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Sharp X68000 General Maintenance

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Sharp X68000 CZ-600C opened (Dave Jones)
A CZ-600C with the main board K5023DE (left) and the video board K5024DE (right) out of the board tower (photo: Dave Jones, CC BY 2.0)

The original Sharp X68000 (CZ-600C) went on sale in March 1987, so the newest examples are well over thirty years old.[1] Owners' repair notes for it concentrate on the nickel-cadmium backup cell on the main board, the electrolytic capacitors, and the two motorised floppy drives. Poe, a Japanese repairer, has published his complete service of a CZ-600C, and the steps below follow it.[2] The capacitor values are in the Sharp X68000 Capacitor Replacement Guide and fault-finding is in the Sharp X68000 Troubleshooting Guide.

Before you start

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  • The X68000 runs from 100 V Japanese mains.[3] In countries with 220 to 240 V mains, use a step-down transformer and check its output before connecting the computer.
  • The tower models have a main switch on the rear as well as the power switch on the front.[3] Sharp's manual says to switch on at the rear main switch first and then the front switch, to switch off in the reverse order, and to leave at least 10 seconds between switching off and on again.[4] Switch both off and unplug the machine before opening it.
  • The power supply sits in the drive tower beside the floppy drives, and the main board and video board sit in the other tower.[5][6] Photograph the cables before you unplug them.

Memory backup cell

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A rechargeable nickel-cadmium cell, GS GB50H-3 (3.6 V, 50 mAh), on the main board keeps the 16 KB SRAM's settings while the machine is unplugged.[2][1] Poe found these cells leaking.[2]

  1. Unsolder the cell. Poe fitted a 3.6 V 60 mAh replacement.[2]
  2. Clean the leaked electrolyte from the board. Nickel-cadmium electrolyte is alkaline; the clean-up is described in Battery Explosion, Capacitor or Corrosion Damage.
  3. Look at the crystal oscillator near the cell. On later-lot boards it lies flat and corrodes when the cell leaks; on early grey boards it stands upright, which Poe says keeps it clear of the leak.[2] Replace it if its legs or case are corroded.
  4. Check the tracks around the cell for breaks before powering up. Poe does this after removing a leaked cell from an ACE board.[7]

Without a working cell the settings are lost each time the machine is unplugged. A corrupt SRAM can be cleared from Human68k as described in the Troubleshooting Guide.

Capacitors

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Every board, the power supply, the memory board, both floppy drives and the keyboard have electrolytic capacitors. Poe's lists give 15 in the supply, 20 on the main board, 2 on the video board, 33 on the board in the base and 1 on the memory board, with more in each drive and two in the keyboard.[2] Replace the supply's first. See the Sharp X68000 Capacitor Replacement Guide.

Case and keyboard cleaning

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Sharp's manual says to wipe the case with a soft cloth dampened with water or a neutral detergent, and never to use benzine or thinner, which strip or discolour the finish.[4] Bring a machine stored somewhere cold up to room temperature before switching on: the manual warns that condensation inside causes malfunctions.[4]

Floppy drives

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The CZ-600C has its own drive unit, the K-61432-71.[8] Service each drive in this order:

  1. Blow the dust out with compressed air.[2]
  2. Remove the old grease with cotton buds and re-grease the spiral head-seek shaft, the guides, the head-load rollers, the auto-load mechanism on the underside, and the eject motor and its gears.[2]
  3. Replace the drive's electrolytic capacitors. Poe found one of his two drives leaking near the power connector.[2]
  4. Leave the spindle motor and the stepping motor where they are. The head position depends on their alignment.[8]
  5. If you remove an E-ring, refit it with the sharp-edged face upwards; the other face is rounded from punching.[8]
  6. Check the contacts in both drive cable connectors for corrosion.[2]

Sharp's manual adds that nothing but discs and the protective sheet should go into the drives, and that thick or lifting labels can stop a disc ejecting.[9] More general advice is in Floppy Drive Repair.

Discs

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Poe checks his master discs for mould by turning the hub by hand and looking for white specks. He removes it with isopropyl alcohol on chamois swabs, and avoids ethanol and cotton buds, which can damage the magnetic coating.[2]

Keyboard

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The CZ-600C keyboard uses Alps green switches and has two electrolytics, C3 (2.2 µF 50 V) and C8 (100 µF 10 V).[2] Poe blew the dust out with compressed air and cotton buds, cleaned the case and replaced both capacitors.[2]

Power supply

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  • Poe replaced the supply's fan with an X-FAN RDL6025S (12 V, 0.07 A).[2]
  • The cable harness uses 3.96 mm VH crimp contacts. Poe keeps contacts and a crimping tool for repairs.[2]
  • If the supply cannot be repaired, Poe points to the AC adapter set and ATX power cable made by Mashiron and sold by the shop BEEP, and antongale has published a PicoPSU adapter board for the CZ-600C.[2][10]

Checklist

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  1. Confirm the mains supply is 100 V, through a step-down transformer if needed.
  2. Remove or replace the backup cell and inspect the crystal oscillator beside it.
  3. Recap the power supply, then the boards.
  4. Service both floppy drives: clean, re-grease, recap, check the cable connectors.
  5. Recap the keyboard.
  6. Check discs for mould before use.
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References

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  1. ↑ 1.0 1.1 X68000, IPSJ Computer Museum, Information Processing Society of Japan—link(accessed 2026-10-21) First sold March 1987; 16 KB SRAM; two 5-inch 2HD floppy drives.
  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 2.12 2.13 2.14 2.15 SHARP X68000 修理– Poe—link(accessed 2026-10-21) CZ-600C service notes: GS Ni-Cd GB50H-3 3.6 V 50 mAh SRAM cell and a 3.6 V 60 mAh replacement; early and late main board lots, the late lot with the crystal oscillator lying flat where the cell's leak corrodes it; floppy drive service (dust blown out, dried grease replaced on the spiral head-seek shaft, the guides, the head-load rollers, the auto-load mechanism underneath and the eject motor and gears; capacitors replaced, one of two drives leaking near the power connector); corrosion inside the drive cable connectors; mould on discs removed with isopropyl alcohol and chamois swabs; keyboard cleaned and its two capacitors replaced, early keyboards with Alps green switches and later ones with Alps yellow; supply spares: 250 V 4 A glass fuse in PCB clips, 3.96 mm VH connector contacts and crimping pliers, X-FAN RDL6025S 12 V 0.07 A fan; alternatives to a supply repair from the BEEP shop (AC adapter set and ATX cable made by Mashiron).
  3. ↑ 3.0 3.1 X68000, Wikipedia (Japanese)—link(accessed 2026-10-21) Specification sheet: power 100 V AC, 50/60 Hz. Tower models have a main power switch on the rear as well as the front switch; the two towers hold the floppy drives and power supply on one side and the main board and two expansion slots on the other; 16 KB battery-backed SRAM for the memory switch settings.
  4. ↑ 4.0 4.1 4.2 Sharp, X68000 取扱説明書 (File:Sharp X68000 CZ-600C Owner's Manual.pdf), 4.2 and 4.3, printed pages 19 and 20 (PDF pages 33 and 34): clean the case with a soft cloth and water or neutral detergent, never benzine or thinner, which strip or discolour the finish; condensation can cause malfunctions; use 100 V AC; wait at least 10 seconds between switching off and on; do not pull the plug while the machine is on; switch on with the rear main switch and then the front switch, and off with the front switch and then the rear main switch. Described at Sharp X68000 CZ-600C Owner's Manual.
  5. ↑ Sharp X68000 Personal Computer Teardown (17265064954)– Dave Jones, Wikimedia Commons—link(accessed 2026-10-21) Front of a black CZ-600C with the main board K5023DE and the video board K5024DE taken out beside it.
  6. ↑ Sharp X68000 Personal Computer Teardown (17267174853)– Dave Jones, Wikimedia Commons—link(accessed 2026-10-21) Inside of the drive tower: floppy drive and the power supply labelled UADP-0047CEZZ.
  7. ↑ X68000 ACE/ACE-HD CZ-601C CZ-611C / FDDユニット K61432-72 / 電源 SH2･SH3 修理– Poe, 2021-01-20—link(accessed 2026-10-21) After removing the leaked Ni-Cd cell from the ACE control board, check how far the leak has spread and look for broken tracks.
  8. ↑ 8.0 8.1 8.2 SHARP X68000 FDD K61432-71 K61432-72 K-61435-72 修理– Poe, 2020-02-23—link(accessed 2026-10-21) K-61432-71 drive used only in the CZ-600C; capacitors at the centre of the drive and on the spindle motor driver board found leaking; E-rings are punched and have one rounded side, so fit them with the sharp-edged side up; the head position is set against the index sensor timing and depends on the spindle and stepping motor alignment, so do not remove those motors just to change capacitors.
  9. ↑ Sharp, X68000 取扱説明書 (File:Sharp X68000 CZ-600C Owner's Manual.pdf), 6.3, printed page 42 (PDF page 56): if a foreign object or a program crash stops a disc ejecting, use the DISK EJECT button on the rear, which forces the disc out regardless of the drive's state and can corrupt the disc if used during normal operation; do not block the disc as it ejects; put nothing but discs and the protective sheet in the drives; thick or lifting labels can stop a disc ejecting.
  10. ↑ OGX68KPicoPSU– antongale, GitHub—link(accessed 2026-10-21) Repository description: "X68000 PicoPSU for the original CZ-600 model."


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