Atari 600XL Troubleshooting Guide

Atari 600XL

Troubleshooting for the Atari 600XL, built on Atari's 600XL Computer Field Service Manual: its pin-assignment figures, symptom checklist, Super SALT error codes, parts list and schematic.

Power supply and voltage checks

The 600XL has no voltage regulator on the board. The external supply, Atari part CA061982, takes 110 V AC and delivers +5 V DC ±2 %, 1.5 A maximum, through a 7-pin DIN (J4) with +5 V on pins 1, 4 and 6. In volts that is 4.90–5.10 V.

On the board the 5 V passes through the power switch SW1 and splits into three filtered rails, +5A, +5B and +5C. For a black or grey screen Atari's checklist says to "verify all voltages +5A, B, C" on the PCB and to verify the 3.58 MHz frequency at Y1.

Where to measure

Supply pins (Atari figures 1-3 to 1-6)

Device	Location	VCC pin	Ground pin
6502C CPU	U10	8	1 and 21
ANTIC	U9	21	1
GTIA	U17	27	3
POKEY	U20	17	1

Pin 40 of the 6502C is RESET, not the supply; it idles high, so a reading there near 5 V says nothing about VCC.

Rail low at the chips but correct at the connector

1. Inspect the solder joints at J4 and the power switch SW1.
2. Measure from the connector to the chip supply pins. The rails pass through ferrite beads, L1–L3 and L6–L8 in Atari's parts list.
3. Check the input reservoirs C95 and C96 (470 µF); see the Atari 600XL Capacitor Replacement Guide.
4. Look for a short between +5 V and ground caused by a failed IC.

Chip map

Atari 600XL integrated circuits (FSM parts list, p. 5-7)

Location	Device	Atari part number
U1	74LS138 decoder	C061428
U2	MMU 1B memory management unit	C061618
U3	OS ROM, 16K × 8 (27128)	C061598
U4	BASIC ROM Rev. B, 8K × 8 (2764)	C060302
U5, U6	74LS158 multiplexer	C014345
U7	74LS08 quad AND	C017097
U8	LM358 dual op-amp	C061702
U9	ANTIC, Rev. E	C021697 (domestic), C021698 (international)
U10	6502C CPU	C014806
U11, U12	DRAM, 16K × 4	C061505
U13	Delay module	C060472
U14	74LS14 hex inverter	C061850
U15	74LS375 latch	C060613
U16	74LS51 AND-OR-invert	C060474
U17	GTIA	C014805
U18	74S32 quad OR	C061622
U19	CD4050B hex buffer	C010816
U20	POKEY	C012294
U21	PIA	C014795
U22, U23	CD4051B multiplexer	C014336

Every position is socketed, from the 8-pin socket at U8 to the 40-pin sockets at U9, U10, U17, U20 and U21. Y1 is a 3.579545 MHz crystal, R43 the 500 kΩ colour trimpot, SW1 the power switch and SW2 the channel switch.

Symptom checklist

Atari's table, reproduced in full.

Atari 600XL symptom checklist (FSM section 3)

Symptom	Possible causes	Remedy
Snowy screen	Switch box, DC power supply and ON/OFF switch, channel select switch, RF modulator, damaged RF cable	Adjust or replace
Black/grey screen	+5 V from supply; verify all voltages +5 A, B, C; verify Y1	Troubleshoot the power supply; verify 3.58 MHz at Y1
Green/yellow screen	U1–U7, U11–U16	Repair or replace
Solid blue screen	U1, U2, U4	Troubleshoot the BASIC ROM and its support circuit
No colour or bad colour	U17, U19, Q6, Q1, CR1–CR2, Y1, RF modulator or TV tuner, R43, U8	Adjust or replace; RF modulator or colour adjustment
No power light	Power LED, power supply, cables	Repair or replace
No grey bars, or a missing bar	U17, U19	Repair or replace
Upside-down alphanumerics on the player field	U9	Replace
Some keyboard keys fail	U20, keyboard, U22, U23, cable	Repair or replace
All keyboard keys fail	U20, U22, U23	Repair or replace
Tone or tones missing during tone test	U20, 4.5 MHz audio carrier frequency, U8, TV volume	Repair; adjust 4.5 MHz on the RF modulator
Console game switches will not function	U17, switches, cable	Repair or replace
ROM test failed	U1–U4	Repair or replace
RAM test failed	U1, U2, U5–U7, U11–U16, U18	Verify U18 is a 74S32; verify ANTIC is Rev. E (Tech Tip 20)
Video or ANTIC stress fail	U7, U9	Repair or replace
I/O port test fail, joystick, paddle	U20, U21, Q4, J1, J5, J6	Repair or replace
CPU test fail	Y1, Q2, Q3, U10	Verify the 3.58 MHz oscillator frequency
2-way clock fail	J1, U20	Repair or replace
External audio	J1, U8	Repair or replace
GTIA fail	U17	Replace

The I/O port, 2-way clock, external audio, joystick and paddle tests need the Super SALT extended hardware test module and jumper cables; the keyboard and console switch tests need someone to press the keys.

U18 must be a 74S32, not a 74LS32. Atari's footnote to "RAM test failed" reads "MUST have correct IC's or RAM will fail." Tech Tip 20 (17 August 1983) adds that early international machines, and possibly some domestic ones, were built with the wrong ANTIC and fail the Super SALT RAM test and software that turns the screen off; the international part is C021698 and the domestic part C021697, both at U9.

RAM

The 16 KB is two 16K × 4 DRAMs, Atari part C061505, at U11 and U12 in 18-pin sockets. U9 and U10 are ANTIC and the CPU. Atari's checklist widens a RAM failure to U1, U2, U5–U7, U11–U16 and U18, because the address multiplexers (U5, U6), the latch (U15) and the decoding logic are all in the path.

The Atari 1064 memory module expands the 600XL to 64 KB, and some late 600XLs were sold with 64 KB fitted. Software that needs 48 KB or 64 KB will not run on a standard 16 KB machine, so establish how much memory a machine has before treating a load failure as a fault.

Self test and Super SALT

Hold OPTION while switching on, with no cartridge and no powered disk drive, to run the built-in self test. It covers memory (RAM, ROM and ANTIC), audio-visual (ANTIC, GTIA and POKEY) and the keyboard (POKEY and ANTIC).

Atari's field procedure used the Super SALT cartridge and its extended hardware test module, FA100332: a performance test, 2-way clock test, 6502 test, ANTIC stress test, bar and colour bar tests, GTIA diagnostic, I/O port test with a voltage check on each port, OS ROM checksum, RAM test, tone, video and external audio tests, joystick, keyboard and paddle tests and a maintenance mode, each documented screen by screen in section 2. Error summaries are cleared by SYSTEM RESET and at power-up, so do not use SYSTEM RESET to return to the menu if you want accurate error reporting.

Super SALT fatal system error codes (FSM p. 2-31)

Code	Test in progress
04	Refresh check on pattern "FF"
05, 06	Address line check (could not get unique addresses using each line)
07	Data line check (one or more stuck high or low)
08	Refresh test in pattern "00"
09	RAM somewhere in page 0 or 1
10, 11	RAM failure in page 0 (about $004D–$007F)
17	RAM failure in page 0 (address $0038)
18	RAM failure in page 0 (address $0019)
97	Not enough stack entries present (same as 98)
98	Stack overflow or underflow
99	RAM error in page 1, or the system has taken a bad branch

Atari notes that codes 97, 98 and 99 relate to address decoding on ROM or RAM or to a bad CPU. Pot line failures in the error summaries decode as A, possible short to +5 V; B, time constant too long; D, time constant too short; E, both A and D.

Parallel Bus Interface

The PBI is a 50-pin edge connector giving an unbuffered, direct connection to the address, data and control signals. Figure 1-12 gives the assignment:

Atari 600XL PBI connector (Atari figure 1-12, looking into the computer)

Pin	Signal	Pin	Signal
1	GND	2	EXTSEL' (external select)
3	A0	4	A1
5	A2	6	A3
7	A4	8	A5
9	A6	10	GND
11	A7	12	A8
13	A9	14	A10
15	A11	16	A12
17	A13	18	A14
19	GND	20	A15
21	D0	22	D1
23	D2	24	D3
25	D4	26	D5
27	D6	28	D7
29	GND	30	GND
31	Bø2 (phase 2 clock, output)	32	GND
33	Reserved	34	RST' (reset output)
35	IRQ (interrupt request input)	36	RDY (ready input)
37	Reserved	38	EXTENB (external decoder output)
39	Reserved	40	REF (refresh output)
41	CAS' (column address strobe output)	42	GND
43	MPD' (math pack disable input)	44	RAS' (row address strobe output)
45	GND	46	LR/W' (latched read/write output)
47	Reserved in figure 1-12; +5 V on the schematic	48	Reserved in figure 1-12; +5 V on the schematic
49	AUDIO IN	50	GND

Figure 1-12 marks pins 47 and 48 as reserved and not connected, but the schematic in the same manual ties both to the +5A rail. The comp.sys.atari.8bit FAQ also lists them as +5 V on the 600XL and not connected on the 800XL. Treat them as live +5 V when connecting or probing a PBI device.

Controller jacks

Both jacks are 9-pin D-type male connectors and are functionally and electrically identical.

Atari 600XL controller jack (Atari figure 1-10)

Pin	Function	Pin	Function
1	Joystick forward input	6	Trigger input
2	Joystick back input	7	+5 volts
3	Joystick left input	8	Ground
4	Joystick right input	9	A potentiometer input
5	B potentiometer input

Pin 7 is the only +5 V on the jack, so a joystick that stops the machine when it is plugged in is shorting that pin to ground.

Outside the computer

Rule these out before opening the case:

• The power supply. Measure it unloaded, then under load if possible.
• The SIO chain. Unplug everything from SIO and retest.
• A cartridge. A cartridge in the slot takes precedence over the built-in BASIC by hardware, so dirty cartridge contacts can give blank screens and lock-ups. With the slot empty, a working 600XL starts in Atari BASIC Rev. B.
• The TV switch box and RF cable, which Atari's checklist puts first for a snowy screen.
• The monitor socket. North American 600XLs have none; PAL 600XLs have one, without separate luminance and chrominance.

Sockets

Every IC in the 600XL is in a socket. Before replacing anything, remove each socketed IC, clean the pins and reseat it, checking orientation.

Disassembly

Atari's sequence:

1. Turn the unit upside down and remove the four screws from the bottom cover.
2. Turn it upright and tip the cover by raising the left side to reach the keyboard cable. Remove the keyboard cable from its socket and set the top aside.
3. With a magnetic screwdriver, remove the screw reached through the RF shield at the top left.
4. Remove the four remaining screws and lift out the PCB and RF shield together.
5. Remove the six bolts and nuts from the RF shield.

Related pages

• Atari 600XL
• Atari 600XL General Maintenance
• Atari 600XL Capacitor Replacement Guide
• Atari 600XL Field Service Manual
• Atari 800XL Troubleshooting Guide