Atari 400 General Maintenance

Atari 400

The Atari 400 was Atari's entry-level 8-bit computer and the only 8-bit Atari with a membrane keyboard. It is built from separate modules: a motherboard, a power supply board, a plug-in CPU card and a plug-in RAM card. Atari's field service manual documents it at component level, with parts lists, schematics and service procedures.

Cleaning

Case and keyboard

• Wipe the case with a damp microfibre cloth and mild soap. Avoid solvents on the plastic.
• The keyboard is a sealed membrane panel (Atari CA014031, later replaced by CA017999). There are no keycaps to remove. Clean the surface only, and use isopropyl alcohol on a lint-free cloth for heavy soiling, kept away from the edges of the panel.

Inside

• Unplug the adaptor, then open the case.
• Remove dust from the boards, the cartridge slot and the card edge connectors with compressed air or an anti-static brush.
• Look for insect debris and corrosion, particularly around the power supply board.

Card edge contacts

The CPU and RAM cards plug into the motherboard at J109 (22/44-way) and J110 (28/56-way); the cartridge socket is J108. Atari's procedure for cleaning and lubricating the card contacts, with the cards removed from the motherboard:

1. Tools: ZEP Safety Solvent or isopropyl alcohol, a dust-free cloth, an acid brush, a board-cleaning brush, and Lubriplate DS-ES lubricant.
2. Inspect the contact fingers for flux, dirt, foreign material and deep scratches.
3. Moisten the cloth with solvent and clean both sides of the fingers until they are free of old lubricant and contamination. Avoid touching the contacts afterwards.
4. If corrosion or residue remains, moisten the board-cleaning brush with solvent and brush the contacts firmly, four to six strokes, without touching the rest of the board. Then repeat the cloth step.
5. With the acid brush, apply an even coat of lubricant to the fingers on both sides of the board, thick enough to protect them from corrosion, and keep it off the rest of the board.

Atari's Tech Tip 10 on intermittent lock-ups also lists dirty or corroded edge connectors among the hardware causes, with heat-related failure of components (particularly the 6502) and outright component failure.

Power supply

The 400 is powered by an external transformer, Atari C014319, the same adaptor as the Atari 800. It supplies 9 V AC, not DC; the rectifiers and regulators are inside the machine on the power supply board.

• Measure the adaptor output with the meter set to AC volts.
• Inspect the power jack J204 and its joints, and the power switch S203.

Atari: the power supply board "receives 9VAC from an external power adaptor (transformer) and provides +5Vdc, +12Vdc, and -5Vdc for the Console system".

Atari 400 supply rails

Rail	J201 pin	Use	Regulated by
+5 V A	5, 6	Logic boards	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)
+5 V B	7	Video circuitry	+5 V A through L201
+12 V	2	Dynamic RAM	A201, 78M12 (Atari C014349), fed from a voltage doubler
−5 V	18	Dynamic RAM	CR202/CR203 with the 5.1 V zener CR201; the power LED CR210 is in this circuit

Both regulators sit on the heatsink assembly CA014708. The Console5 TechWiki identifies A202 as a 7805 and A201 as a 7812; Atari's schematic prints 78M12 for A201. Atari gives no tolerance band for the internal rails; its flowcharts check for their presence at J201 (see the Atari 400 Troubleshooting Guide).

The interlock switch

S202 is a power interlock switch on the power supply board. It breaks power when the top panel is opened to change a cartridge. A 400 that is dead on the bench with the lid up is usually just this switch. Atari's visual inspection checks it: with the console on and Memo Pad on screen, open the cartridge door, hold the plunger down with a small screwdriver, and release it. When the plunger returns to its open position the console should power down.

Connectors and sockets

Atari's visual inspection list: controller jacks and the peripheral jack for bent or broken pins, the RF cable for cracked or pinched insulation, the cartridge slot for foreign objects, and the power jack for bent or damaged pins. Also check:

• The SIO port, J202 on the power supply board, a 13-pin connector.
• The four controller jacks J101–J104. The 400 and 800 are the only 8-bit Ataris with four ports.
• The keyboard ribbon at its motherboard connector.
• The 40-pin sockets on the CPU card (A301–A303) and the sockets on the RAM card.

Capacitors

The 400 has eight aluminium electrolytics. Seven are on the power supply board: C201 and C202 (470 µF 16 V), C204 and C205 (2200 µF 16 V), C207 (10 µF 16 V), C209 (4700 µF 16 V) and C210 (22 µF 16 V). The eighth is C192 (10 µF 16 V) on the motherboard. The full table is on the Atari 400 Capacitor Replacement Guide.

Common failure points

Atari's symptom checklist for the 400 and 800. Designators A1xx/Z1xx are on the motherboard, A2xx/C2xx/CR2xx on the power supply board, A3xx/Z3xx on the CPU card, and A4xx on the 800's ROM board.

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	See the RAM test matrix (Table 5-2)	p. 5-31
Hand controllers do not work	See the port test matrix (Table 5-3)	p. 5-33

RAM card

The 400 was first sold with 8 KB, and most were sold with 16 KB; Atari also sold a 48 KB expansion kit for it, to dealers only, which needed some soldering. The 16K card (CA014802) carries eight RAM chips at Z505–Z512 with Z501 (74LS10), Z502 (74LS244) and Z503–Z504 (74LS158). Reseat the card and clean its edge before suspecting the chips.

CPU card

A301 is the video chip, A302 ANTIC and A303 the 6502B (Atari C014377). Early 400s have the CTIA (Atari C012295) at A301 and most have the GTIA (C014805). Atari's Upgrade Bulletin 1 (28 May 1982) covers the swap. The GTIA adds three graphics modes and runs software written for the CTIA, but software that uses the GTIA's modes does not display correctly on a CTIA machine. Late-production 400s use a revised CPU board with Atari's SALLY 6502.

Operating system ROMs

On the 400 the two 4 KB operating system ROMs are at A103 (C014599) and A104 (C012499) on the motherboard, with a 2 KB ROM (C012399) at A105. Atari's Tech Tip 2 (28 May 1982) says Revision A and Revision B ROMs must not be mixed: "System will not operate if Revision A is paired with Revision B." Revision B parts carry a copyright symbol followed by 1981 Atari. The same tip gives A401 and A403 on the ROM module as the locations in the Atari 800, and says to replace the ROMs if a printer overprints ("hiccups").

Faults outside the computer

• The adaptor: measure it on the AC range.
• The SIO chain: unplug everything from SIO and retest.
• The cartridge: clean the cartridge as well as the slot. With no cartridge and no powered disk drive, a 400 starts in Memo Pad.
• Software: Atari's Tech Tip 10 lists user software, third-party software and a lock-up in the Atari BASIC cartridge while editing as more likely causes of lock-ups than hardware. Its test for hardware is a burn-in of at least 12 hours on the SALT 2.05 continuous RAM test, then six presses of SYSTEM RESET, each of which should bring up the power-up screen.

Diagnostics

The 400 and 800 have no built-in self test. Atari's field procedure uses the SALT II diagnostic cartridge: colour bar, grey bar, keyboard, tone, console switch, verify ROM, RAM and port tests, each with a flowchart in Section 5. The clock check points (Atari Table 5-1) are on the Atari 400 Troubleshooting Guide. X101 is a 3.579575 MHz crystal on the motherboard.

Tools

See Recommended Tools. Specific to this machine: a meter that reads AC volts for the adaptor, contact cleaner, and a lubricant for card edge contacts (Atari used Lubriplate DS-ES).

Maintenance checklist

1. Check the adaptor output on the AC range.
2. Check the interlock switch S202 and the power switch S203.
3. Clean and lubricate the CPU and RAM card contacts.
4. Clean the cartridge slot and the SIO connector.
5. Look for domed or leaking electrolytics on the power supply board, and replace any that have failed (see Atari 400 Capacitor Replacement Guide).
6. Inspect the keyboard ribbon for cracks where it folds.
7. Keep the vents clear.

Related pages

• Atari 400
• Atari 400 Capacitor Replacement Guide
• Atari 400 Troubleshooting Guide
• Atari 800 General Maintenance
• Recommended Tools