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Atari Mega ST General Maintenance

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Atari Mega ST with SM124 monitor and Megafile hard disk. The clock battery housing is on the top of the case, behind the left fan vent.

The Atari Mega ST is a two-piece machine: a system unit with an internal switching power supply and fan, and a detached keyboard that also carries the mouse and joystick ports. The parts that need regular attention are the clock batteries, the power supply, the floppy drive and the keyboard cable. Atari's Mega Service Manual and the owner's manual are both hosted here and are cited throughout.

Safety

Warning: the power supply is inside the system unit and its primary side runs at rectified mains voltage. Unplug the machine before removing the cover. The supply's reservoir capacitors hold their charge after unplugging; measure them before touching the primary side. See Atari Mega ST Capacitor Replacement Guide.

The custom chips are static-sensitive. Atari's service manual asks for them to be kept in their carriers or on conductive foam when out of the board.[1]

Opening the case

Atari's procedure:[2]

  1. Unplug the keyboard from the left side of the case and turn the unit upside down.
  2. Remove the nine screws in the square holes. If the board or the floppy drive is coming out, also remove the three screws in the round holes, which hold the drive.
  3. Turn the unit upright. Lift the back of the top cover slightly and unplug the clock battery connector from underneath its left rear corner, then lift the cover off.
  4. Straighten the six twist tabs of the upper shield (one is under the floppy drive), lift the back of the shield, push the drive up and pull the shield forward and out.
  5. To remove the supply, take out the two screws at its front corners, unplug the harness at its right front corner and lift it out.
  6. To remove the board, take out the six studs holding the I/O shield to the case and lift the assembly out from the front; then straighten the six twist tabs of the lower shield.

On reassembly, put the insulation panel back on the bottom shield before the board, push the battery connector up through the opening at the left rear corner of the top shield, and use the three longer screws in the round holes. Atari recommends retesting the machine once it is back in its case, to catch shorts to the shield.[2]

Cleaning

Keep the fan vents on the top and back panel clear. Clean the outside of the case with a soft, slightly damp, lint-free cloth with the machine switched off; Atari's manual rules out cleansers, abrasives and solvents.[3] Inside, remove dust from the board and the supply with a soft brush and low-pressure air.

Clock batteries

The Ricoh RP5C15 real-time clock is backed by 3 V from two AA cells.[4] The cells sit in a housing on the top of the case at the rear of the left fan vent. To change them, press the housing cover and pull it towards the front of the computer, fit two AA cells the way round marked in the slots, then push the cover down and back until it locks.[5] Set the date and time from the Control Panel afterwards.

Take the cells out if the machine is going into storage. Leaked alkaline cells leave an alkaline residue: neutralise it with white vinegar, then clean with isopropyl alcohol, and check the housing contacts and the lead that runs to the board. See Battery Refurbishment and Battery Explosion, Capacitor or Corrosion Damage.

A clock that loses time only when the machine is switched off points to the cells or the power-off sense circuit; one that does not run at all points to the RTC PAL (U37), the RP5C15 (U25) or its crystal.[6][7]

Power supply

The supply is Atari's ASP34-1 with its own fan. Its output connector carries +5 V on pins 1 and 2, ground on pins 3 to 5 and +12 V on pin 6.[8] Atari rates it at +5 V 3 A, +12 V 1 A and −12 V 30 mA, 95 W maximum;[9] its documents give no voltage tolerance. Check the rails with the board connected, since a switching supply is not meant to run unloaded. The supply is the first thing to recap on a machine of this age; see Atari Mega ST Capacitor Replacement Guide.

The input is set for 110 V or 220 V by a wire link, W1, on the supply board, not by a switch.[8] Check it before powering a machine brought in from another country.

Floppy drive

Atari fitted Newtronics and Chinon drives (parts C070350-003, C070352-003 and C103047-001).[7] Clean the heads with isopropyl alcohol on a lint-free swab or a cleaning disk; exxos lists head cleaning among the fixes worth doing on every machine.[10] If a replacement drive such as the Sony MPF920 is fitted, which lacks the pull-up resistors of the original drives, add pull-ups to the WD1772's output lines on the board.[10] Atari's diagnostic cartridge measures the rotation period, which should be between 196 and 204 ms.[11] See also Floppy Drive Repair.

Keyboard, cable and mouse

The keyboard plugs into the jack on the left side of the computer; the mouse goes into mouse/joystick port 0 on the underside of the keyboard.[12] The keyboard socket is a six-way modular jack.[13] The two outer contacts at each end carry +5 V and ground and the middle two carry the data lines,[14] so a replacement cord must be a six-wire cord wired pin 1 to pin 1.[15] A reversed telephone cord puts the supply on the wrong pins of the keyboard.

Atari's service manual notes that a keyboard plugged in while the machine is on can leave the keys dead while the mouse still works; switch off and on again.[6] A mouse that moves slowly or erratically may need cleaning.[3]

Sockets

The Glue (U17), MMU (U30) and Blitter (U5) sit in 68-pin LCC sockets, the DMA controller (U27) and Shifter (U31) in 40-pin sockets, and the TOS ROMs in 28-pin sockets.[7] Atari's dead-unit procedure checks the clocks to the Glue and memory controller and then swaps those chips where they are socketed.[16] If a machine has intermittent faults that change when the board is flexed, reseat the socketed chips and inspect the socket contacts.

Maintenance checklist

  1. Check the fan turns and the vents are clear.
  2. Remove or replace the clock cells; inspect the housing for leakage.
  3. Measure +5 V and +12 V with the machine running.
  4. Recap the power supply if it has not been done.
  5. Clean the floppy heads and test a read and a write.
  6. Check the keyboard cord is a straight six-wire cord and the plugs are clean.
  7. Add the DMA and 1772 pull-ups and the reset fix if the machine is used with hard disks or a replacement drive.[10]

References

  1. ↑ Atari Corporation, Mega Service Manual (Mega 2 and Mega 4), undated, section 6, "Diagnostic flowcharts": replacement procedures and handling of integrated circuits, p. 6.1. Hosted as File:Atari Mega ST Service Manual.pdf.
  2. ↑ 2.0 2.1 Atari Corporation, Mega Service Manual, section 4, "Disassembly/assembly", pp. 4.1–4.3. Hosted as File:Atari Mega ST Service Manual.pdf.
  3. ↑ 3.0 3.1 Atari Corporation, Computer MEGA ST Owner's Manual, 1987, chapter 1 (fan vents) and appendix B, "Troubleshooting and preventive maintenance", pp. 103–107. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  4. ↑ Atari Corporation, Mega Service Manual, section 2, "Real Time Clock with Battery Backup", p. 2.7. Hosted as File:Atari Mega ST Service Manual.pdf.
  5. ↑ Atari Corporation, Computer MEGA ST Owner's Manual, 1987, "Clock Batteries", pp. 6–7, and "The Computer's Top Panel", p. 18. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  6. ↑ 6.0 6.1 Atari Corporation, Mega Service Manual, section 5, "Symptom checklist", pp. 5.1–5.2. Hosted as File:Atari Mega ST Service Manual.pdf.
  7. ↑ 7.0 7.1 7.2 Atari Corporation, Mega Service Manual, section 7, parts list (PDF pp. 62–64). Hosted as File:Atari Mega ST Service Manual.pdf.
  8. ↑ 8.0 8.1 Atari Corporation, Mega Service Manual, section 8, "Schematic diagram (ASP34-1)" and "Power supply assy ASP34-1 (117V)" (PDF pp. 73 and 81). Hosted as File:Atari Mega ST Service Manual.pdf.
  9. ↑ Atari Corporation, Computer MEGA ST Owner's Manual, 1987, appendix C, pp. 109–110. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  10. ↑ 10.0 10.1 10.2 exxos, "MANDATORY FIXES", exxos's Atari, Amiga & retro forum, 31 January 2019. Community source; items 4 and 10.
  11. ↑ Atari Corporation, Mega Service Manual, section 3, "Testing", floppy disk tests, pp. 3.12–3.14. Hosted as File:Atari Mega ST Service Manual.pdf.
  12. ↑ Atari Corporation, Computer MEGA ST Owner's Manual, 1987, chapter 1, "Getting started": mouse and keyboard connection. Hosted as File:Atari Mega ST Owner's Manual (1987).pdf.
  13. ↑ "Desktop-Modelle: Der Atari Mega ST", atari-computermuseum.de, last edited 13 July 2024, https://www.atari-computermuseum.de/megast.htm, interface table. Secondary source.
  14. ↑ Dennis Salzner, "Atari ST Mega keyboard and mouse adapter", 13 June 2023, section "Atari Connector". Community source.
  15. ↑ "Mega ST keyboard cable replacement", exxos's Atari, Amiga & retro forum, January 2020. Community source.
  16. ↑ Atari Corporation, Mega Service Manual, section 3, "Troubleshooting a dead unit", pp. 3.2–3.3. Hosted as File:Atari Mega ST Service Manual.pdf.


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