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Sharp X68000 EXPERT General Maintenance

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Sharp X68030 and X68000 Expert
An X68000 EXPERT (right) beside an X68030 (photo: Manupkp, CC BY-SA 4.0)

The Sharp X68000 EXPERT (CZ-602C, CZ-612C) and EXPERT II (CZ-603C, CZ-613C) were sold from 1989 and 1990.[1] Their power supply, the SH4, is the same board as the Sharp X68000 XVI's, so Sharp's XVI service manual covers it.[2][3] Poe, a Japanese repairer, has written up EXPERT and EXPERT II repairs: a seized floppy drive head, the memory settings and a worn reset button.[4]

Before you start

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  • The X68000 series runs from 100 V AC.[5] In countries with 220 to 240 V mains, use a step-down transformer and check its output before connecting the computer.
  • The EXPERT has a main switch on the rear as well as the front switch.[5] Sharp's manuals for the original X68000 and for the XVI give the same routine: switch on at the rear and then the front, switch off at the front and then the rear, and leave at least 10 seconds between switching off and on.[6][7]
  • The SH4's primary side holds more than 100 V DC after the plug is pulled.[8]

Power supply

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Recap the SH4 first. Its 13 electrolytics, the polarity errors on Sharp's diagram and the full procedure are in the Sharp X68000 EXPERT Capacitor Replacement Guide and the Sharp X68000 XVI Capacitor Replacement Guide.[3][9] In Fram's SH4 the leaked electrolyte had shorted parts of the standby circuit, burst a power transistor and blown the fuse.[8]

Floppy drives

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Poe found an EXPERT's drive (K61432-72, version 05) with the lubricant on its head motor shaft set hard, so that the head did not move at all.[4]

  1. Clean the old lubricant off the head drive shaft and re-grease it.[4]
  2. Replace the drive's electrolytic capacitors. On version 05 the ones at the spindle motor are easy to reach.[4]
  3. Leave the spindle motor and the stepping motor in place; the head position is set against the index timing.[10]
  4. If you remove an E-ring, refit it with the sharp-edged face upwards; the other face is rounded from punching.[10]

See also Floppy Drive Repair.

Reset button

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If the reset button responds poorly, Poe replaces it. The replacement he uses is the Alps tactile switch SKHCBFA010 (12 × 12 × 4.3 mm, through-hole), sold by RS as stock number 119-2015.[4]

Memory settings

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The 16 KB SRAM holds the memory switch settings.[5] If they are corrupt, they can be cleared from Human68k; see the Troubleshooting Guide.[4]

Case cleaning

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Sharp's manuals for the original X68000 and for the XVI both say to wipe the case with a soft cloth dampened with water or a mild neutral detergent, and never to use benzine or thinner, which strip or discolour the finish.[6][7]

Checklist

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  1. Confirm the mains supply is 100 V.
  2. Recap the SH4, fitting C27, C28 and C31 by the board silkscreen.[9]
  3. Record and recap the main, base, analogue and slot boards.
  4. Clean, re-grease and recap both floppy drives.
  5. Replace a worn reset button.
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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: EXPERT CZ-602C, March 1989; EXPERT II CZ-603C, March 1990.
  2. ↑ 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) EXPERT and EXPERT II use the SH4, as do the SUPER, XVI and X68030.
  3. ↑ 3.0 3.1 Sharp, service manual CZ-134 for the X68000 XVI (File:Sharp X68000 XVI Service Manual.pdf), 22, power supply basic wiring diagram, printed pages 97 and 98 (PDF page 85): SH4 board UADP-0069CEZZ, 13 electrolytics. See Sharp X68000 XVI Service Manual.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 X68000 EXPERT CZ-602C CZ-612C / EXPERT II CZ-603C CZ-613C / FDDユニット K61432-72 修理備忘録– Poe—link(accessed 2026-10-21) Repair of an EXPERT (CZ-602C) drive K61432-72, version 05: the lubricant on the head motor shaft had hardened and the head did not move at all; cleaned, re-greased and all capacitors replaced; the spindle motor capacitors of version 05 are easy to change. SRAM initialisation with DB.X. A poorly responding reset button is replaced with an Alps SKHCBFA010 tactile switch, SPST-NO, through-hole, 12 × 12 × 4.3 mm, RS stock number 119-2015.
  5. ↑ 5.0 5.1 5.2 X68000, Wikipedia (Japanese)—link(accessed 2026-10-21) Power 100 V AC; the tower models have a main power switch on the rear as well as the front switch; 16 KB battery-backed SRAM keeps the memory switch settings.
  6. ↑ 6.0 6.1 Sharp, X68000 取扱説明書 for the CZ-600C (File:Sharp X68000 CZ-600C Owner's Manual.pdf), 4.2 and 4.3, printed pages 19 and 20 (PDF pages 33 and 34): soft cloth with water or neutral detergent, no benzine or thinner; 100 V AC; at least 10 seconds between off and on; rear main switch then front switch to switch on, front then rear to switch off.
  7. ↑ 7.0 7.1 Sharp, X68000 XVI・XVI HD 取扱説明書 (File:Sharp X68000 XVI Owner's Manual.pdf), 4.2, printed page 23 (PDF page 39): soft cloth with water or diluted neutral detergent, no benzine or thinner; 4.3, printed page 24 (PDF page 40): rear then front to switch on, front then rear to switch off, 10 seconds or more between off and on.
  8. ↑ 8.0 8.1 X68kXVI HDの電源リペア– Fram, 2015-05-23—link(accessed 2026-10-21) The SH4's primary side holds more than 100 V DC after the plug is pulled. Leaked electrolyte shorted the zener diode, R33/R34 and a transistor; a power transistor burst and the fuse blew.
  9. ↑ 9.0 9.1 【訂正】X68kの電源ユニットのシルク印刷に間違いがあった件&X68kの電源修理再び(未完)3– Fram, 2024-03-24—link(accessed 2026-10-21) The SH4 diagram draws C31, C27 and C28 with reversed polarity; the silkscreen is correct.
  10. ↑ 10.0 10.1 SHARP X68000 FDD K61432-71 K61432-72 K-61435-72 修理– Poe, 2020-02-23—link(accessed 2026-10-21) E-rings are punched and have one rounded side; fit the sharp-edged side up. Do not remove the spindle or stepping motor just to change capacitors, because the head position is set against the index timing.


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