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Sharp X68000 XVI General Maintenance

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Sharp X68000 XVI CZ-634C rear panel (htomari)
Rear of an X68000 XVI: the main POWER switch, mains inlet and switched outlet are on the right-hand tower, below the ventilation grille (photo: htomari, CC BY-SA 2.0)

The Sharp X68000 XVI (CZ-634C and CZ-644C) went on sale in May 1991.[1] Both Sharp's owner's manual and Sharp's service manual for it are hosted here, so most of the advice below comes from Sharp; the repair experience comes from Fram and Poe, two Japanese repairers who have written up their work on XVI machines. The repairs they describe concern the power supply's electrolytic capacitors, the backup battery and the floppy drives.[2][3]

Before you start

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  • The XVI runs from 100 V AC only. Sharp says that a supply voltage well above or below that causes faults.[4] In countries with 220 to 240 V mains, use a step-down transformer and check its output before connecting the computer.
  • There are two power switches: the front switch and the main switch on the rear. Sharp's order is rear switch on, then front switch on; to switch off, turn the front switch off, wait until the 16MHz or 10MHz lamp turns red, then turn the rear switch off. Leave at least 10 seconds between switching off and on, and do not pull the plug with the machine switched on.[4]
  • With only the front switch off, the machine is not fully off: the timer and the TV control of a Sharp display still work.[4][5]
  • The service outlet on the rear (AC100V OUT, up to 300 W) is live whenever the mains plug is in, whatever the state of either switch.[5] Unplug the machine itself before working on it, and unplug anything powered from that outlet.
  • The power supply's primary side holds more than 100 V DC after the plug is pulled.[2]

Opening the case

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The service manual's disassembly drawings show the order: take out the three screws on the rear of a tower, slide its side cover off, then remove the screws and connectors in the numbered sequence for the part you need.[6] The power supply, with its fan, is in the left-hand tower seen from the front and comes out after two screws and its connectors.[2][6] The control board, which carries the backup battery, is in the base.[7][2]

Backup battery

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The 16 KB SRAM, which holds the settings made with SWITCH.X, is backed up by a 3.0 V battery on the control board.[8][7][2]

  1. Fram found the battery in his XVI-HD completely flat. He replaced it with a CR2032 in a holder, so that it can be changed later without taking the base board out.[2]
  2. After fitting the new battery he initialised the SRAM, installed the boot menu and set the machine up again with SWITCH.X.[2]
  3. Fram observed that the battery does not seem to be used while the rear main switch is left on.[2]

If the SRAM contents are corrupt, they can be cleared from Human68k; see the Troubleshooting Guide.

Capacitors

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Sharp's circuit diagrams show 13 electrolytics in the power supply, 4 on the main board, 36 on the control board, 8 on the analogue board, 4 on the expansion slot section and 2 in the keyboard, with more in each floppy drive.[7][9] The supply is the part that fails: in Fram's XVI-HD its capacitors had leaked, and the electrolyte had shorted parts of the circuit, burst a power transistor and run down the cable onto the base board.[2] Replace the supply's capacitors first, then the boards. Values, polarity errors in the service manual and the procedure are in the Sharp X68000 XVI Capacitor Replacement Guide.

Case cleaning and handling

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  • Wipe the case with a soft cloth dampened with water or with water and a little neutral detergent. Never use benzine or thinner: Sharp warns that they strip or discolour the finish.[10]
  • Condensation inside the machine can cause malfunctions. Sharp lists humid or steamy rooms, bringing the machine from a cold place into a warm one, a room just after heating, and cold air from an air conditioner or fan.[10]
  • Sharp says to use the machine standing upright and not to move it while it is working. Before moving it, switch off at the front, then at the rear, and unplug it.[10]
  • If liquid gets inside, switch off at the front and the rear and unplug it straight away.[10]

Floppy drives

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Poe has repaired the drives from XVI machines. From one XVI he replaced the write-protect sensor in drive 0, replaced every capacitor and re-greased both drives, and fitted a new E-ring to drive 0; the drives were version 06.[3]

  1. Replace the drive's electrolytic capacitors; Poe lists them by drive version in the capacitor guide.[3]
  2. Leave the spindle motor and the stepping motor in place. The head position is set against the index timing, and Poe warns that shifting the clamp hub moves the heads inwards or outwards.[3]
  3. If you remove an E-ring, refit it with the sharp-edged face upwards; the other face is rounded from punching.[3]

Sharp's manual adds that nothing but discs and the protective sheet should go into the drives, that nothing should block a disc as it ejects, and that thick or lifting labels can stop a disc ejecting.[11] More general advice is in Floppy Drive Repair.

Discs

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Sharp's manual says to keep discs in their envelopes, not to touch the recording surface, to keep them away from magnets, not to bend them, and to keep them out of direct sunlight and away from temperatures of 50 °C or more.[12]

Keyboard

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The keyboard has two electrolytics, C3 (2.2 µF 50 V) and C8 (100 µF 10 V).[7] Sharp's manual warns that plugging the keyboard in while the rear main switch is on can make the machine misbehave; with the keyboard plugged in, switch off at the front, wait for the red lamp, switch off at the rear, wait about 10 seconds and switch on again.[4]

Power supply

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  • The supply is the SH4, board UADP-0069CEZZ, also used in the EXPERT, EXPERT II, SUPER and Sharp X68030.[13][9]
  • Its fuse is F1, rated 125 V 4 A.[9]
  • The supply switches its outputs on when its own main switch is on, so it can be tested on the bench out of the machine.[2]

Checklist

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  1. Confirm the mains supply is 100 V, through a step-down transformer if needed.
  2. Unplug the machine and anything on its service outlet before opening it.
  3. Recap the power supply, fitting C27, C28 and C31 by the board silkscreen.
  4. Recap the control, main, analogue and slot boards.
  5. Replace the backup battery and reset the SRAM settings.
  6. Service both floppy drives: capacitors, grease, E-rings, leaving the motors in place.
  7. Recap the keyboard.
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References

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  1. ↑ X68000, IPSJ Computer Museum, Information Processing Society of Japan—link(accessed 2026-10-21) Series table: X68000 XVI (CZ-634C) and XVI-HD (CZ-644C), May 1991.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 X68kXVI HDの電源リペア– Fram, 2015-05-23—link(accessed 2026-10-21) The primary side holds more than 100 V DC after unplugging. The supply is in the left-hand tower seen from the front. Leaking capacitors; electrolyte shorted the zener diode, R33, R34 and a transistor, burst a power transistor and ran down the cable to the base board; the fuse had blown. The supply turns its outputs on when its own main switch is on. The backup battery was completely flat; he fitted a CR2032 in a holder so that it can be changed without removing the base board, then initialised the SRAM, installed the boot menu and set up the machine with SWITCH.X. The battery appears not to be used while the main switch is left on.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 SHARP X68000 FDD K61432-71 K61432-72 K-61435-72 修理– Poe, 2020-02-23—link(accessed 2026-10-21) XVI drive version 06, repaired 23 May 2020: write-protect sensor replaced (drive 0), all capacitors replaced (drives 0 and 1), E-ring replaced (drive 0), regreased (drives 0 and 1). E-rings are punched and have one rounded side; fit the sharp-edged side up. The head position is set against the index timing; do not remove the spindle or stepping motor just to change capacitors, because a shifted clamp hub moves the heads inwards or outwards.
  4. ↑ 4.0 4.1 4.2 4.3 Sharp, X68000 XVI・XVI HD 取扱説明書 (File:Sharp X68000 XVI Owner's Manual.pdf), 4.3, power, printed pages 24 and 25 (PDF pages 40 and 41): use 100 V AC; a voltage far too high or too low causes faults; switch on and off at intervals of 10 seconds or more; do not pull out or push in the plug with the switch on; on: rear main switch then front switch; off: front switch, confirm the power is fully off (16MHz or 10MHz lamp lit red), then rear main switch. Plugging in the keyboard with the rear main switch on can cause misbehaviour; in that case, with the keyboard plugged in, confirm the front switch is off (lamp red), turn the main switch off, wait about 10 seconds and turn it on again. The front switch alone does not cut the power fully; the timer and TV control still work. For long periods of non-use switch off at the front, then the rear, and unplug. Described at Sharp X68000 XVI Owner's Manual.
  5. ↑ 5.0 5.1 Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 3.2, rear of the computer, printed pages 19 and 20 (PDF pages 35 and 36): main power switch (with it on, TV control and timer control work even with the front switch off; normally left on; to cut the power fully, switch off at the front, confirm the power is off, then switch off at the rear); service outlet AC100V OUT, up to 300 W, live whenever the mains plug is in regardless of the front and rear switches.
  6. ↑ 6.0 6.1 Sharp, service manual CZ-134 (File:Sharp X68000 XVI Service Manual.pdf), 28, disassembly of the printed circuit boards, printed pages 111 and 112 (PDF pages 94 and 95): remove the circled screws and connectors in numbered order; three screws on the rear of each tower; the power supply with its fan held by two screws and two connectors.
  7. ↑ 7.0 7.1 7.2 7.3 Sharp, service manual CZ-134 (File:Sharp X68000 XVI Service Manual.pdf); see Sharp X68000 XVI Service Manual. Main board diagrams, printed pages 75 to 84 (PDF pages 73 to 77); control board diagram with the 3.0 V backup battery, printed pages 85 and 86 (PDF page 78); I/O diagrams, printed pages 89 and 90 (PDF page 80); analogue board diagram, printed pages 93 and 94 (PDF page 83); keyboard diagram with C3 2.2 µF 50 V and C8 100 µF 10 V, printed pages 101 and 102 (PDF page 87).
  8. ↑ Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), B.2 specifications, printed pages 127 and 128 (PDF pages 143 and 144): static RAM 16 KB.
  9. ↑ 9.0 9.1 9.2 Sharp, service manual CZ-134 (File:Sharp X68000 XVI Service Manual.pdf), 22, power supply basic wiring diagram, printed pages 97 and 98 (PDF page 85): board UADP-0069CEZZ, 13 electrolytics, fuse F1 125 V 4 A.
  10. ↑ 10.0 10.1 10.2 10.3 Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 4.2, everyday use, printed pages 22 and 23 (PDF pages 38 and 39): use it standing upright; avoid knocks; do not move it while working; to move it, confirm it is not working, switch off at the front, then the rear, and unplug it; if liquid gets in, switch off front and rear, unplug and contact the dealer; clean with a soft cloth and water or water with diluted neutral detergent, never benzine or thinner, which strip or discolour the finish; condensation can cause malfunctions, especially in humid or steamy places, after moving from cold to warm, just after heating a room, and in direct cold air from an air conditioner or fan.
  11. ↑ Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 6.3, floppy drive precautions, printed page 47 (PDF page 63): do not block a disc as it ejects; put nothing but discs and the protective sheet in the drives; thick labels or labels with lifting corners can stop a disc ejecting.
  12. ↑ Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 6.3, handling precautions, printed page 45 (PDF page 61): keep discs in their envelopes when not in use; do not touch the recording surface; keep away from magnets; do not bend; avoid direct sunlight and temperatures of 50 °C or more.
  13. ↑ X68000シリーズ電源のハンダ面から見た抵抗配置図– Kunihiko Ohnaka (kunichiko), 2020-10-10—link(accessed 2026-10-21) SH4 supply in the EXPERT, EXPERT II, SUPER, XVI and X68030.


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