFFFF)
# TOS performs basic hardware initialisation (RAM test on TOS 2.06 only; earlier TOS versions do not test RAM at boot)
# GEM desktop loads and displays the green (or grey, depending on TOS version) desktop with drive icons

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No video output at all (blank screen, no sync signal on monitor) || GLUE not generating clock/sync signals; dead CPU; dead MMU; failed Shifter; no +5 V; ROM completely dead || Confirm +5 V is present. Check for 8 MHz clock on CPU pin 15 (CLK). If no clock, suspect GLUE (C025915) or the 32 MHz oscillator (which is divided to produce the 8 MHz CPU clock and 2 MHz bus clock). If clock is present but CPU /HALT is asserted (pin 17 active low), the CPU has taken a double bus fault — suspect ROM or MMU.
|-
| White or grey screen, no desktop || CPU is running but display initialisation has failed. Typically ROM, RAM, or Shifter fault. || Re-seat ROMs. Swap-test ROMs with known-good set. Check DRAM — a failed DRAM chip can prevent TOS from initialising the display. Swap-test the Shifter (C025914) if available.
|-
| Screen shows "garbage" or random characters at boot || Shifter malfunction; RAM fault causing corrupted video data; MMU generating incorrect video DMA addresses || Swap-test Shifter. Run RAM diagnostics (use a diagnostic cartridge or boot from a diagnostic floppy). Check MMU (C025912).
|-
| 2 bombs immediately on boot || Bus error during TOS initialisation — hardware not responding at an expected address || Re-seat all socketed ICs. Check ROM sockets (corroded contacts cause bus errors when ROM addresses fail). Check DRAM. On the C070789 board, verify that the RAM configuration jumpers (if any) match the installed RAM (8 chips for 520ST, 16 chips for 1040ST).
|-
| 4 bombs immediately on boot || Illegal instruction — CPU executing corrupt data from ROM or RAM || Verify correct TOS ROM set is installed (matched pair, correct HI/LO orientation). Re-seat ROMs. If ROMs are correct, suspect RAM corruption in the low memory area (vector table region $000000–$000400).
|-
| Machine boots to desktop but freezes within seconds || RAM fault (intermittent); PSU voltage marginal; thermal fault in custom chip || Run RAM diagnostics. Check +5 V under load. If the fault appears after warm-up, try cooling individual chips with freeze spray to isolate a thermal intermittent (common with GLUE and MMU).
|}

== Video Troubleshooting ==

The 520ST and 1040ST output video through the 13-pin DIN monitor connector and (on STFM models) through an RF modulator. The Shifter chip (C025914) generates the video signal, the GLUE chip (C025915) generates sync timing, and the MMU (C025912) generates the video DMA addresses to fetch pixel data from RAM.

=== Monitor Detection ===

The ST detects whether a colour or monochrome monitor is connected via pin 4 of the 13-pin DIN connector (active low = monochrome). The SM124 monochrome monitor grounds this pin; colour monitors (SC1224, SC1435, or a SCART-adapted TV) leave it floating or high.

If the wrong monitor type is detected, the ST will boot in the wrong resolution and the display will be unviewable. This is a common issue with homebrew monitor cables that have incorrect wiring.

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No picture on colour monitor, but SM124 mono works fine || Colour video signals (R, G, B on DIN pins 7, 8, 10) not reaching monitor; Shifter analogue output failure; bad cable || Test with a known-good cable. Measure RGB signals at the DIN connector with an oscilloscope (should see ~0.7 V peak-to-peak analogue video). If signals are present at the DIN but not at the monitor, the cable is faulty.
|-
| No picture on SM124 mono monitor || Monochrome output (DIN pin 1) dead; mono detect pin (4) not grounded || Check that pin 4 is grounded by the monitor cable. Measure mono video signal at DIN pin 1. If absent, suspect Shifter.
|-
| Picture rolls vertically || Incorrect sync signal; wrong vertical frequency for the monitor || Verify the monitor matches the region (PAL 50 Hz or NTSC 60 Hz for colour; ~71.2 Hz for mono). If using a modern TV or capture device, confirm it supports the ST's non-standard sync rates.
|-
| Colour fringing, blurry picture on RF output (STFM) || RF modulator failing or mistuned || The RF modulator has a tuning coil — adjustment may improve the picture. However, RF output quality on the ST was never high. Using the RGB monitor output is strongly recommended.
|-
| Distorted or shifted display, wrong colours || Shifter fault; video RAM corruption || Swap-test Shifter. Run RAM diagnostics. Check for cold solder joints on the Shifter socket or DIN connector.
|-
| Every other 16-pixel group is blank or garbled || MMU address generation fault — video DMA is reading incorrect memory locations || Swap-test MMU (C025912). This specific symptom (16-pixel-wide columns of missing data) is characteristic of an MMU fault because the MMU generates the word-aligned video DMA addresses.
|}

== RAM Fault Diagnosis ==

The 520ST uses eight 41256 (256K×1) DRAM chips providing 512 KB. The 1040ST uses sixteen of the same chips providing 1024 KB. On some board revisions, higher-density chips (1M×1 or 256K×4) are used in smaller quantities.

=== Symptoms of RAM Failure ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| 2 bombs on boot || One or more DRAM chips returning incorrect data during TOS initialisation || Run a RAM diagnostic (e.g., from a diagnostic cartridge — the cartridge ROM executes without needing working RAM in the main bank). Note which addresses fail; cross-reference with the board layout to identify the faulty chip.
|-
| Machine detects less RAM than expected (e.g., 1040ST shows 512 KB in system info) || Failed DRAM chips in the upper bank; MMU configuration issue; solder bridge or trace break || Check DRAM chip continuity. On unified boards (C070789), verify that all 16 DRAM positions are populated for 1 MB, or 8 for 512 KB.
|-
| Random crashes during use, especially in memory-intensive applications || Intermittent DRAM fault; marginal cell that passes at cold but fails when warm || Run extended RAM diagnostics that cycle for several hours. Use freeze spray to isolate the failing chip — spray one chip at a time while the test is running to see if the errors shift or stop.
|-
| Corrupted graphics (random pixels, flickering areas) || Video RAM area corrupted — the ST uses main RAM for video (screen memory typically at top of RAM) || Same as above. The specific DRAM chips responsible for the video area can be determined from the board layout and the screen memory base address.
|}

== Custom Chip Failures ==

The four custom Atari ASICs are the most difficult components to replace (no longer manufactured). Diagnosis is typically done by swap-testing with known-good chips from a donor machine.

=== GLUE (C025915-38A) ===

The GLUE chip handles address decoding, interrupt management, and sync/timing generation. GLUE failures can produce a wide range of symptoms:

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No video sync at all (monitor cannot lock on) || GLUE not generating H-sync / V-sync || Check 32 MHz master oscillator output. If present, swap-test GLUE.
|-
| No interrupts firing (keyboard dead, no cursor blink, system hangs after boot) || GLUE interrupt priority encoder or IPL output fault || Swap-test GLUE.
|-
| Bus errors at specific address ranges || GLUE address decode fault — peripheral select signals (ROM, I/O, etc.) not asserting correctly || Swap-test GLUE.
|-
| Machine works intermittently, especially after warming up || Thermal fault in GLUE die || Apply freeze spray to GLUE while fault is present. If machine recovers, GLUE has a thermal defect.
|}

=== MMU (C025912-38) ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No boot, CPU /HALT asserted || MMU not generating DTACK or RAS/CAS for RAM, causing double bus fault || Swap-test MMU.
|-
| Partial or corrupted display (16-pixel columns missing) || Faulty video DMA address generation || Swap-test MMU.
|-
| RAM test fails in a pattern (e.g., every other bank) || Faulty RAS/CAS line for specific DRAM bank || Check RAS/CAS signals with logic probe or oscilloscope. If lines from the MMU are dead or stuck, replace MMU.
|}

=== Shifter (C025914) ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No video output (but system otherwise boots — floppy drive activity is normal, keyboard LEDs respond) || Shifter not serialising video data or analogue output stage dead || Swap-test Shifter.
|-
| Wrong colours, colour palette corrupted || Palette register fault in Shifter || Swap-test Shifter.
|-
| Display works in mono but not in colour (or vice versa) || Partial Shifter failure affecting one video mode || Swap-test Shifter.
|}

=== DMA (C025913) ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Floppy drive does not read or write (drive spins but no data transfer) || DMA chip not transferring data between WD1772 and RAM || Verify WD1772 is functioning (check WD1772 signals with logic probe). If WD1772 appears to operate but data does not appear in RAM, swap-test DMA.
|-
| ACSI hard disk not detected or produces read errors || DMA fault on the ACSI channel || Swap-test DMA. Note: on some boards, a separate ACSI DMA IC may be used.
|-
| Data corruption on disk reads/writes || DMA transferring incorrect data || Swap-test DMA.
|}

== Floppy Drive Troubleshooting ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Drive does not spin at all || +12 V absent (drive motor needs 12 V); floppy cable disconnected or faulty; WD1772 not issuing motor-on signal || Check +12 V at the floppy power connector. Check ribbon cable continuity. If +12 V is present and cable is good, check WD1772.
|-
| Drive spins but cannot read any disks || Dirty or worn head; head alignment out of spec; WD1772 fault; DMA fault || Clean the drive head. Try a known-good disk. Try a different drive. If the replacement drive also fails, suspect WD1772 or DMA.
|-
| Drive reads some disks but not others || Head partially dirty; ageing drive with marginal alignment; disks themselves may be degraded || Clean head. Try the disks in another machine. If the drive consistently fails on disks that work elsewhere, the drive mechanism is likely worn.
|-
| Write operations fail (verify errors) || Write-protect tab set; head dirty; WD1772 write precompensation issue; DMA fault || Check write-protect tab. Clean head. Try a different drive. If the replacement drive also fails on write, suspect DMA or WD1772.
|}

== Keyboard and Mouse Troubleshooting ==

The keyboard assembly contains an HD6301V1 (or compatible, e.g., HD6303) microcontroller that scans the key matrix, reads the mouse quadrature signals, and reads the joystick ports. It communicates with the mainboard via the keyboard ACIA (MC6850) at 7812.5 baud.

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Keyboard completely dead (no keys respond, mouse does not move cursor) || Keyboard ribbon cable disconnected or damaged; keyboard ACIA (MC6850) failed; HD6301 controller in keyboard assembly failed || Re-seat the keyboard ribbon cable. Check for +5 V at the keyboard connector (the keyboard assembly is powered from the mainboard). If +5 V is present and cable is good, swap-test the keyboard ACIA on the mainboard. If ACIA is good, the HD6301 in the keyboard assembly may be dead.
|-
| Some keys do not work || Membrane trace break; dirty contacts; faulty key switch || See [[Atari ST General Maintenance#Keyboard Maintenance]].
|-
| Keys repeat or produce wrong characters || HD6301 ROM corruption (very rare); keyboard matrix short || Check for conductive debris under the keys. If the matrix scan is producing incorrect results, the keyboard PCB may be damaged.
|-
| Mouse works but keyboard does not (or vice versa) || Partial HD6301 failure; ACIA serial link error || The HD6301 handles both keyboard and mouse — a partial failure can affect one but not the other. Swap-test with a known-good keyboard assembly.
|}

== MIDI Port Troubleshooting ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| MIDI OUT does not transmit || MIDI ACIA (MC6850) failed; MIDI OUT buffer/driver circuit fault; broken solder joint on MIDI DIN connector || Check the MIDI ACIA (there are two MC6850s on the board — one for keyboard, one for MIDI). Swap the two ACIAs to test. Check for +5 V at the MIDI ACIA. Inspect solder joints on the MIDI OUT DIN connector.
|-
| MIDI IN does not receive || Opto-isolator (typically 6N138 or CNY17) on MIDI IN circuit failed; MIDI ACIA fault || The MIDI IN circuit uses an opto-isolator for electrical isolation as per the MIDI specification. Test or replace the opto-isolator. Swap-test MIDI ACIA.
|-
| MIDI timing is erratic (notes delayed, jitter) || Excessive interrupt latency; software issue; less commonly, ACIA clock fault || The ST's MIDI timing is generally excellent (hardware ACIA driven at 31.25 kbit/s from a fixed clock). If timing is poor, check for software conflicts. If the problem persists with different software, check the 500 kHz ACIA clock signal.
|}

== Serial and Parallel Port Troubleshooting ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| RS-232 serial port does not communicate || MC68901 MFP USART fault; SN75188/SN75189 (or MC1488/MC1489) line driver/receiver failed; incorrect baud rate or handshaking settings || Check software configuration first. RS-232 requires proper handshaking — many issues are cable/null-modem related. If configuration is correct, check the line driver and receiver ICs (these are standard, cheap, and easy to replace). The MFP is less likely to fail but can be swap-tested.
|-
| Parallel port does not print || Software driver issue; data lines not driven; failed output buffer || Check with different software. Measure data lines (active during a print operation) at the DB-25 connector. Some boards buffer the parallel port through an LS244 or LS245 — check this chip if data lines are dead.
|}

== Diagnostic Tools ==

Several diagnostic aids are available for the 520ST and 1040ST:

* '''Diagnostic cartridges:''' ROM-based test suites that execute from the cartridge port, bypassing the need for working main RAM. Useful for testing RAM, ROM, and basic I/O.
* '''Diagnostic floppy disks:''' Software-based test suites (e.g., "STD TEST" and similar). Require a working boot path (ROM, basic RAM, floppy drive).
* '''Oscilloscope:''' Essential for PSU ripple measurements, clock verification, and signal integrity checks on bus lines.
* '''Logic probe:''' Useful for quick checks on chip select lines, clock signals, and bus activity.
* '''Freeze spray:''' For isolating thermal intermittent faults — spray individual components to cool them and observe whether the fault changes.

== See Also ==

* [[Atari ST General Maintenance]]
* [[Atari ST Capacitor Replacement Guide]]
* [[Atari 520ST]]
* [[Atari 1040ST]]

== References ==
<references />

{{Navbox-AtariVintageComputers}}

[[Category:Atari Troubleshooting Guides]]

← Older revision		Revision as of 17:22, 29 March 2026
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	[[Category:Atari Troubleshooting Guides]]		[[Category:Atari Troubleshooting Guides]]
			[[Category:Atari Systems]]

Atari ST Troubleshooting Guide - Revision history

Josh at 16:22, 29 March 2026

Josh at 16:13, 29 March 2026

Josh: Remove infobox from supplementary page

Josh: Create Atari ST Troubleshooting Guide covering 520ST and 1040ST

Josh: Created shared troubleshooting guide covering 520ST and 1040ST