Atari Mega ST Troubleshooting Guide

Mega ST2 main board C100167, labelled in German (atari-computermuseum.de): the CPU (Prozessor) is at the left, the Glue at the right, the DRAM array (Arbeitsspeicher) at the bottom right.

Fault-finding on the Atari Mega ST starts from Atari's own Mega Service Manual, which gives a dead-unit procedure, a symptom checklist, and the tests and error codes of the Mega/ST field service diagnostic cartridge (revision 4.0). The cartridge is still the quickest way to isolate a fault; without it, a terminal on the RS-232 port and a scope get a long way.

Before you start

Warning: the power supply is inside the system unit and its primary side runs at rectified mains voltage. Unplug the machine before opening it; see Atari Mega ST General Maintenance for Atari's disassembly steps.

Atari's list of equipment for the job is a colour and a monochrome monitor, an external floppy drive, the port and expansion test fixtures, RS-232 and MIDI loop-back connectors, the diagnostic cartridge, an RS-232 terminal (or a second ST running the VT52 emulator), a 100 MHz oscilloscope and a digital multimeter.

Preliminary checks

1. Check that the power light on the keyboard, the drive light and the monitor's power light come on. If the keyboard light stays out, go to the power supply section below.
2. Check the monitor. A monochrome monitor pulls pin 4 of the 13-pin monitor socket (monochrome detect) low; if the machine does not see that line low at power-up it starts in colour mode.
3. Check the keyboard cord. It must be a six-wire cord wired pin 1 to pin 1. The outer contacts at each end carry +5 V and ground, so a reversed telephone cord puts the supply on the wrong pins of the keyboard.
4. Boot from the original system disk with nothing else connected. A bad desk accessory or a damaged DESKTOP.INF stops the desktop appearing on a machine with no hardware fault; with no disk in drive A the desktop takes about 40 seconds to appear.

Dead unit: no display, no terminal output

Atari's procedure for a machine that powers up but shows nothing:

1. Connect a terminal to the RS-232 port: unit pin 2 to terminal pin 3, pin 3 to pin 2, pin 7 to pin 7, at 9600 bit/s, 8 data bits, 1 stop bit, no parity. Fit the diagnostic cartridge and power up. If the cartridge's messages appear on the terminal, use the cartridge.
2. If nothing appears, open the machine. Check for the 8 MHz clock at pin 15 of the 68000.
3. Check pin 17 (HALT). It should be a TTL high. If it is low the CPU is halted, for one of three reasons: a bad reset circuit, a double bus error, or a bad CPU. Check the reset circuit at the inputs of the inverters on the HALT line.
4. Watch pin 22 (BERR) at power-up. It should stay high. Low pulses mean the Glue is signalling a bus error: check the clocks to the Glue and the memory controller and swap those chips if socketed. If the fault remains, the CPU cannot read the ROMs or a device is not answering, most likely the MFP (which should answer MFPCS with DTACK) or the DMA controller (which should answer FCS with RDY).
5. If the CPU is not halted, the address and data lines should be toggling. Check the MFP's serial output at pin 8 at power-up with the cartridge fitted, and trace it through the 1488 driver; RS-232 needs both +12 V and −12 V.
6. Check the Shifter outputs. BLANK must go high for any picture. In monochrome, check Shifter pin 30 and MFP pin 29 (MONOMON).
7. A Shifter that produces a signal but an unreadable picture points to screen RAM; use the cartridge with the terminal as the display.

A CPU too hot to touch for more than a second is very likely faulty. The reset controller is a TL7705A at U1. A machine that only starts after the reset button is pressed is also a known fault with a community fix; see below.

Symptom checklist

Atari's symptom checklist

Symptom	Probable cause	Action
Black screen	No power, Glue, Shifter	Check the LED; follow the dead-unit procedure
White screen	Shifter, Glue, memory controller, DMA controller, 68000	Run the cartridge with a terminal on the RS-232 port
Dots or bars on screen	RAM, memory controller, Shifter	Run the cartridge
One colour missing	Video summing network, buffer transistor, Shifter	Check the signals with a scope (see video faults below)
Scrambled screen	Glue, memory controller	Run the cartridge
Disk will not boot	Power supply, WD1772, DMA controller, sound chip (drive select), drive	Check the drive light comes on; if not, check the YM2149 outputs. Listen for the motor; if silent, check the supply. Try an external drive
Disk will not format	WD1772, DMA controller, drive	Run the cartridge floppy tests
Crash after loading files	Disk, drive, WD1772, DMA controller, memory controller	Try another disk, then the cartridge
Keys dead	Keyboard controller, 6850 ACIA, MFP
Keys dead, mouse works	Keyboard plugged in with the power on	Switch off and on again
MIDI: no data	PC900 opto-isolator, 6850 ACIA, inverters	Run the cartridge MIDI test with a loop-back cable
RS-232: no data	MFP, 1488 driver, 1489 receiver, YM2149, ±12 V supply	Run the cartridge RS-232 test with a loop-back plug
Printer: no output	YM2149, MFP
Printer: works with some printers only	Printer input impedance below 3 kΩ	Change the pull-up resistors in the printer
DMA port dead	DMA controller, memory controller, a WD1772 loading the bus
Clock does not run	RTC PAL (U37), RP5C15 (U25), crystal
Clock loses time when switched off	Batteries, power-off sense circuit	See Atari Mega ST General Maintenance
No video once a Blitter is fitted	Jumpers below and to the right of the Blitter socket not cut	Cut the jumpers
Blitter does not work	Blitter	Replace the chip

Diagnostic cartridge error codes

The cartridge (Mega and ST Field Service Diagnostic Test, revision 4.0) tests on power-up, then offers a menu. Everything it prints on the screen also goes to the RS-232 port, and it accepts keystrokes from a terminal when the keyboard has failed. The G (Blitter), L (clock) and X (expansion connector) tests are highlighted only when the hardware is found; X needs the expansion test fixture fitted inside the case.

Error codes (Atari quick reference)

Group	Code	Meaning
Start-up	11	RAM data line stuck
	12	RAM disturbance: a location changes when another is written
	13	RAM addressing: wrong location addressed
	14	MMU error: no DTACK after a RAM access
	15	RAM sizing error: top address fails
Exceptions	E6	Autovector error: IPL0 grounded or bad 68000
	E7	Spurious interrupt: a device interrupted without supplying a vector
	E8, E9	Internal exception; bad instruction fetch
	EA	Address error, usually after a bus error or bad fetch
	EB	Bus error, usually a device not responding; the address is displayed
RAM	R0–R4	R0 low 2 KB; R1 walking ones/zeros (replace the chip for the failing bit); R2 address error (RAM chip or memory controller); R3 64 KB boundary (memory controller); R4 video RAM area
Keyboard	K0–K2	K0 stuck key; K1 keyboard controller not responding; K2 keyboard reports a self-test error (replace the keyboard)
MIDI	M0–M4	M0 no data (trace from the 6850 through drivers and receivers); M1 mismatch (6850); M2–M4 framing, parity and overrun
RS-232	S0–SB	S0 no data (MFP pin 8 to port pin 2 through the 1488, pin 3 to MFP pin 9 through the 1489); S1–S8 MFP faults; S9, SA, SB control-line loop-back (DTR–RI, DTR–DCD, RTS–CTS)
DMA	D0–D3	D0 time-out; D1 byte count (memory controller or DMA); D2 data mismatch (DMA controller, port wiring, or a WD1772 loading the bus); D3 DMA controller not responding
Timing	T0–T6	T0 MFP timers; T1 vertical sync and T2 horizontal sync (Glue); T3 display enable; T4 video counter (memory controller); T5 YM2149 and T6 WD1772 holding the bus too long
Printer and joystick	P0, P1, J0–J4	P0 printer port data; P1 BUSY input; J0–J4 joystick ports and buttons through the keyboard (needs the port test fixture)
Floppy	F0–FD	F0 drive not responding to restore; F4 seek; F5 write protect; F6 data compare; F7 DMA; F8 DMA count (memory controller); F9 CRC; FA record not found; FB lost data; FC side select; FD not ready
Blitter	G0–G13	Replace the Blitter

RAM errors give the failing address and bits. On the Mega 2 all RAM (0–1FFFFF) is bank 0; on the Mega 4, 200000–3FFFFF is bank 1. Each bank is sixteen 1M × 1 DRAMs, one per data bit: U40–U55 for bank 0 and U60–U75 for bank 1. The chips are soldered to the board.

The number of bombs on the screen is the 68000 exception number: two bombs are a bus error, three an address error, four an illegal instruction. Errors while loading files from disk also crash the system, so Atari advises checking the disk and the drive before the computer.

Video faults

Each colour is produced by three Shifter outputs summed through a resistor network with weights in the ratio 4:2:1, then buffered by a transistor: Q6 for blue, Q7 for green and Q8 for red. On the later Shifter, C101608, the resistor network is inside the chip. Using the cartridge's colour test:

Colour test faults

Symptom	Cause	Action
One primary colour missing	Transistor stage or Shifter output	Look for an eight-step staircase at Q6, Q7 or Q8. Trace forward to the monitor socket if present, back to the Shifter if not
Primaries present, secondaries wrong	Shifter	Replace the Shifter
Coarse steps instead of a smooth ramp	Shifter, or a short on one of the three outputs for that colour	Check the outputs and the summing resistors
Specks or lines	RAM, or the Shifter if RAM tests good
Wavering display	Extra interrupts reaching the CPU	Check MFP pin 32 for an interrupt every 126 µs from display enable (pin 20); MFP inputs on pins 22–29 should all be high. A disconnected keyboard causes continual interrupts

Too low a brightness setting on the monitor can make eight intensity steps look like four.

Floppy drive

The cartridge's floppy menu has a quick test, a read-track mode for alignment disks (default track 40), an interchange test, an exerciser, and a speed test. The rotation period should be 196–204 ms. For F0, check that the drive light comes on and the head seeks; TR00 (WD1772 pin 23) should then go low and the WD1772 should interrupt on pin 28. If the drive is not selected at all, check YM2149 pin 20 (drive A) and pin 19 (drive B). Replacement drives without their own pull-up resistors need pull-ups adding to the WD1772 outputs. See also Floppy Drive Repair.

Power supply

The supply's six-way connector carries +5 V on pins 1 and 2, ground on pins 3 to 5 and +12 V on pin 6. Atari rates it at 3 A on +5 V and 1 A on +12 V; the 30 mA −12 V rail for RS-232 is produced on the main board. Measure the rails with the board connected.

Power faults

Symptom	Possible cause	Action
No LED, no fan	Mains lead, switch or fuse, or a failed supply	Check the lead and switch with the machine unplugged, then the supply
Fan runs, no picture	+5 V low or missing	Measure at the connector; recap the supply
Picture dims when the floppy starts; random crashes	Aged output capacitors	Recap the supply; see Atari Mega ST Capacitor Replacement Guide
RS-232 dead, everything else works	±12 V supply to the RS-232 drivers	Measure +12 V and −12 V at the 1488 driver (U20)
Supply dead after use on another country's mains	Link W1 fitted (110 V setting) on a 230 V supply	W1 is fitted for 110 V and omitted for 220 V; check it before connecting a machine from another country

Community fixes

The exxos forum lists fixes for recurring faults on the Mega ST: recapping the supply, a 10 kΩ pull-up network on the DMA data bus for hard disk and floppy errors, pull-ups on the WD1772 outputs for replacement drives, a reset-circuit fix for machines that need the reset button to start, and 2.2 kΩ in place of the 10 kΩ bus resistors for instability with cartridges or a different CPU or ROM brand.

Related pages

• Atari Mega ST
• Atari Mega ST General Maintenance
• Atari Mega ST Capacitor Replacement Guide
• Atari Mega STE Troubleshooting Guide
• Floppy Drive Repair