Atari TT Troubleshooting Guide

Left side of a TT030: reset button, LAN, MIDI Out and In, cartridge port and keyboard socket

Fault-finding for the Atari TT030 follows Atari's own field service manual, which gives a dead-unit procedure, the TT diagnostic cartridge and its error codes, a symptom checklist and the meaning of the bomb messages. Atari notes that the more expensive parts are socketed, so swapping them is often the quickest test.

Power

Atari's first check on a dead machine is the power LED at the front left corner. If it is not lit, measure the supply voltages first. The supply is rated at +5 V 7 A, +12 V 2 A, −5 V 0.4 A and −12 V 0.3 A from 100–240 V AC. Measure all four rails, and Power Good, at main board connector J100:

J100

Pin	Signal	Pin	Signal
1	Power Good	5–8	Ground
2	+5 V	9	−5 V
3	+12 V	10–12	+5 V
4	−12 V

The RS-232 ports need both +12 V and −12 V: Atari's procedure points out that a serial fault can be a missing supply to the 1488 line driver.

Warning: the supply is a mains switching unit with charged primary capacitors. Unplug the machine before opening the supply.

Dead unit

Atari's procedure for a correctly configured machine that powers on but shows no display:

1. Connect a terminal to the RS-232 port. Atari suggests an STE running its VT52 emulator, set to 9600 bit/s, 8 data bits, 1 stop bit, no parity. Cross the transmit and receive lines and connect ground to ground. (Atari's text gives ground as pin 7, which is the ground pin of a 25-pin port; on the TT's 9-pin serial ports transmit is pin 3, receive pin 2 and ground pin 5.)
2. Fit the diagnostic cartridge and switch on. If its messages appear on the terminal, use the cartridge to find the fault. If there is no activity on the terminal or the screen, open the machine.
3. With the board exposed and running, check the 32 MHz clock at the 68030's clock input (pin 6) with an oscilloscope, and replace the oscillator if it is missing. Then check pin 74 (STATUS) of the 68030. It should be a TTL high. If it is not, the CPU has halted, because of a bad reset circuit, a double bus error or a bad CPU.
4. Check the HALT input, which should be held high, and watch the bus error (BERR) line at power-on, which should stay high. Low pulses on BERR mean a component is failing to respond and the PAL at U112 is generating bus errors. Check the clocks and swap the socketed parts.
5. If it still fails, the CPU cannot read ROM or a device is not answering a read or write, probably the 68901 MFP or the DMA controller. A 68030 too hot to touch for more than a second strongly suggests a bad CPU.
6. If the CPU is not halted, it should be fetching from the cartridge, and its address and data lines should be toggling. If they are not, replace the CPU. Check the MFP's serial output with the cartridge fitted and follow it through the 1488 driver.
7. Check the TTVIDEO chip's R, G and B outputs on a colour monitor. If DE (display enable) never goes high there will be no picture. With a monochrome monitor, check the XMONO input at MFP pin 32: if the CPU does not read a low there at power-on, the TTVIDEO will produce RGB output instead.
8. If the TTVIDEO is producing a signal but the picture is unreadable, suspect the screen RAM, and use the cartridge with the terminal as the display.

Diagnostic cartridge

Atari's TT030 diagnostic cartridge (revision 1.4) is fitted with its label facing up, or with the chips facing down if it has no plastic case. At power-up it prints a series of progress messages, ending with "Checking Bus Error Handling", before the menu appears; they show how far initialisation got if the machine crashes. A red screen means an initialisation error. Every message also goes to the RS-232 port, and if the keyboard fails the cartridge takes commands from the terminal. Atari's test set-up adds loop-back connectors for the serial, MIDI and LAN ports; the RS-232 loop-back must not be fitted until the main menu is on screen.

The menu offers ROM (O), colour (C), keyboard (K), MIDI (M), audio (A), timing (T), printer, mouse and joystick ports (P), serial port (S), DMA (D), high-resolution monitor (H), SCC (J), real-time clock (L), RAM (R), floppy (F), SCSI (I) and VME (V) tests, an automatic long run (Q) and an unattended run of the internal tests (Z).

Selected diagnostic cartridge error codes

Code	Meaning (Atari)
11	RAM data line stuck
12	RAM disturbance: a location is altered by a write to another location
13	RAM addressing: the wrong location is being addressed
14	MMU error: no DSACK after a RAM access
15	RAM sizing error: the uppermost address fails
16	Bus error handling failed: a deliberate bus error caused a crash
17	Bus error not detected: the bus error is not being asserted or is not reaching the 68030
E6	Autovector error: IPL0 is grounded or the 68030 is bad
E7	Spurious interrupt: a device interrupted but did not supply a vector
EB	Bus error from the 68030 or the MCU, usually a device not responding; the address accessed is displayed
R0–R4	RAM errors: R0 in low memory, R1 bad RAM chip, R2 address error (RAM chip or memory controller), R3 address error at a 64K boundary (address line not working), R4 error in the video RAM test
T0–T4	MFP timers; vertical sync; horizontal sync; display enable interrupt; MCU video address counter
T5, T6	PSG or floppy controller staying on the data bus too long and causing a bus error
K0–K2	Stuck key; keyboard controller not responding; keyboard controller reports an error
M0–M4	MIDI: data not received, wrong data, framing error, parity error, overrun at the 6850
50–5C	MFP serial port: data, framing, parity, overrun, interrupt and handshake-line errors
D0, D1, D3	DMA time-out; DMA count error (possible MCU fault); DMA controller not responding
F0, F4–FD	Floppy: drive off-line, seek, write-protect, data compare, DMA, DMA count, CRC, record not found, lost data, side select, drive not ready
P0, P1, J0–J4	Printer port data and BUSY line; joystick ports 0 and 1, keyboard controller time-out, left and right button lines
C0, C1	No real-time clock; clock increment error

The quick reference also lists the SCC, LAN, SCSI and SCSI DMA messages in words.

Symptom checklist

Atari's symptom checklist (FSM section 5)

Symptom	Probable cause	Action
Black screen	No power; MCU; TTVIDEO	Check the supply, then follow the dead-unit procedure above
White screen	TTVIDEO, MCU, DMA controller, 68030	Run the diagnostic cartridge with a terminal on the RS-232 port
Dots or bars on the screen	RAM, MCU, TTVIDEO	Run the diagnostic cartridge
One colour missing	Video cable, TTVIDEO	Atari: check the signals at video connector pins 3, 6 and 10 with an oscilloscope, then the cable and the TTVIDEO. Red, green and blue are on pins 1, 2 and 3, with returns on 6, 7 and 8.
Monochrome monitor fails to sync but a colour monitor works	Monochrome detect line; MFP	Check that the monochrome detect line (connector pin 9) is not open with the monochrome monitor connected, check the connector and the MFP XMONO pin, then replace the MFP
Scrambled screen	MCU	Run the diagnostic cartridge
Disk will not boot	Supply, floppy controller, DMA controller, PSG, drive	Check whether the drive select light comes on; if not, check the PSG outputs. Listen for the motor; if it does not turn, check the supply. Swap the drive or try an external drive, then test the DMA and floppy controllers with the cartridge
Disk will not format	Floppy controller, DMA controller, drive	Test with the cartridge
Crash after loading files	Disk, drive, floppy controller, DMA, MCU	Try another disk, then test with the cartridge
Keyboard faults	Keyboard, 6850 ACIA, MFP
MIDI faults	Opto-isolator, 6850 ACIA, inverter
RS-232 faults	MFP, line receiver or driver, PSG, supply
Printer port faults	PSG, MFP
Hard disk port faults	DMA controller, MCU, floppy controller loading the bus, SCSI controller

Source: Atari service manual section 5, pp. 72–74.

Bombs

When TOS shows bombs, their number is the 68030 exception number. Atari's list:

Bombs	Exception
2	Bus error, asserted by the MCU or detected by the CPU. Atari lists a faulty MFP, RAM, ROM or PSG, or a short or open circuit on the address or data lines, as causes
3	Address error: word or long-word access at an odd address
4	Illegal instruction
5	Divide by zero
6	CHK instruction
7	TRAPV instruction
8	Privilege violation
9	Trace
10, 11	Line 1010 and line 1111 emulator
24	Spurious interrupt

Errors while loading a file can also crash the system, so Atari's manual says to check the disk and the drive before repairing the computer.

Known hardware issues

• Atari's 32 MHz main board assembly drawing includes a rework (ECO 1323) that insulates fuse F61 and ferrite bead L601 with tubing and tape so that they cannot short to the power supply's mounting foot. If a TT has power problems after the supply has been taken out and refitted, check that insulation.
• The same drawing lists ECOs 1269 and 1322, which bond the ground planes and add cuts and jumper wires around the MCU, the DMA controllers and their support logic. Each procedure ends by marking the board with its new assembly revision, so jumper wires matching them are Atari's factory rework.
• A 1.44 MB drive needs the AJAX floppy controller at U409, TOS 3.06 and segment 7 of the configuration switch on.
• The keyboard's +5 V supply passes through fuse F31 (0.5 A) beside the keyboard connector J302, and the cartridge port's through fuse F61 (5.0 A) and ferrite bead L601. A dead keyboard, or a cartridge that is not seen, can be one of these fuses.
• Jumper W101 must be at 1–2 for 32 MHz; at 2–3 the processor runs at 16 MHz.
• If the date, time and settings are lost at every power-up, the 3.6 V lithium clock battery is flat. See Atari TT General Maintenance#Clock battery.

Related pages

• Atari TT030
• Atari TT General Maintenance
• Atari TT Capacitor Replacement Guide
• Atari TT030 Computer Field Service Manual
• Atari TT030 Schematic
• Capacitor Failure Symptoms