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Atari Mega STE Clock Battery Replacement

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Atari Mega STE clock battery circuit (schematic rev 2 sheet 4)
The clock battery circuit on sheet 4 of Atari's Mega STE schematic: the battery connector J401 at the right, the diode D401, the VBAT line and the RP5C15 clock chip U402.

The Atari Mega STE keeps its date and time in a Ricoh RP5C15 real-time clock, U402, with a 32.768 kHz crystal, Y401. With the machine switched on, the clock's supply line, VBAT, is fed from VCC through transistor Q401; when the machine is off it runs from a battery plugged into J401, through diode D401.[1] The original battery is a Tadiran TL-5242/W, a non-rechargeable 3.6 V lithium battery in a sealed block on a twisted lead with a plug, held inside the case with a Velcro pad.[2]

When the TL-5242/W is hard to obtain, a 3.6 V lithium cell in a holder can be wired to the original plug. Keeping the plug housing keeps the polarity as it was.[2]

Symptoms

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  • The date and time are lost whenever the machine is switched off.[2]
  • Corrosion on or around the battery or its lead.

See also the Atari Mega STE Troubleshooting Guide.

Safety

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The Mega STE's power supply runs from the mains. Switch off and disconnect the mains lead and every other cable before opening the case.[3] The label on the original battery warns of fire, explosion and severe burn hazard, and says not to recharge, disassemble, heat it above 100 °C, incinerate it or expose its contents to water. Do not cut into the sealed block.[2] If the old battery has leaked, see Battery Explosion, Capacitor or Corrosion Damage.

Tools and parts

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  • A Phillips screwdriver and a slotted screwdriver.[3][4]
  • A replacement: a Tadiran TL-5242/W, or a 3.6 V lithium cell with a holder that has leads.[2]
  • New crimp contacts and a crimping tool, if the original plug housing is to be reused.[2]
  • A multimeter.
  • A Velcro pad.[2]
Battery connector J401
Pin Connection
1 Battery positive, through D401 to VBAT
2 Ground
3 Joined to pin 1

Source: Atari's Mega STE schematic, sheet 4.[1]

Procedure

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Open the case

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  1. Switch off and disconnect all cables from the sides and back, including the keyboard and mains lead.[3]
  2. If a hard disk is fitted, remove it.[4]
  3. Remove the accessory cover and the top cover.[3] The floppy drive is mounted in the top cover: unplug its cables before lifting the cover away.[4]

Replace the battery

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  1. Find the battery, held by its Velcro pad inside the case, and follow its lead to J401 on the main board.[2][1]
  2. Note which way round the plug sits, then unplug it from J401.
  3. Pull the old battery off its Velcro pad and check it, its lead and the board around J401 for leakage or corrosion. Clean any residue as described in Battery Explosion, Capacitor or Corrosion Damage.
  4. If a TL-5242/W is fitted as the replacement, plug it straight into J401 the same way round as the old one and go to the last step.[2]
  5. Otherwise, release the metal contacts from the original plug housing one wire at a time, so that the red and black wires cannot be swapped.[2]
  6. Crimp a new contact onto the matching wire of the new holder and push it into the place the old contact came from. Repeat for the second wire.[2]
  7. Fit the cell in the holder and measure the voltage across the plug's contacts; positive must be on the contact that goes to pin 1 or 3 of J401, and negative on pin 2.[1]
  8. Plug the battery into J401.
  9. Stick a Velcro pad on the new battery or holder and fix it where the old one was.[2]

Check the battery path

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If the clock still loses the time with a good battery fitted, check the path from J401 to the clock chip with the machine switched off and unplugged.[1]

  1. Measure the battery voltage between J401 pin 1 and pin 2.
  2. Measure the voltage on the VBAT line at U402 pin 18, the clock chip's supply pin.
  3. If the battery voltage is present at J401 but not at U402 pin 18, test diode D401 between them.
  4. Check C402 (100 µF) and C401 (0.1 µF), which decouple the VBAT line. C402 is listed on the Atari Mega STE Capacitor Replacement Guide.

Close the case

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  1. Reconnect the floppy drive's cables and refit the top cover and the accessory cover.[3][4]
  2. Refit the hard disk, if one was removed.[4]

Testing

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  1. Switch on, open the Control Panel and set the date and time: select the date or time, clear it with Esc or Backspace, type the new value and click on it.[5]
  2. Switch off, disconnect the mains lead, and check the date and time at the next power-up.[1]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Atari Corporation, Mega STE Schematic, revision 2, April 1990, sheet 4: real-time clock U402 (RP5C15, VDD on pin 18), crystal Y401 (32.768 kHz) and trimmer C403 (5–30 pF), VBAT switch Q401 (2N3906) and Q402 (2N3904), C401 (0.1 µF) and C402 (100 µF) on VBAT, chip-select gates U401 (4011) from PWRGOOD, battery connector J401 (three pins: 1 and 3 joined to D401, 2 to ground) and diode D401 (1SS108). Hosted here: Atari Mega STE Schematic (PDF).
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 François Planque, "How to replace the RTC battery of an Atari Mega STE in 2024", fplanque.com, 14 September 2024: photographs of the battery's position in the case and of the original TL-5242/W with its label; replacement with a 3.6 V Tadiran cell in a holder with leads; reuse of the original plug housing with new crimped contacts; Velcro pad.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Atari Corporation, Coprocessor Installation Kit for the Atari MEGA STE, C302228-001 Rev A, 1991, steps 1–2 and 12: disconnect all cables; remove the accessory and top covers; refit them. Hosted here: Atari Mega STE Coprocessor Installation Kit (PDF).
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Atari Corporation, Atari MEGA STE 1.44 Megabyte Floppy Drive Installation Manual, C302953-001 Rev A, 1991, tools and steps 1–2, 13–14: remove the hard disk, if fitted, and the top cover; refit them. Hosted here: Atari Mega STE 1.44MB Floppy Drive Installation Manual (PDF).
  5. ↑ Atari Corporation, Atari MEGA STe Owner's Manual, C302016-001 Rev B, chapter 6, "Desk accessories": setting the clock and calendar in the Control Panel (printed pp. 6-2 to 6-3, PDF p. 53). Hosted here: Atari Mega STE Owner's Manual (PDF).


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