Atari 600XL Troubleshooting Guide

This is the troubleshooting guide for the Atari 600XL home computer. It covers voltage test points, IC diagnostics, RAM chip identification, PBI connector testing, and common failure modes. A multimeter, oscilloscope, and logic probe are recommended for advanced diagnostics.

Power Supply and Voltage Verification

The Atari 600XL has no onboard voltage regulator for the main +5V rail. The external power supply must provide clean, regulated +5V DC. Incorrect voltage will damage ICs.

Voltage Test Points

Atari 600XL. Source: Wikimedia Commons.

Primary Voltage Test Points

Test Point	Expected Voltage	Tolerance	Notes
+5V rail (Pin 40 of CPU / U1)	+5.0V DC	±0.25V	Main logic supply — measure directly at the CPU VCC pin
+5V rail (Pin 24 of ANTIC / U7)	+5.0V DC	±0.25V	Verify supply reaches custom chips
+5V rail (Pin 28 of POKEY / U19)	+5.0V DC	±0.25V	POKEY supply — also affects keyboard scanning
+5V rail (Pin 20 of GTIA / U4)	+5.0V DC	±0.25V	GTIA supply
GND (any ground pin)	0V	—	Reference ground
+5V at power connector	+5.0V DC	±0.1V	Measure at the barrel jack before board insertion

Power Rail Integrity

If the +5V rail reads low (below +4.75V) at the CPU but correct at the power connector:

1. Check for dry or cracked solder joints at the power connector pads.
2. Measure resistance along the +5V trace from the power connector to the CPU. Resistance should be less than 0.5Ω.
3. Inspect the main filter capacitor C95 (470µF 16V) — a failed capacitor here will cause voltage sag under load.
4. Check for short circuits between +5V and GND caused by failed ICs or tantalum capacitors.

CPU Diagnostics (6502C / SALLY)

The CPU is located at position U1 on the motherboard.

6502C Pinout Reference (40-pin DIP)

Key 6502C (SALLY) Pins

Pin	Signal	Description	Expected State
1	VSS	Ground	0V
2	RDY	Ready	HIGH (pulled up); LOW when ANTIC halts CPU
3	Φ1 (OUT)	Phase 1 clock output	1.79 MHz clock signal (NTSC)
4	IRQ	Interrupt Request	HIGH (inactive) unless interrupt pending
5	—	Not connected	—
6	NMI	Non-Maskable Interrupt	HIGH (inactive); pulled LOW by ANTIC for DLI/VBI
7	SYNC	Opcode fetch indicator	Pulses HIGH during opcode fetch cycles
21	VSS	Ground	0V
37	Φ0 (IN)	Phase 0 clock input	1.79 MHz from system oscillator
39	Φ2 (OUT)	Phase 2 clock output	1.79 MHz (system bus timing reference)
40	RES	Reset	HIGH during normal operation; LOW during reset

CPU Failure Symptoms

• Completely dead (no video output, no activity on address/data bus): If +5V is confirmed at pin 40 and GND at pin 1, and the clock is present at pin 37, the CPU has likely failed.
• Boots to self-test but fails: Indicates the CPU is partially functional. Suspect RAM or ROM before replacing the CPU.
• Random crashes during operation: May indicate an intermittent socket contact. Reseat the CPU; if symptoms persist, replace the socket with a machine-pin type.

ANTIC Diagnostics (U7)

The ANTIC chip is the display co-processor, responsible for generating the display list and fetching screen data from RAM.

Key ANTIC Signals

Key ANTIC Pins (40-pin DIP)

Pin	Signal	Description	Expected State
1	VSS	Ground	0V
14	HALT	CPU halt request	Pulses LOW during DMA cycles
15	NMI	Non-Maskable Interrupt output	Pulses LOW for Display List Interrupts (DLI) and Vertical Blank Interrupts (VBI)
24	VCC	+5V supply	+5.0V DC
32	Φ0	Clock output to CPU	1.79 MHz

ANTIC Failure Symptoms

• Blank screen (no video output at all): ANTIC is not generating display data. Verify clock signal at pin 32. If the clock is absent, ANTIC has failed.
• Screen full of random characters/garbage: ANTIC is running but reading corrupt data. Check RAM first, then ANTIC.
• No colour (monochrome only): This is a GTIA issue, not ANTIC.
• Self-test screen displays but system hangs: ANTIC is functional; issue is elsewhere (typically RAM or ROM).

GTIA Diagnostics (U4)

The GTIA generates the final colour video output, handles player/missile (sprite) graphics, and manages collision detection.

GTIA Failure Symptoms

• No colour output / monochrome display: GTIA colour generation has failed. Verify +5V at pin 20.
• Garbled colours or incorrect palette: Suspect a cracked solder joint under the GTIA socket or partial chip failure.
• No player/missile graphics (sprites missing in games): GTIA sprite registers are not being updated. Could be GTIA failure or address bus issue.

POKEY Diagnostics (U19)

POKEY handles audio output, keyboard scanning, serial I/O (SIO), paddle inputs, and random number generation.

POKEY Failure Symptoms

• No sound: Verify +5V at pin 28. If supply is good, POKEY audio section has failed.
• Keyboard completely dead: POKEY drives the keyboard scan. However, check the CD4051 multiplexers (U22/U23) first — these fail more often than POKEY.
• SIO devices not detected: POKEY handles serial communication. Verify the SIO port connections first, then suspect POKEY.
• No random numbers (games behave deterministically): POKEY random number generator failure.

RAM Diagnostics

RAM Configuration by Revision

The 600XL's 16 KB of RAM is implemented differently depending on the board revision:

RAM Chip Configurations

Board Revision	RAM Chips	Type	Positions
Early revisions	2 × 4164	64K×1-bit DRAM (only 16 KB used)	U9, U10
Later revisions	1 × 4416	16K×4-bit DRAM	U10
64 KB upgrade (typical)	2 × 4464	64K×4-bit DRAM	U9, U10 (with wiring mod)

Built-in Self-Test

The Atari 600XL includes a built-in self-test in ROM, accessible by holding the OPTION key during power-on. The self-test performs:

1. RAM test — Tests all available RAM and reports errors by address.
2. ROM checksum — Verifies the integrity of the OS and BASIC ROMs.
3. POKEY audio test — Plays a sequence of tones through each channel.
4. Keyboard test — Displays key presses on screen.

If the system boots directly to the self-test screen without holding OPTION, this typically indicates a ROM or BASIC failure — the OS cannot find a valid BASIC ROM and falls through to self-test.

RAM Testing with a Logic Probe

For units that do not boot at all:

1. Verify +5V and GND at the RAM chip power pins.
2. Check for activity on the RAS (Row Address Strobe) and CAS (Column Address Strobe) lines — these should show regular pulsing during operation.
3. Verify address line activity (A0–A7 should show toggling patterns during memory access).
4. If no activity is present on the RAM address/data lines but the CPU is running (verified by address bus activity at the CPU), suspect the address decoder logic.

4164 DRAM Pinout (16-pin DIP)

4164 DRAM Pinout

Pin	Signal
1	NC (Not Connected)
2	DIN (Data In)
3	WE (Write Enable)
4	RAS (Row Address Strobe)
5	A0
6	A2
7	A1
8	VCC (+5V)
9	A7
10	A5
11	A4
12	A3
13	A6
14	DOUT (Data Out)
15	CAS (Column Address Strobe)
16	VSS (Ground)

Compatible DRAM Replacements

The following DRAM types are confirmed compatible with the 600XL:

• 4164 (direct replacement) — µPD4164, HM4864, KM4164, TMS4164, MB8264, MT4264
• 4264 (backward compatible with 4164)
• 41256 (will function at 64K density, operates as a drop-in for 4164)
• 4464 (for 64 KB upgrades, requires wiring modification)

Speed ratings of -10 (100ns), -15 (150ns), or -20 (200ns) are all suitable. Mixed speed ratings on the same board will function correctly, with all chips operating at the speed of the slowest IC.

PBI (Parallel Bus Interface) Diagnostics

The PBI is a 50-pin edge connector providing direct access to the system bus.

PBI Connector Pinout

PBI Edge Connector Pinout (50 pins)

Pin (Top)	Signal	Pin (Bottom)	Signal
1	D0	2	D1
3	D2	4	D3
5	D4	6	D5
7	D6	8	D7
9	A0	10	A1
11	A2	12	A3
13	A4	14	A5
15	A6	16	A7
17	A8	18	A9
19	A10	20	A11
21	A12	22	A13
23	A14	24	A15
25	GND	26	GND
27	R/W	28	Φ2
29	IRQ	30	RDY
31	HALT	32	MATH (active = external device selected)
33	EXTSEL	34	RST (active low reset)
35	MPD (Memory Present Detect)	36	REF (Refresh)
37	S4 (ROM select)	38	S5 (ROM select)
39	CCTL (cartridge control)	40	RD4 (RAM select)
41	RD5 (RAM select)	42	CAS
43	CASINH (CAS Inhibit)	44	EXTENB (External Enable)
45	+5V	46	+5V
47	+5V	48	+5V
49	GND	50	GND

PBI Diagnostic Checks

1. Verify +5V at pins 45–48 and GND at pins 25–26, 49–50.
2. Check for address bus activity at pins 9–24 using a logic probe or oscilloscope. All address lines should show toggling during CPU operation.
3. Check for data bus activity at pins 1–8. Data lines should toggle during read/write operations.
4. Verify Φ2 clock at pin 28 — should show a clean 1.79 MHz square wave.
5. Check R/W at pin 27 — should toggle between HIGH (read) and LOW (write) during operation.
6. If EXTSEL (pin 33) is stuck LOW: No external device is being selected. Check that any PBI peripheral is properly seated.
7. If CASINH (pin 43) is not asserted during external memory access: The expansion memory will conflict with internal RAM.

Known Issues

16 KB RAM Limitation

The most significant issue with the 600XL is its base 16 KB RAM. Many Atari 8-bit programs require 48 KB or 64 KB to run. Symptoms include:

• Programs fail to load from disk or cassette — they may start loading but crash immediately or report an "Error 2" (insufficient memory).
• BASIC programs that work on an 800XL crash on the 600XL — the BASIC interpreter reports "ERROR-2" (out of memory).
• Games display a blank screen after loading — the game initialises but cannot allocate sufficient memory for graphics and game data.

Resolution: Upgrade the RAM to 64 KB using the Atari 1064 module or an internal modification (see Atari 600XL General Maintenance#RAM Expansion Care).

CD4051 Multiplexer Failure (Keyboard)

The CD4051 analogue multiplexers at U22 and U23 are the most common point of failure for the keyboard. Symptoms range from individual dead keys to a completely non-functional keyboard. These are standard CMOS ICs and readily available as replacements (CD4051BE or CD4051BCN).

OS ROM Boot to Self-Test

If the system boots directly to the self-test screen without the OPTION key being held:

1. Check the BASIC ROM at U3 — remove and reseat. If the issue persists, the ROM may have failed.
2. Check the OS ROM at U2 — reseat in socket.
3. Verify the OPTION key is not stuck or shorted — a stuck OPTION key contact in the membrane can trigger self-test on every boot.

Oxidised IC Sockets

The factory-fitted single-wipe IC sockets are a known reliability issue on XL-series computers. Over time, oxidation builds up on the contact surfaces, causing intermittent failures. Symptoms include:

• Random crashes or lockups
• Failure to boot on some attempts
• Garbled graphics or missing colours

Resolution: Remove all socketed ICs, clean contacts with IPA, and preferably replace the sockets with machine-pin (dual-wipe) types.

Diagnostic Flowchart

System will not power on:

1. Verify PSU output is +5.0V DC → If not, replace PSU
2. Verify +5V at CPU pin 40 → If not, trace power path; check for shorts
3. Verify clock at CPU pin 37 → If not, check system oscillator
4. Verify RESET (CPU pin 40) goes HIGH after power-on → If stuck LOW, check reset circuit

System powers on but no display:

1. Check for CPU activity (address bus toggling) → If no activity, CPU or clock fault
2. Check ANTIC clock output (pin 32) → If absent, ANTIC failure
3. Check composite/RF video output with oscilloscope → If no signal, check video output circuit and RF modulator
4. Reseat all socketed ICs → Retry
5. Run self-test (hold OPTION during power-on) → Identifies RAM/ROM failures

System boots but with garbled display:

1. Run self-test → Check RAM test results
2. Replace DRAM chips one at a time → Test after each replacement
3. Reseat GTIA → Check for improved colour output
4. Check for cold solder joints under IC sockets with a magnifying glass

Related Pages

• Atari 600XL
• Atari 600XL General Maintenance
• Atari 600XL Capacitor Replacement Guide