Atari TT030 Clock Battery Replacement

The clock battery circuit on sheet 4 of Atari's TT030 schematic: the battery connector J400 at the right, its two diodes CR402 and CR401, the VBAT line and the MC146818A clock chip U401.

The Atari TT030 keeps its date, time and 50 bytes of configuration in a Motorola MC146818A real-time clock, U401. With the machine switched on the clock runs from the main board supply; when it is off, or the mains fails, it runs from a lithium battery. Atari's parts list gives the battery as part C301020-001, lithium, 3.6 V, 400 mAh. It plugs into a four-pin connector, J400, on the main board.

Not every TT030 has the same battery. Owners report that early machines, with the processor in a PGA socket, have a 6 V lithium pack, and later ones a 3.6 V cell; they report that the clock works from either voltage. Mark Nias describes the cell in his TT030 as a 3.6 V AA lithium thionyl chloride cell, and pictures the Tadiran TL-5242/W pack beside a Tadiran SL-360 AA cell; another owner also gives the original as a TL-5242/W.

Symptoms

• The date and time are lost whenever the machine is switched off.
• The settings kept in the clock's battery-backed RAM, such as the volume, are lost at power-up.
• The diagnostic cartridge's real-time clock test reports C0 (no real-time clock) or C1 (increment error).
• Corrosion on or around the battery or its lead.

See also the Atari TT Troubleshooting Guide.

Safety

The TT030's power supply runs from the mains. Switch off and disconnect the mains lead and every other cable before opening the case. Do not short, open or heat a lithium cell. If the old cell has leaked, see Battery Explosion, Capacitor or Corrosion Damage.

Tools and parts

• A screwdriver for the ten bottom-cover screws, and a slotted screwdriver for the accessory cover's retaining screw.
• A replacement battery of the same voltage as the one fitted: 3.6 V in Atari's parts list, or 6 V in early machines. It needs a lead and plug that fit J400: positive to pin 1, negative to pin 3 or 4, with pin 2 as the key. Owners have moved the original plug across to the new cell or pack.
• A multimeter.
• A soldering iron, if the plug is to be moved to the new battery.
• Atari's TT030 diagnostic cartridge, for the clock test (optional).

One owner reports that a CR2032 coin cell, at about 3.2 V, did not run the clock correctly.

Battery connector J400

Pin	Connection
1	Battery positive, through CR402 and CR401 to VBAT
2	Key
3	Ground
4	Ground

Source: Atari's TT030 schematic, sheet 4.

Procedure

Remove the top cover

Atari's sequence:

1. Switch off and disconnect all cables from the sides and back, including the keyboard, mains lead and any external drive.
2. Turn the computer over and rest it on its top.
3. Remove the ten screws in the square holes in the bottom cover.
4. Loosen the retaining screw of the accessory cover with a slotted screwdriver.
5. Holding the top cover, base and accessory cover together, turn the unit back over onto its base.
6. Lift the front of the accessory cover and remove it. If a hard disk is fitted, stand it on its side on the top cover and disconnect its power and interface cables.
7. Lift the top cover off and stand it on its back behind the base. Mind the floppy drive cables and the power LED cable.
8. Disconnect the power, interface and LED cables from the floppy drive and set the top cover aside.

Replace the battery

1. Find the battery and follow its lead to the four-pin connector J400 on the main board.
2. Note which way round the plug sits, then unplug it from J400.
3. Remove the old battery and check it, its lead and the board around it for leakage or corrosion. Clean any residue as described in Battery Explosion, Capacitor or Corrosion Damage.
4. Measure the voltage of the new battery before fitting it.
5. If the new battery has no plug, solder the original lead and plug to it, positive to the wire that goes to pin 1.
6. Plug the new battery into J400 with the key at pin 2, so that positive is on pin 1.
7. Secure the battery where the old one was.

Check the battery path

If the clock still loses the time with a good battery fitted, check the path from J400 to the clock chip with the machine switched off and unplugged.

1. Measure the battery voltage between J400 pin 1 and pin 3.
2. Measure the voltage on the VBAT line at U401 pin 24, the clock chip's supply pin.
3. If the battery voltage is present at J400 pin 1 but not at U401 pin 24, test the two 1N4148 diodes, CR402 and CR401, between them.
4. Check C402 (10 µF) and C403 (0.01 µF), which decouple the VBAT line. C402 is listed on the Atari TT Capacitor Replacement Guide.

Refit the top cover

Atari's sequence:

1. Stand the top cover behind the base and reconnect the floppy drive's power, interface and LED cables.
2. Set the top cover back on the base. Mind the floppy drive, hard disk and power LED cables.
3. Reconnect the hard disk, if fitted, and fit the back of the accessory cover into its opening with its tabs in the slots.
4. Holding the covers together, turn the unit over onto its top.
5. Refit the ten screws in the square holes in the bottom cover.
6. Tighten the accessory cover's retaining screw with a slotted screwdriver.

Testing

1. Switch on and set the date and time. One owner sets them with the XCONTROL control panel, as on an ST.
2. Switch off, disconnect the mains lead, and check the date and time at the next power-up.
3. If the diagnostic cartridge is available, run the real-time clock test, L. It saves the current time and date, writes a new time, waits one second and checks that the hours, minutes, seconds and the other registers have rolled over, then repeats with another date. It reports C0 if no clock is found and C1 for an increment error.

Related pages

• Atari TT030
• Atari TT General Maintenance
• Atari TT Troubleshooting Guide
• Atari TT Capacitor Replacement Guide
• Atari TT030 Power Supply Repair
• Atari TT030 Computer Field Service Manual
• Battery Explosion, Capacitor or Corrosion Damage