Atari 800 Troubleshooting Guide

Atari 800

Fault-finding on the Atari 800 follows Atari's own procedure: the power-up flowcharts in section 7 of the ATARI Home Computer Field Service Manual 400/800 (June 1982), the section 6 symptom checklist, the SALT II diagnostic cartridge and the clock table. Designators: A1xx, Z1xx and Q1xx are on the motherboard, A2xx, C2xx, CR2xx and S2xx on the power supply board, A3xx and Z3xx on the CPU board, A4xx on the ROM personality module and Z5xx on the RAM modules.

Before you start

• The adaptor (Atari C014319) is a transformer. It delivers 9 V AC to the power jack J206, and the power supply board in the console makes +5 V, +12 V and −5 V from it. Measure the adaptor on the AC range.
• Two switches sit in the AC input: the power switch S202 and the interlock S201, which opens when the cartridge door is lifted. An 800 that is dead with a good adaptor may simply have an interlock that is not closing.
• There is no fuse in the 800. Atari's parts list has none and the power supply schematic draws none.
• Take out any cartridge and unplug all SIO devices. With no cartridge and no powered disk drive the 800 starts in Memo Pad. The 800 has no built-in self-test.
• The ROM personality module must be in the slot nearest the keyboard; up to three RAM modules go in the other slots.

Power-up test

Connect the adaptor and a television, leave the cartridge slots empty and switch on. "ATARI COMPUTER - MEMO PAD" in the upper left of the screen means the machine has passed the catastrophic-failure stage. Atari then checks the special graphics characters from the keyboard (CTRL with each key) and moves on to the SALT II cartridge.

If Memo Pad does not appear, the flowchart to use depends on the screen.

Snowy screen

Chart AA (p. 7-3):

1. Check, adjust or replace the AC adaptor, the channel select switch and the TV switch box.
2. Check for +5 V at J201 pin 17 on the power supply board.
3. If it is missing, check the bridge diodes CR206–CR209 (Atari says to replace 1N4001-type diodes there with 1N5391, C017988), check that the regulator A202 has +5 V, and check C208, C209, R203 and the power switch S202. The 400's chart names the same two parts, which on the 400 are the 0.22 µF C208 and the 4700 µF reservoir C209; on the 800 the two 4700 µF reservoirs are C207 and C208, and C209 is a 0.1 µF ceramic, so check C207 too.
4. If it is present, check the ground at pin 1 of the RF module and whether the module is mistuned or faulty.

Black or grey screen

Chart BB (pp. 7-4 and 7-5):

1. Check for −5 V at J201 pin 1.
2. Check for +12 V at J201 pin 21. If it is missing, check the regulator A201, CR204 and C203–C205.
3. Check the CPU board clock against Table 7-1 (below): start at the top and trace back until the signal appears.
4. Check for +5 V on the reset pins, A302 (ANTIC) pin 36 and A303 (6502) pin 40.
5. Replace the CTIA or GTIA (A301).
6. On the motherboard, swap the clock circuit parts and Z101–Z104, and swap the operating system ROMs by substitution.

Atari's box for missing −5 V repeats the +12 V parts (A201, CR204, CR205, C203–C205, C212). On the schematic the −5 V supply is CR202 and CR203 with C201 and C202 and the zener CR201, with the power LEDs CR210 and CR211 in the same circuit; check those parts first.

Green or yellow screen

Chart CC (p. 7-6): look for a faulty RAM module, starting with the lowest RAM slot. Then swap the ROMs A401–A403 on the personality module, and then A303, Z303 and Z304 on the CPU board.

Solid blue screen

Chart DD (p. 7-7): swap POKEY (A101). If the screen is strictly black and white, swap A104 and the crystal X101 on the motherboard.

Supply rails

Atari 800 supply (Figure 2-8 and p. 1-6)

Rail	Check at	Made by	Feeds
+5 V A	J201 pins 18, 19	A202, drawn as a 7805 (Atari C014348), with R203 (33 Ω 2 W) across it	Logic boards
+5 V B	J201 pin 17	+5 V A through L201 (1 µH), with C209 and C210	Video circuitry
+12 V	J201 pin 21	Doubler CR204/CR205 with C204/C205, then A201, printed 78M12 (Atari C014349)	Dynamic RAM; SIO pin 12
−5 V	J201 pin 1	CR202/CR203 with C201/C202, zener CR201	Dynamic RAM

The flowcharts check that each rail is present and give no tolerance band. The electrolytics are listed on the Atari 800 Capacitor Replacement Guide.

Clock circuit

Table 7-1 gives the clock at named pins, for a scope or frequency counter:

Clock check points (Atari Table 7-1, 800)

Point	Board	Frequency (MHz)
Pins 21 and 22 of J101	CPU board	1.789
Pins 8, 9, 10 and 13 of Z302	CPU board	1.789
Pin 37 of A303	CPU board	1.140 (as printed)
Pin 6 of Z301	CPU board	1.789
Pin 34 of A302 (ANTIC)	CPU board	1.789
Pin 35 of A302 (ANTIC)	CPU board	3.579
Pins 28 and 29 of A301 (GTIA)	CPU board	3.579
Collector of Q102	Motherboard	3.579
Pin 7 of A101 (POKEY)	Motherboard	1.789
Pin 25 of A102 (PIA)	Motherboard	1.789

X101 is the crystal on the motherboard; Atari's 800 parts list prints its frequency as "3.59575 MHz". 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.

SALT II RAM test

The SALT II RAM test shows failures as red 1s. A pattern that keeps changing points to a pair of faulty 74LS158 multiplexers on one RAM module; replace them with parts of the same manufacturer and date code. To find the module, switch off, remove one module and restart the test from Display Options.

With a stable pattern, line B columns 4 to 7 give the failing 8 KB block (Table 3-2), and a difference between rows D and E in one column gives the chip (Table 3-3). If more than one column differs, replace the 74LS244 (Z502) and retest.

Defective RAM identifier (Atari Table 3-3)

Column	8 KB module	16 KB module
D0	Z512	Z512
D1	Z511	Z511
D2	Z510	Z510
D3	Z508	Z509
D4	Z509	Z508
D5	Z507	Z507
D6	Z505	Z506
D7	Z506	Z505

The SALT "M" test checks the video buffer chip, which is A103 on the 800.

Symptom checklist

Section 6 is shared by the 400 and the 800. Several of its designators follow the 400's numbering: A111 is the 400's CD4050 video buffer, which is A103 on the 800, and on the 800 the operating system ROMs are A401–A403 on the personality module.

Atari 400/800 symptom checklist (Section 6)

Symptom	Possible cause (as printed)	800 flowchart
Snowy screen	Switch box, AC adaptor, channel select switch, modulator adjustment, RF cable damage	p. 7-3
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. 7-4
Green or yellow screen	"Z103-5, A03, Z303 & 4, all on ROM Board"	p. 7-6
Solid blue screen	A101, Q107, X101	p. 7-7
No colour or bad colour bars with SALT II	R309 adjustment, A301–A302 on the CPU board; A111, Q107, X101, Q103, Q104 on the motherboard; modulator adjustment	pp. 7-9, 7-12
No colour bars on the TIA test	A301 on the CPU board	p. 7-12
No grey bars	A301, A111, Q107, R309 adjustment, X101, Q103, Q104, modulator	p. 7-14
Upside-down characters or player field	A302 on the CPU board, A301, A111, Q107; modulator	pp. 7-14, 7-21
Some keyboard keys fail	Keyboard	p. 7-22
All keyboard keys fail	A101, Z103, Z104	p. 7-22
Tones missing in sound or games	A101, R160, R161, R166, R170, C164, C179, C184, Q105, L101, Q106, L118, A203	p. 7-24
Console switches do not work	A301 on the CPU board, R119–R122, C121–C124	p. 7-28
ROM test failed	A401–A403 on the ROM board, Z403	p. 7-29
RAM test failed	RAM test matrix (Figure and Table 7-2)	p. 7-31
Hand controllers do not work	Port test matrix	p. 7-33

R309 on the CPU board is the colour adjustment. On the 800 the console keys are S204–S207 on the power supply board, and the keyboard's key-in and key-out multiplexers are Z103 and Z104, reached through J106.

Lock-ups

Atari's Tech Tip 10 (22 February 1983) ranks the causes of intermittent lock-up: editing a program in Atari BASIC (a fault in the BASIC cartridge, cleared by switching off and on), the user's own software, 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, then six presses of SYSTEM RESET, each of which should bring up the power-up screen. If the machine fails, check the wiring and connectors between the power supply board and the motherboard and where the ROM, RAM and CPU boards plug in, look for cold solder joints, replace the 6502 (C014377), and retest with a Star Raiders cartridge. If it comes back still failing, the tip lists ANTIC (C012296), GTIA (C014805) and the multiplexers Z503 and Z504 on the RAM modules (C014345, with matching vendor and date codes).

Other known faults

• Operating system ROMs: Revision A and Revision B ROMs must not be mixed; a mixed pair will not run. On the 800 they are A401 (C014599) and A403 (C012499) on the personality module, and Revision B parts are marked with a copyright symbol and "1981 Atari". The same tip says to replace the ROMs if a printer overprints ("hiccups").
• CTIA or GTIA: early units have the CTIA (C012295) at A301 and later ones the GTIA (C014805). Software written for the GTIA's extra modes does not display correctly on a CTIA machine.

Related pages

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