Atari Mega ST Troubleshooting Guide

This article provides a structured troubleshooting guide for the Atari Mega ST computer, covering common failure modes, diagnostic procedures, and remediation steps. This guide assumes a basic understanding of electronics and soldering. Mains voltages inside the PSU can be lethal — observe all safety precautions.

Diagnostic tools

The following tools are recommended for Mega ST troubleshooting:

• Digital multimeter (DMM) — For voltage, continuity, and resistance measurements.
• Oscilloscope — For checking clock signals, bus activity, and PSU ripple.
• Atari Diagnostic Cartridge — A ROM-based hardware diagnostic that tests RAM, custom chips, floppy drives, ports, and the real-time clock. Plugs into the cartridge port and runs independently of TOS.
• Logic probe — For checking digital signal levels on the bus.
• Spare known-good PSU — For isolating PSU-related failures.
• Spare TOS ROMs — For isolating ROM failures.

Symptom table

Symptom	Possible Cause	Recommended Action
Machine completely dead (no power LED, no fan)	PSU failure; blown fuse in PSU; mains cable fault	Check mains cable and fuse. Test PSU output voltages with a dummy load. Recap or replace PSU.
Power LED on, but no video output (black screen)	Failed TOS ROMs; CPU failure; GLUE or MMU failure; bad RAM; PSU +5V out of spec	Check +5V rail (should be 4.95–5.10V). Reseat TOS ROMs and CPU. Try diagnostic cartridge. Swap TOS ROMs if available.
White screen only (no desktop, no RAM test)	TOS ROM failure; address bus fault; MMU failure	Reseat ROMs. Check for bent/damaged pins on ROM sockets. Verify address bus signals with logic probe.
Bombs on boot (row of bomb icons)	Software or hardware fault depending on bomb count. 2 bombs = bus error; 3 = address error; 4 = illegal instruction	Note bomb count. 2 bombs often indicate RAM fault or bus issue. Reseat SIMMs/DRAM. Run diagnostic cartridge to identify failing address.
Machine requires multiple power cycles or reset button to start	Reset circuit capacitor degradation	Replace the reset circuit capacitor. Apply exxos reset fix modification.
Random crashes during operation	PSU capacitor degradation; parity SIMMs; bus resistor values; DMA pull-up issue	Recap PSU. Check for parity SIMMs (remove parity chips). Replace 10K bus resistors with 2.2K. Add 10K DMA pull-ups.
Floppy drive does not read/write	Dirty heads; drive belt failure; 1772 controller fault; PSU voltage sag during motor spin-up	Clean heads with IPA. Check drive belt. Verify PSU +5V does not sag below 4.8V during floppy access. Add 1772 pull-up resistors if using non-original drive.
Floppy reads some disks but not others	Head alignment; dirty heads; drive incompatibility	Clean heads. Try known-good disks. Check drive type matches disk format (DD vs HD).
Hard disk (ACSI) not detected at boot	DMA controller fault; ACSI cable issue; hard disk power; DMA pull-up needed	Check hard disk power supply and SCSI/ACSI cabling. Add 10K DMA data bus pull-up resistors. Check DMA chip (C025913).
Missing colour on RGB output (e.g., no red, green, or blue)	Failed video output transistor (Q6=blue, Q7=green, Q8=red); broken trace	Check output of video transistor amplifiers Q6/Q7/Q8. Look for a staircase waveform (8 levels) at each output. Trace forward to video connector or backward to Shifter chip.
No sound from monitor speaker	YM2149 failure; broken trace to audio output; monitor fault	Test audio pin on video connector with oscilloscope. Verify YM2149 is clocked (2 MHz). Check continuity to 13-pin DIN audio pin.
MIDI In/Out not working	ACIA 6850 failure; broken MIDI connector solder joints	Check solder joints on MIDI DIN connectors (cold joints common with age). Test ACIA with diagnostic software. Check for +5V on MIDI Out pin 4.
Keyboard unresponsive	Cable fault; HD6301 keyboard controller failure; corrosion on RJ-11 connector	Check keyboard cable continuity. Clean RJ-11 connector. Try a known-good keyboard. Check +5V at keyboard connector.
Mouse erratic or unresponsive	Dirty mouse ball/encoder; bad mouse port on keyboard; IKBD fault	Clean mouse mechanism. Try a different mouse. Check keyboard IKBD controller.
Date/time lost on every power cycle	Dead RTC battery; RTC chip (RP5C15) failure	Replace CR2032 battery. If date still lost, check RP5C15 chip.
Video noise, wavering or flickering display	PSU capacitor degradation; poor video cable; ground loop	Recap PSU (primary suspect). Check video cable integrity. Ensure good ground connection.
BLiTTER-related crashes	BLiTTER chip fault; bus contention	Test with BLiTTER disabled (set system variable at $0044C to 0). If stable without BLiTTER, the chip may need replacement.
Cartridge port causes instability	Bus resistor values too high (10K stock)	Replace bus resistors with 2.2K.

Voltage test points

Test point	Expected voltage	Notes
PSU output: +5V	4.95–5.10V	Most critical rail. All logic runs on +5V. Out-of-spec causes widespread instability.
PSU output: +12V	11.8–12.2V	Used by serial port drivers, floppy drive motor. Machine will run without it but serial and floppy fail.
PSU output: -12V	-11.8 to -12.2V	Used by RS-232 line drivers.
PSU output: -5V	-4.90 to -5.10V	DRAM negative bias voltage on some board revisions.
CPU pin 14 (VCC)	+5V	Verify CPU is powered.
CPU pin 15 (CLK)	8 MHz clock	Square wave at 8 MHz. Check with oscilloscope.
RESET pin (active low)	Momentary low, then high	Should go low at power-on and return high within ~200 ms.

Chip identification

Designation	Chip	Function
U1 (or similar)	Motorola MC68000	Main CPU
—	Atari C025912	BLiTTER graphics co-processor
—	Atari C025913	DMA controller
—	Atari C025914	GLUE (bus logic / address decode)
—	Atari C025915	MMU (memory management)
—	Atari Shifter	Video display generation
—	WD1772	Floppy disk controller
—	MC6850 (×2)	ACIA: MIDI and keyboard interface
—	MFP 68901	Multi-function peripheral (timers, serial, interrupts)
—	Yamaha YM2149F	Sound generator (PSG)
—	Ricoh RP5C15	Real-time clock
—	HD6301V1	Keyboard microcontroller (in keyboard unit)

Recommended preventive modifications

The following modifications are recommended by the Atari repair community to prevent known failure modes:

1. PSU recapping — Replace all electrolytic capacitors in the PSU. See Atari Mega ST Capacitor Replacement Guide.
2. DMA bus pull-ups — Add 10K pull-up resistor network to DMA data bus to prevent hard drive and floppy failures.
3. 1772 pull-ups — Add pull-up resistors to WD1772 floppy controller output lines (required for non-original replacement drives).
4. Reset circuit fix — Replace aged reset capacitor to ensure reliable cold boot.
5. Bus resistor replacement — Change 10K address/data bus resistors to 2.2K for improved stability with cartridges and alternative CPUs/ROMs.

All modifications are documented on the exxos forum.

See also

• Atari Mega ST
• Atari Mega ST General Maintenance
• Atari Mega ST Capacitor Replacement Guide