Atari ST Troubleshooting Guide

A long-case ST with its internal supply and drive

Fault-finding for the original Atari ST family: the 520ST, 260ST, 520ST+, 520STM, 520STF, 520STFM, 1040ST, 1040STF and 1040STFM. Most of what follows is from Atari's 520ST/1040ST Computer Field Service Manual, which covers both case styles. For the STE range see Atari STE Troubleshooting Guide. Routine care, disassembly and the clock diagram are on Atari ST General Maintenance, and capacitors on Atari ST Capacitor Replacement Guide.

Warning: the long-case machines have a mains switching supply inside the case. Unplug the machine and let the supply discharge before working on it.

Tools

Atari's procedures assume a multimeter, an oscilloscope, the Atari ST diagnostic cartridge (the manual describes revision 3.6), the test fixture and loopback connectors for the ports, and a terminal on the RS-232 port. The terminal can be another ST running the VT52 emulator, set to 9600 baud, 8 data bits, 1 stop bit and no parity.

Power supply

Atari's supply figures

	Short case (external linear supply)	Long case (internal switching supply)
Outputs	+5 V, +12 V, −12 V	+5 V and +12 V
Rating	3 A at 5 V, 30 mA at ±12 V	3 A at 5 V, 0.9 A at +12 V
Adjustment	None	One preset for both outputs; set 5 V within ±5 % (4.75–5.25 V)
Typical demand	1.5 A at 5 V	2.2 A at 5 V, 160 mA at +12 V
Maximum demand	Not given	2.3 A at 5 V, 340 mA at +12 V, during disk access
Fuse	Not given	2 A

Source: Atari's manual, section 1. Atari says a blown fuse in the internal supply probably means a catastrophic failure such as a shorted primary, so find the fault before fitting a new one.

Do not measure a switching supply without a load. exxos warns that an ST supply disconnected from the computer will probably malfunction, and that testing one on the bench needs a load on each output: a 2.2–4.7 Ω resistor of at least 10 W on 5 V and a 22 Ω, 10 W resistor on 12 V.

Supply faults

Symptom	Likely cause	Action
No power LED	Supply, fuse or switch; on a short-case machine, the external supply	Measure the outputs under load. On a long-case machine, if the 2 A fuse has blown, look for a shorted primary component before replacing it
Random crashes	Ripple on +5 V from aged capacitors	exxos measured about 1 V of ripple on the 5 V output of an unrecapped Mitsumi SR98 and says this causes random crashes. Recap the supply
Picture dims when the floppy motor starts	Slow regulation response in the Mitsumi SR98	exxos's fix: replace R202 with 1 kΩ and R201 with 1.2 kΩ, then reset the 5 V preset
Disk will not boot, motor does not spin	Supply	Atari lists the supply first for a disk that will not boot

The long-case supply has no −12 V output, although the RS-232 drivers need ±12 V.

Dead unit

Atari's procedure, for a machine that is correctly connected, with the LED lit and a working monitor, but no display:

1. Connect a terminal to the RS-232 port: pin 2 of the ST to pin 3 of the terminal and vice versa, pin 7 to pin 7. Fit the diagnostic cartridge and switch on. If the cartridge's messages appear on the terminal, diagnose from there.
2. If nothing appears, expose the board and check the 8 MHz clock at 68000 pin 15 with an oscilloscope. Replace the oscillator if it is missing.
3. Check pin 17 (HALT). It should be at a TTL high. If it is low the CPU is halted by a bad reset circuit, a double bus error or a bad CPU:
   • reset circuit: look at the input of the two inverters on the HALT line;
   • double bus error: watch pin 22 (BERR) at power-up. It should stay high. Low pulses mean a component is failing and GLUE is generating bus errors; check the clocks to GLUE and the memory controller, and substitute them if they are socketed;
   • otherwise the CPU cannot read ROM or a device is not answering, most likely the MFP or the DMA controller. The MFP should answer MFPCS with DTACK; the DMA chip should answer FCS by asserting RDY. A bad CPU can only be found by elimination, but a CPU too hot to touch for more than a second is very likely bad.
4. If the CPU is not halted, its address and data lines should be toggling; if not, replace the CPU. Check the MFP's serial output on pin 8 at power-up with the cartridge fitted and trace it through the 1488 driver, which needs ±12 V.
5. Check the Video Shifter's outputs to the R, G and B summing resistors. If BLANK does not go high there is no picture. For monochrome, check Shifter output pin 30 and the MONOMON input on MFP pin 29: if the CPU does not read it low at power-up, the Shifter puts out colour.
6. A Shifter signal with an unreadable picture points at the screen RAM. Use the cartridge with the terminal as the display.

The clock tree (32 MHz into the Shifter, 16 MHz to the memory controller, 8 MHz to the CPU, GLUE, DMA and WD1772) is tabulated on Atari ST General Maintenance#Clocks.

Symptom checklist

Atari's symptom checklist

Symptom	Probable cause
Black screen	No power (check the LED), GLUE, Video Shifter
White screen	Video Shifter, GLUE, memory controller, DMA controller, 68000
Dots or bars on screen	RAM, memory controller, Video Shifter
One colour missing	Video summer, buffer, Video Shifter
Scrambled screen	GLUE, memory controller
TV output bad	Modulator, phase-locked loop
Disk will not boot	Supply, WD1772, DMA controller, PSG, drive. Check that the drive select light comes on (if not, check the PSG outputs) and that the motor spins (if not, check the supply)
Disk will not format	WD1772, DMA controller, drive
Crash after loading files	Diskette, drive, WD1772, DMA, memory controller
Keyboard	Keyboard, 6850, MFP
MIDI	Opto-isolator, 6850, inverter
RS-232	68901 MFP, receiver, driver, PSG, ±12 V supply
Printer port	PSG, MFP
Hard disk port	DMA controller, memory controller, WD1772 (loading the bus)

Source: Atari's section 5.

Video faults

The Video Shifter has three outputs per colour (R0–R2, G0–G2, B0–B2), weighted by a resistor network so that each colour has eight levels, then buffered by a transistor amplifier for each colour: Q3 blue, Q4 green, Q5 red. The cartridge's colour test shows seven colour bands, each in eight levels. Atari's fixes:

Colour test faults

Symptom	Action
One primary colour missing	Look for the eight-step staircase at Q3, Q4 or Q5. If it is there, trace forward to the video connector; if not, trace back to the Shifter
Primaries present, secondaries missing or wrong	Replace the Video Shifter
Coarse steps in intensity	Replace the Shifter, or look for a short on one of that colour's three outputs
Specks or lines on screen	Bad RAM; if the RAM tests good, replace the Shifter
Wavering display	Extra interrupts. The MFP interrupt request (pin 32) should pulse every 126 µs (two lines) from Display Enable (pin 20), and MFP inputs 22–29 should all be high. If no outside source is found, replace the MFP. With the keyboard unplugged the 6850 input sits low and causes continual interrupts

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

RAM faults

The 520ST has one bank of sixteen 256K × 1 DRAMs; the 1040ST has a second bank of sixteen. The cartridge tests RAM in three stages and reports an error code with the address, the data written and read, and the bits that disagreed, for example "R2 45603E W:603E R:613C bad bits: 1,8". Replace the chip for the failing bit in the bank that holds the address: on a 520 everything up to 7FFFF is bank 0; on a 1040, 80000–FFFFF is bank 1.

RAM error codes

Code	Meaning
R0	Low memory failed while setting up the test
R1	Failed walking ones or zeros
R2	Failed address test (counting pattern)
R3	Failed 64 KB boundary test; probably the memory controller
R4	Failed while displaying the tested area (video RAM)

Source: Atari's manual, section 3. Except for R3, the repair is the RAM chip for the bit shown.

Other cartridge error codes

Error codes

Codes	Area	Atari's notes
K0–K2	Keyboard	K0 stuck key; K1 no response (keyboard or the 6850 channel); K2 the keyboard reports a self-test error, replace the keyboard
M0–M4	MIDI	M0 trace the signal from the 6850 through drivers, loopback and receivers; M1 replace the 6850; M2–M4 the 6850 or a noisy driver or receiver
S0–SB	RS-232	Loopback and handshake tests through the MFP
T0–T6	Timing	T0 MFP timers; T1 and T2 GLUE vertical and horizontal sync; T3 Display Enable; T4 memory controller video counter; T5 PSG and T6 WD1772 defective
D0–D3	DMA port	DMA time-out, count error, data mismatch, controller not responding
F0–FD	Floppy	F5 write-protected disk; F7 DMA controller not answering FDRQ; F8 count error, memory controller; FC side select on a single-sided drive; FD drive not ready
P0, P1, J0–J4	Printer and joystick ports	P0 check PSG data pins 6–13, chip select and the 2 MHz clock; J0 and J1 replace the keyboard, then check continuity from J11 to J12

Source: Atari's manual, section 3. If no sound is heard in the audio test, check the PSG's output with a scope and trace it to the monitor connector; a missing channel means a bad PSG.

Bombs

A row of bombs on screen is the 68000 reporting an exception: two bombs for a bus error, three for an address error, four for an illegal instruction. Errors while loading files can crash the machine, so check the diskette and drive before the computer.

Related pages

• Atari 520ST
• Atari 1040ST
• Atari ST General Maintenance
• Atari ST Capacitor Replacement Guide
• Atari STE Troubleshooting Guide
• Atari 520ST and 1040ST Computer Field Service Manual
• Floppy Drive Repair