Atari 400 Troubleshooting Guide

Atari 400

Fault-finding on the Atari 400 follows Atari's ATARI Home Computer Field Service Manual 400/800 of June 1982: its power-up flowcharts, its symptom checklist, the SALT II diagnostic cartridge and the clock table. Designators: A1xx and Z1xx are on the motherboard, A2xx, C2xx and CR2xx on the power supply board, A3xx and Z3xx on the CPU card, and Z5xx on the RAM card.

Before you start

• The external adaptor (Atari C014319) is a transformer and delivers 9 V AC to the power jack J204. The power supply board inside the console makes +5 V, +12 V and −5 V from it. Measure the adaptor on the AC range; there is no DC output to measure.
• The console has two switches in the AC input: the power switch S203 and the power interlock switch S202, which breaks power when the top panel is open. A 400 that is dead with a good adaptor is often an interlock that is not closed.
• The 400 has no fuse; none is listed in Atari's parts list or drawn on the power supply schematic.
• Remove any cartridge and unplug all SIO devices. With no cartridge and no powered disk drive, a 400 starts in Memo Pad.

Power-up test

Atari's first test needs no equipment: with the adaptor and television connected and no cartridge fitted, switch on. A clear ATARI COMPUTER - MEMO PAD screen means the machine has passed the catastrophic-failure stage; the special graphics mode of the keyboard is then checked, and testing moves on to the SALT II cartridge. The 400 has no built-in self test.

The Memo Pad flowcharts branch on what the screen shows:

Snowy screen (no picture)

Atari's chart AA (p. 5-3):

1. Check, adjust or replace the AC adaptor, the channel select switch and the TV switch box.
2. If there is still no Memo Pad, check for +5 V at J201 pins 5, 6 and 7 on the power supply board.
3. If +5 V is missing: check and replace the rectifier diodes CR206–CR209 (replace 1N4001-type diodes there with 1N5391, Atari C017988), check that the regulator A202 has +5 V, and check C208, C209, R203 and the interlock switch S202.
4. If +5 V is present: check for a good ground at pin 1 of the RF module, and whether the RF module is mistuned or defective.

Black or grey screen

Atari's chart BB (pp. 5-4 and 5-5):

1. Check for −5 V at J201 pin 18. If it is missing: CR206–CR209 (replace 1N4001-type diodes with 1N5391), CR201–CR203, C201, C202 and C210.
2. Check for +12 V at J201 pin 2. If it is missing: the regulator A201, CR204 and C203–C205.
3. Check the clock circuit on the CPU card against Table 5-1 (below), starting at the top and tracing back until a missing signal identifies the failed part.
4. Check the reset lines at the chips (Atari's chart names ANTIC, A302 pin 36).
5. Swap the CTIA/GTIA (A301).
6. On the motherboard, swap A103, A111, Z103–Z105, Q103, Q104 and X101.
7. Check and replace the discrete components in the video summing circuit.

On the power supply schematic the power LED CR210 is part of the −5 V supply, with R201 and the 5.1 V zener CR201.

Green or yellow screen

Chart CC (p. 5-6): check for a defective RAM card, starting with the lowest RAM slot; then swap the operating system ROMs A103, A104 and A105; then swap A303, Z303 and Z304 on the CPU card.

Solid blue screen

Chart DD (p. 5-7): swap POKEY (A101). If the screen is then strictly black and white, swap Q107 and X101 on the motherboard.

Supply rails

Atari 400 supply (Figure 2-4 and p. 1-6)

Rail	Check at	Made by	Feeds
+5 V A	J201 pins 5, 6	A202, drawn as a 7805 (Atari C014348), with a 33 Ω 2 W resistor across it (R203 on the schematic, R202 in the parts list)	Logic boards
+5 V B	J201 pin 7	+5 V A through L201 (1 µH)	Video circuitry
+12 V	J201 pin 2	Voltage doubler CR204/CR205 with C204/C205, then A201 (78M12, Atari C014349)	Dynamic RAM; SIO pin 12
−5 V	J201 pin 18	CR202/CR203 with C201/C202, 5.1 V zener CR201, LED CR210	Dynamic RAM

Atari's flowcharts check for the presence of each rail and give no tolerance band. The electrolytics are listed on the Atari 400 Capacitor Replacement Guide.

Clock circuit

Atari's Table 5-1 lists the clock at named pins, measured with a scope or frequency counter:

Clock check points (Atari Table 5-1, 400)

Point	Board	Frequency (MHz)
Pins 21 and 22 of J101	CPU card	1.789
Pins 8, 9, 10 and 13 of Z302	CPU card	1.789
Pin 37 of A303	CPU card	1.140 (as printed)
Pin 6 of Z301	CPU card	1.789
Pin 34 of A302 (ANTIC)	CPU card	1.789
Pin 35 of A302 (ANTIC)	CPU card	3.579
Pins 28 and 29 of A301 (GTIA)	CPU card	3.579
Collector of Q104	Motherboard	3.579
Pin 7 of A101 (POKEY)	Motherboard	1.789
Pin 25 of A102 (PIA)	Motherboard	1.789

X101 is the 3.579575 MHz crystal on the motherboard. On the 6502 (A303), Atari's pin-assignment figure gives Vcc at pin 8, Vss at pins 1 and 21, φ0 in at pin 37, φ2 out at pin 39 and RES at pin 40.

Symptom checklist

Atari 400/800 symptom checklist (Section 6)

Symptom	Possible cause	400 flowchart
Snowy screen	Switch box, AC adaptor, channel select switch, modulator adjustment, RF cable damage	p. 5-3
Black or grey screen	+5 V, −5 V and +12 V supplies; CR201–CR209, C201–C205, C210, A201, A302, A303, A301, A103, A111, Z103–Z105, Q103, Q104, X101	p. 5-4
Green or yellow screen	Z103–Z105, A103, Z303, Z304	p. 5-6
Solid blue screen	A101, Q107, X101	p. 5-7
No colour or bad colour bars with SALT II	R309 adjustment, A301–A302 on the CPU card; A111, Q107, X101, Q103, Q104 on the motherboard; modulator adjustment	pp. 5-9, 5-12
No colour bars on the TIA test	A301 on the CPU card	p. 5-12
No grey bars	A301, A111, Q107, R309 adjustment, X101, Q103, Q104, modulator	p. 5-14
Upside-down characters or player field	A302 on the CPU card, A301, A111, Q107; modulator	pp. 5-1, 5-21
Some keyboard keys fail	Keyboard	p. 5-22
All keyboard keys fail	A101, Z103, Z104	p. 5-22
Tones missing in sound or games	A101, R160, R161, R166, R170, C164, C179, C184, Q105, L101, Q106, L118, A203	p. 5-24
Console switches do not work	A301 on the CPU card, R119–R122, C121–C124	p. 5-28
RAM test failed	RAM test matrix (Table 5-2)	p. 5-31
Hand controllers do not work	Port test matrix (Table 5-3)	p. 5-33

R309 on the CPU card is the colour adjustment.

Lock-ups

Atari's Tech Tip 10 (22 February 1983) lists the probable causes of intermittent lock-ups in order of likelihood: editing a program in Atari BASIC (a fault in the BASIC cartridge, cleared by switching off and on), errors in the user's own software, errors in third-party software, and last, hardware. The hardware causes it names are dirty or corroded edge connectors, heat-related failure of components, particularly the 6502, and outright component failure.

Its hardware test is a burn-in of at least 12 hours on the continuous RAM test of the SALT 2.05 cartridge, followed by six presses of SYSTEM RESET, each of which should bring up the power-up screen correctly. If the unit fails: check the wiring and connectors between the power supply and the motherboard and where the ROM, RAM and CPU boards plug in, look for cold solder joints, replace the 6502 (Atari C014377), then retest with a Star Raiders cartridge.

Other known faults

• Operating system ROMs: Revision A and Revision B ROMs must not be mixed; a mixed pair will not run (Tech Tip 2, 28 May 1982). The same tip says to replace the ROM pair if a printer overprints ("hiccups").
• CTIA or GTIA: early units have the CTIA (C012295) at A301 and most have the GTIA (C014805). Software written for the GTIA's extra modes does not display correctly on a CTIA machine (Upgrade Bulletin 1, 28 May 1982).
• Keyboard: some dead keys point at the keyboard; all keys dead points at POKEY (A101), Z103 and Z104.

Related pages

• Atari 400
• Atari 400 General Maintenance
• Atari 400 Capacitor Replacement Guide
• Atari 800 Troubleshooting Guide
• Atari 400 800 Field Service Manual June 82