Atari ST Machine Hub
520ST, 1040STFM & STE workbench reference — "Bombs on Screen" 68000 exception decoder, Mitsumi/ASP PSU recapping, 13-pin DIN RGB/Mono SCART wiring, MIDI & ACSI pinouts, and TOS ROM upgrades.
Internal Switching PSU Recapping & 3.5″ Floppy Drive Maintenance
On all 520STFM, 1040STF, 1040STFM, and 520/1040STE machines, an open-frame switching power supply (typically a Mitsumi SR98, ASP, or Phihong unit) sits under a perforated metal cage at the rear-left of the motherboard. After 35–40 years, dried-out secondary electrolytic capacitors cause severe +5 V ripple, random 2-bomb/4-bomb crashes, wavy jailbars on RGB video, or refusal to cold-boot. Worse still, the primary mains filter stage holds ~325 V DC across its reservoir cap even after unplugging!
1 Internal PSU Rail & Recap Checklist
The 6-pin Molex power header feeds only two DC rails (+5 V and +12 V) to the ST motherboard:
| Molex Pin | Signal | Target Spec |
|---|---|---|
| Pins 1, 2, 3 | GND (Black) | 0.00 V DC Return |
| Pins 4, 5 | +5 V DC (Red) | 4.95–5.15 V DC (@ 3.0 A) |
| Pin 6 | +12 V DC (Yellow/Blue) | 11.6–12.4 V DC (@ 0.9 A — FDD & video amp) |
- Mitsumi SR98 / ASP Recap Priority: Replace all secondary low-ESR caps (typically 2200 μF 16 V, 1000 μF 16 V, and 470 μF 25 V) PLUS the small primary start-up capacitor (22 μF–47 μF 50 V) with 105 °C Nichicon PW / Panasonic FR parts.
- Trim the +5 V Pot (VR1): With the motherboard connected, adjust the small trimmer pot on the PSU PCB so +5.05 V is measured directly across the 68000 CPU VCC/GND pins.
2 External Brick 520ST / 260ST Pinout
Early non-FM 520ST and 260ST models use an external power brick with a 7-pin DIN female plug:
| DIN Pin | Signal | Notes |
|---|---|---|
| Pin 1, 6 | +5 V DC | Regulated +5 V (3.0 A combined) |
| Pin 2, 4, 5 | GND | Common DC ground |
| Pin 3 | −12 V DC | RS-232 serial port negative rail |
| Pin 7 | +12 V DC | RS-232 & video buffer rail |
Modern drop-in MeanWell internal PSU replacements (e.g. Exxos PSU or RD-35A) run cool and add overvoltage crowbar protection.
3 Internal 3.5″ Floppy Drive Care
- Epson SMD-380 / SMD-480 & Sony MP-F110 (Direct Drive vs Belt): Most STFM/STE drives are direct-drive, but early Chinon/Matsushita mechanisms use a flat spindle belt underneath that stretches, causing `Drive A is not responding` errors.
- Sticking Eject Mechanism & Disk Shutter Catch: Clean the sliding metal disk-caddy plate with IPA and apply dry PTFE spray. Never use thick grease, which traps dust and prevents the disk-in microswitch from closing!
- Dirty Track 00 & Write-Protect Microswitches: The two miniature mechanical switches at the front corners of the drive oxidise badly — flood with a drop of DeoxIT and click 30 times if the ST refuses to format or write to disks.
Model Selector & Hardware Revisions
Compare the original 520ST, the all-in-one 520STFM / 1040STFM workhorses, and the multimedia-enhanced 520STE / 1040STE.
520ST / 520STFM & 1040STF / 1040STFM 1985–1989 • 8 MHz 68000 • 512 color Palette
Jack Tramiel's legendary 16/32-bit computer line. The original 520ST had an external PSU and no internal floppy; the STF added an internal 3.5″ floppy and internal PSU; and the STFM added an internal RF TV modulator (`M`). Built around a tightly coupled quartet of Atari custom VLSI chips (GLUE, MMU, Shifter, and DMA) allowing the 68000 CPU and video Shifter to share main RAM with zero wait states via interleaved 4 MHz bus phases.
Core Silicon Architecture
- CPU: Motorola MC68000P8 @ 8.021 MHz (DIP-64 or PLCC-68 on late boards)
- GLUE (C025915): PLCC-68 — address decoding, bus arbitration, video sync timing & blanking
- MMU (C025912): PLCC-68 — DRAM refresh, RAS/CAS multiplexing & CPU/Shifter bus interleaving
- Video Shifter (C025914): DIP-40 — pixel serialiser & 16-colour LUT from a 512-colour (9-bit, 3-bit R/G/B) palette
- DMA Controller (C025913): DIP-40 — high-speed DMA transfers for the WD1772 floppy and ACSI hard disk port
- MFP (MC68901): DIP-48 — Multi-Function Peripheral (timers, RS-232 UART, monochrome monitor detect & interrupts)
- Sound / Floppy Select: Yamaha YM2149F (DIP-40) — 3-channel audio + floppy side/drive select + parallel printer port
- Dual ACIAs: 2× Motorola MC6850P (DIP-24) — one for the IKBD intelligent keyboard/mouse/joystick processor (31.25 kbaud), one for hardware MIDI IN/OUT
ST / STFM Motherboard Revisions (C070243 – C103414)
| Board Revision | Models | RAM & Socket Notes |
|---|---|---|
| C070243 (Early 520ST) | 520ST / 520STM | 16× 41256 (256K×1) DRAMs = 512 KB. External 7-pin DIN PSU. Often has 6-chip TOS 1.00 ROM sockets. |
| C070523 / C070789 | 520STFM / 1040STF | 16× 41256 (512 KB) or 32× 41256 (1 MB on 1040). Solder pads exist for optional PLCC-68 Blitter on late revisions! |
| C070859 / C103414 | Late 520/1040STFM | Uses 4× or 8× 44256 (256K×4) DIP-20 DRAMs. Supports jumper-selectable 2-chip (27C1001) or 6-chip (27C256) TOS 1.02 / 1.04 ROMs. |
Mouse/Joystick Port Solder Cracks (Under Keyboard!): Every ST/STFM/STE suffers from cracked solder joints on the two 9-pin D-Sub mouse/joystick ports positioned underneath the front-right of the keyboard PCB (especially on the `6301` IKBD controller board). Always reflow all 9 pins of both DB9 ports whenever the case is open!
520STE / 1040STE (Enhanced ST) 1989–1992 • 4096 Colours • DMA Stereo Audio • SIMM RAM
The heavily upgraded ST Enhanced (STE) consolidates the GLUE and MMU into a single surface-mount/PLCC GSTMCU, makes the hardware Blitter coprocessor standard, expands the colour palette to 4,096 colours (12-bit) with hardware fine horizontal/vertical scrolling, adds 8-bit stereo PCM DMA digital audio (up to 50 kHz via an LMC1992 digitally controlled mixer with rear RCA jacks), and replaces soldered DRAM with four 30-pin SIMM sockets (up to 4 MB RAM in 2 minutes)!
Key STE Hardware Upgrades
- CPU: PLCC-68 MC68000FN8 (socketed!)
- GSTMCU (C300588): PLCC-84 combined GLUE + MMU + DMA sound address generator
- STE Shifter (C300589): PLCC-84 — 4,096-colour palette + 8-bit stereo DMA PCM audio DAC
- Blitter (C101643): PLCC-68 hardware bit-block transfer coprocessor (separate IC on early STE; integrated inside GSTMCU on late STE!)
- RAM: 4× 30-pin SIMM sockets (supports 512 KB, 1 MB, 2 MB, or 4×1 MB = 4 MB max)
- TOS ROMs: Two 32-pin DIP sockets (HI/LO) running TOS 1.62 (or upgraded to TOS 2.06)
- Enhanced Joystick Ports: Side-mounted 15-pin D-Sub Jaguar-style analog/multi-button paddle ports alongside standard DB9 mouse/joystick ports
STFM vs STE Feature & Bench Comparison
| Feature | 520 / 1040 STFM | 520 / 1040 STE |
|---|---|---|
| Colour Palette | 512 colours (3-bit R/G/B) | 4,096 colours (4-bit R/G/B) |
| Hardware Scrolling | Software only | Hardware fine H & V pixel scrolling |
| Audio Architecture | YM2149F 3-ch PSG (Mono) | YM2149F + 8-bit Stereo DMA PCM + RCA Jacks |
| Blitter Chip | Optional (rarely fitted) | Standard on all STE boards |
| RAM Upgrade Method | Soldering 16/32 DIP DRAMs | Plug-and-play 30-pin SIMMs (512K / 1M / 2M / 4M) |
| TOS ROM Version | TOS 1.00 / 1.02 / 1.04 (Rainbow) | TOS 1.60 (buggy) / TOS 1.62 / TOS 2.06 |
STE 30-Pin SIMM Rules: Always populate STE SIMMs in matching pairs or sets of 4: for 1 MB use 4× 256 KB SIMMs; for 2 MB use 2× 1 MB SIMMs (in Banks 0/1); for 4 MB use 4× 1 MB SIMMs (80–100 ns, 8-chip or 9-chip parity/non-parity both work).
"Bombs on Screen" Decoder & Boot Diagnostics
Atari TOS displays a row of cherry-bomb icons across the screen when the Motorola 68000 hits an unhandled CPU exception vector — count the bombs to pinpoint the fault!
Atari ST "Bombs on Screen" Error Code Decoder (68000 Exception Vectors)
Note: Original TOS 1.00 – 1.62 draw classic round Cherry Bombs with fuses, whereas TOS 2.06 draws Mushroom Clouds! The exact number of icons matches the Motorola 68000 exception vector number:
| Bomb Count | 68000 Exception Name | Hardware / Software Cause & Bench Fix |
|---|---|---|
| 2 Bombs | Bus Error (Vector 2) | The CPU accessed a non-existent or protected hardware register, or GLUE asserted /BERR because a peripheral chip failed to return /DTACK within 64 clock cycles. On boot/disk access, check the DMA chip, WD1772, or unseated MMU/GLUE PLCC sockets. |
| 3 Bombs | Address Error (Vector 3) | The 68000 attempted a 16-bit Word or 32-bit Longword read/write (or instruction fetch) at an ODD memory address (`A0 = 1`). Usually caused by a corrupted RAM bit flipping a pointer to an odd value, or oxidised SIMM/ROM pins! |
| 4 Bombs | Illegal Instruction (Vector 4) | The 68000 fetched an opcode word that does not match any valid 68000 instruction. Common causes: bad DRAM / unseated 30-pin SIMM, bad floppy sector read (DMA/1772 fault), or running STE-only software on an STFM. |
| 5 Bombs | Zero Divide (Vector 5) | Software executed `DIVU` or `DIVS` by zero (or corrupt RAM zeroed out a divisor variable). |
| 6 – 8 Bombs | CHK / TRAPV / Privilege Violation | User-mode code attempted a Supervisor-only instruction (8 bombs) or jumped into garbage memory due to failing upper RAM or unstable +5 V PSU rail. |
| 9 – 11 Bombs | Trace (9) / Line-A (10) / Line-F (11) | 11 Bombs (Line-F / `$Fxxx` Emulator): Triggered when executing coprocessor/FPU instructions without a 68881/68882 present, or when a stuck data bit D12–D15 (high nibble = `$F`) corrupts ROM/RAM reads! |
| 24 Bombs | Spurious Interrupt (Vector 24) | An interrupt request (`IPL0–IPL2`) was asserted to the 68000, but no device acknowledged the interrupt cycle (`IACK`). Check the MC68901 MFP, GLUE, or noisy +5 V PSU lines. |
Black Screen vs. Solid White Screen Boot Decision Tree
Understanding how far the ST gets before hanging:
- Pure Black Screen (No Sync):
- Check +5 V and +12 V from the internal PSU first.
- Verify the 32.084988 MHz master crystal oscillator feeding the MMU and Shifter, which divides down to 16 MHz (Shifter), 8 MHz (68000/GLUE), 4 MHz, and 2 MHz.
- Check the dual 555 / discrete Reset circuit: 68000 Pin 18 (
/RESET) and Pin 17 (/HALT) must both sit at +5 V after 250 ms. If/HALTimmediately drops back low (0 V), the 68000 suffered a Double Bus Fault while fetching the initial SSP/PC reset vectors from TOS ROM `$000000`!
- Solid White Screen (No Atari Logo / No Bombs):
- A solid white screen means the 68000, GLUE, MMU, Shifter, and initial TOS ROM boot code succeeded in initialising the video registers!
- Wait 30–60 seconds! If no floppy disk is inserted (or the internal 3.5″ drive cable is unplugged/faulty), TOS 1.00–1.04 sits on a blank white screen for up to 35 seconds waiting for the WD1772 floppy timeout before drawing the green GEM desktop!
- If it stays white forever, check the WD1772 floppy controller, the DMA chip, or the MC68901 MFP (Timer C 200 Hz system tick interrupt).
PLCC-68 / PLCC-84 Socket Oxidation & Diag Cart
Machine works when pressing down on the motherboard, or crashes once warm.
- The Atari PLCC Socket Curse: The square brown through-hole PLCC-68 and PLCC-84 sockets for the MMU, GLUE, Blitter, and STE GSTMCU/Shifter lose spring tension and oxidise heavily.
- Always use a proper PLCC Extraction Tool: Never pry a PLCC chip out from one corner with a flathead screwdriver — you will crack the plastic socket housing or bend the J-leads!
- Clean & Retension: Extract the PLCC ICs, scrub their J-leads with 99% IPA on a stiff toothbrush, and gently flex each miniature gold/tin contact finger inside the socket ~0.3 mm inward using a fine dental pick.
- Atari Official Diagnostic Cartridge + RS-232 Terminal: Plugging an Atari Diagnostic Cartridge into the left-side ROM cartridge port outputs live RAM/chip error codes at 9600 baud (8N1) out of the rear DB25 RS-232 serial port even when the video Shifter or lower RAM is completely dead!
13-Pin Monitor Video, MIDI & ACSI Pinouts
13-pin DIN Monitor port (Low/Med Colour RGB vs High-Res 71.2 Hz SM124 Monochrome), SCART/OSSC wiring, built-in MIDI, and DB19 ACSI hard drive bus.
13-Pin DIN Monitor Port & RGB SCART / Monochrome Switching DIN-13 Female on Rear
The Atari ST's 13-pin DIN outputs both 15.7 kHz 50/60 Hz Analog RGB (for ST Low 320×200 16-colour & ST Medium 640×200 4-colour modes) AND 35.7 kHz 71.2 Hz High-Resolution Monochrome (640×400)! Which mode the ST boots into is controlled entirely by Pin 4 (/MONO DETECT):
| DIN-13 Pin | Atari ST Signal | RGB SCART Target Pin | SM124 Mono / Multisync Note |
|---|---|---|---|
| 1 | Audio Out (Mono) | SCART Pins 2 & 6 (Right & Left Audio In) | Feeds internal speaker on SM124 / SC1224 |
| 2 | Composite Sync (CSYNC) | SCART Pin 20 (via 330 Ω series resistor on non-modulator STs) | 15.7 kHz CSYNC in Colour mode |
| 3 | General Purpose Out (GPO) | Leave unconnected | Unused on standard cables |
| 4 | /MONO DETECT (to MFP I7) | LEAVE OPEN for Colour RGB! | Short Pin 4 to Pin 13 (GND) to force 640×400 71.2 Hz High-Res Monochrome! |
| 5 | Audio In | Leave unconnected | Pass-through audio on some monitors |
| 6 | GREEN (Analog 0.7 V p-p) | SCART Pin 11 (Green In) | Green channel |
| 7 | RED (Analog 0.7 V p-p) | SCART Pin 15 (Red In) | Red channel |
| 8 | +12 V DC Pull-up (via 180 Ω/1 kΩ) | SCART Pin 16 (RGB Blanking) / Pin 8 | Wait! On early non-FM 520STs, Pin 8 is NC — check before relying on it for SCART blanking! |
| 9 | HSYNC (TTL Horizontal Sync) | Not used for standard SCART (use Pin 2) | 35.7 kHz HSYNC for SM124 Mono / VGA multisync |
| 10 | BLUE (Analog 0.7 V p-p) | SCART Pin 7 (Blue In) | Blue channel |
| 11 | MONO VIDEO (TTL 640×400) | Tie to R+G+B via resistors on dual-mode cables | Pixel-sharp 1-bit 71.2 Hz video for SM124 / OSSC |
| 12 | VSYNC (TTL Vertical Sync) | Not used for standard SCART (use Pin 2) | 71.2 Hz (Mono) or 50/60 Hz (Colour) VSYNC |
| 13 | GND (0 V Shield) | SCART Pins 4, 5, 9, 13, 17, 18, 21 | Video, Sync & Audio Ground |
Important Note for Early 520ST (Non-FM) Owners: Early 520STs without an internal RF modulator do NOT have the composite video encoder chip (MC1377) on the motherboard, meaning Pin 2 (CSYNC) can be missing or unbuffered on very early Rev A/B boards! On those early 520STs, combine Pin 9 (HSYNC) and Pin 12 (VSYNC) using a simple 74LS86 XOR / transistor sync combiner, or use an OSSC / GBS-Control / VGA adapter that accepts separate H+V sync directly from Pins 9 & 12!
Built-in MIDI IN & MIDI OUT/THRU 2× 5-Pin DIN 180°
The Atari ST's built-in 31,250 baud MIDI ports made it the studio sequencer standard (Cubase, Notator). Uniquely, Atari wired hardware MIDI THRU inside the MIDI OUT socket on Pins 1 & 3!
| DIN Pin | MIDI IN Socket | MIDI OUT (/THRU) Socket |
|---|---|---|
| Pin 1 | NC | MIDI THRU Current Source (+5 V via 220 Ω) |
| Pin 2 | NC (Isolated) | GND Shield |
| Pin 3 | NC | MIDI THRU Sink (buffered from MIDI IN) |
| Pin 4 | MIDI IN + (to PC900 / 6N138 optocoupler) | MIDI OUT + (+5 V via 220 Ω) |
| Pin 5 | MIDI IN − (optocoupler return) | MIDI OUT − (TX data via 220 Ω) |
Dead MIDI IN? Check the Sharp PC900 / 6N138 optocoupler IC directly behind the MIDI sockets — it is designed to sacrifice itself to protect the 6850 ACIA from miswired studio cables.
DB19 ACSI Hard Drive DMA Bus
The 19-pin D-Sub HARD DISK (ACSI) port is an 8-bit bi-directional SCSI-like DMA bus (`Pins 1–8 = D0–D7`, `Pin 9 = /CS`, `Pin 10 = /IRQ`, `Pin 11 = GND`, `Pin 12 = /RST`, `Pin 13 = GND`, `Pin 14 = /ACK`, `Pin 15–17 = GND`, `Pin 18 = R/W`, `Pin 19 = /DRQ`). Used today with UltraSatan, ACSI2STM, and BlueSCSI SD-card hard drive emulators!
Chip & TOS ROM Reference
Interactive IC Pinout Finder deep-dives plus Atari custom silicon and 2-Chip vs 6-Chip TOS EPROM upgrades.
Motorola MC68000
8.021 MHz 16/32-bit microprocessor with 24-bit external address bus (16 MB address space) and 16-bit data bus.
View Pinout →Yamaha YM2149F
3-channel programmable sound generator AND I/O controller for floppy drive select (`DS0/DS1`), side select, RTS/DTR, and Centronics printer data.
View Pinout →Motorola MC6850 ACIA (×2)
Asynchronous Communications Interface Adapters — one dedicated to the HD6301 IKBD keyboard/mouse processor, one to MIDI.
View Pinout →Peripheral & Custom ST Silicon Reference
| Chip | Part Numbers | Function & Bench Notes |
|---|---|---|
| FDC (Floppy) | WD1772-PH / WD1772-02-02 (DIP-28) | MFM floppy controller. The -02-02 revision natively supports faster 2/3 ms step rates and can be clocked at 16 MHz for HD (1.44 MB) floppy mods. |
| MFP | MC68901P / MK68901N (DIP-48) | Handles 4 timers (Timer A/B/C/D — raster HBL interrupts use Timer B!), RS-232 serial port, FDC/ACSI IRQ sensing, and Mono monitor detect. |
| GLUE & MMU | C025915 (GLUE) & C025912 (MMU) | 68-pin PLCC custom gate arrays on ST/STFM. Combined into the 84-pin GSTMCU (C300588) on STE models. |
| Shifter | C025914-38A (DIP-40, STFM) / C300589 (PLCC-84, STE) | Sits inside the central metal RF shield cage on STFM boards. Generates pixel streams and palette DAC levels. |
| Blitter | C101643-001 (PLCC-68) | Hardware bit-block transfer engine. Can be retrofitted to many STFM boards that have the empty 68-pin PLCC footprint (requires cutting/bridging bus-grant links BG/BGACK under the socket!). |
TOS ROM Versions & 2-Chip / 6-Chip EPROM Upgrade Guide
Because the 68000 has a 16-bit data bus, Atari ST TOS ROMs are always split into EVEN (High Byte, D8–D15) and ODD (Low Byte, D0–D7) chips:
| TOS Version | Total Size | Target Machines & EPROM Type |
|---|---|---|
| TOS 1.00 / 1.02 (1985–1987) |
192 KB | Stock on early 520ST/1040STF. Has slow floppy access and the notorious 40-folder limit bug. Upgrade to TOS 1.04! |
| TOS 1.04 ("Rainbow TOS") (1989 — Best for ST/STFM) |
192 KB | The ultimate upgrade for any 520ST/1040STFM! Fixes the 40-folder bug, speeds up FAT12 floppy/hard disk I/O, and adds MS-DOS disk compatibility. • 6-Chip Boards: Use six 27C256 (32 KB, 28-pin) EPROMs (`HI-0, HI-1, HI-2` & `LO-0, LO-1, LO-2`). • 2-Chip Boards: Use two 27C1001 / 27C010 (128 KB, 32-pin) EPROMs and set the motherboard ROM solder pads (J2/J3/J4) from 256K to 1M mode. |
| TOS 1.62 / TOS 2.06 (STE Series) |
256 KB | STE models only! Uses two 27C1001 (128K) or 27C2001 / 27C020 (256K, 32-pin) EPROMs in a dual-TOS (1.62 / 2.06) switcher configuration. (Note: TOS 2.06 on a non-STE STFM requires an extra `$E00000` address decoder GAL!). |
| EmuTOS (Open Source) | 192 KB / 256 KB | Modern actively developed open-source TOS replacement supporting FAT16 partitions, ACSI/IDE drives, and built-in diagnostics straight from boot! |
Resources & Further Reading
Blog posts, interactive bench tools, and essential Atari ST hardware communities.
Smiggy's Atari ST Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — 68000, YM2149, 6850 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- C128 Machine Hub — C128, C128D & DCR reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Acorn Atom & Electron Hub — Atom & Electron reference
- Osborne 1 & Kaypro II Hub — CP/M luggable reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
External Atari ST Resources
- Exxos Atari Forum — the premier technical Atari ST/STE repair, PSU & hardware mod community
- Atari-Forum.com — active 16/32-bit Atari hardware, software, and demoscene forum
- Atari ST Hardware Register & Pinout Compendium — DrCoolZic's schematics & IC architecture docs
- EmuTOS Project — free open-source TOS replacement ROM for all Atari ST/STE systems