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Sharp X68000 EXPERT Troubleshooting Guide

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

Fault-finding on the Sharp X68000 EXPERT and EXPERT II starts with the SH4 power supply, which they share with the Sharp X68000 SUPER and Sharp X68000 XVI.[1] Sharp's diagram of the supply, with the voltage on every output pin, is in the Sharp X68000 XVI Service Manual.[2] Poe, a Japanese repairer, has published EXPERT-specific fixes for the floppy drives, the memory settings and the reset button.[3]

Warning: the supply runs from 100 V mains and its primary side holds more than 100 V DC after the plug is pulled.[4]

Quick reference

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Symptoms, likely causes and where to look
Symptom Likely cause Section
Dead, no lamps Rear main switch off, no 100 V supply, blown fuse, leaking supply capacitors No power
Floppy drive will not read, head does not move Hardened lubricant on the head shaft, drive capacitors Floppy drives
Settings wrong or lost Corrupt SRAM, backup cell Settings
Reset button unreliable Worn tactile switch Reset button

No power

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  1. Check the supply voltage: the X68000 series is built for 100 V mains.[5]
  2. Check the main switch on the rear, which the tower models have as well as the front switch.[5]
  3. Take the supply out and check its fuse, F1, rated 125 V 4 A on Sharp's diagram.[2]
  4. Look for leaked electrolyte and burnt parts. In Fram's SH4 the electrolyte had shorted the zener diode, R33, R34 and a transistor, a power transistor had burst, and the fuse had blown.[4]
  5. Recap the supply; see the Sharp X68000 EXPERT Capacitor Replacement Guide. Fit C27, C28 and C31 by the board's silkscreen, not by Sharp's diagram.[6]
  6. Test the supply on the bench: the SH4 switches its outputs on when its own main switch is on.[4] The connector pin-out is tabled on the XVI capacitor guide.[2]
  7. If it is still dead, kunichiko's in-circuit resistance readings from a working SH4 help to find an open transformer winding or a failed transistor, which do not show on inspection.[1]

Floppy drive faults

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  • Head does not move: on an EXPERT drive (K61432-72, version 05) Poe found the lubricant on the head motor shaft set hard. Clean it off and re-grease the shaft.[3]
  • Capacitors: replace the drive's electrolytics; on version 05 the ones at the spindle motor are easy to reach.[3] The list is in the capacitor guide.
  • Disc stuck: from the ACE onwards a stuck disc is forced out from the front of the machine.[5]
  • Do not remove the spindle or stepping motor to reach a capacitor; the head position depends on them.[7]

See also Floppy Drive Repair.

Settings and SRAM

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The 16 KB SRAM keeps the memory switch settings and depends on a backup cell.[5] Poe gives this method for clearing corrupt SRAM with the debugger DB.X under Human68k:[3]

  1. Start Human68k and run db.
  2. Enter -mes e8e00d 31 to allow writes to the SRAM.
  3. Enter -mes ed0000 00 to clear the SRAM's program area.
  4. Enter -mes e8e00d 00 to protect the SRAM again.
  5. Enter -q to leave DB.X, then press reset.

DB.X from versions of the XC compiler before 2.0 did not accept the commands for Poe; use a later version. The program SRAM_CLR.R, from the retropc.net X68000 archive, also clears the SRAM.[3]

Reset button

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If the reset button responds poorly, replace it. Poe uses the Alps SKHCBFA010 tactile switch (12 × 12 × 4.3 mm), RS stock number 119-2015.[3]

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References

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  1. ↑ 1.0 1.1 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. Open transformer windings and failed transistors do not show on inspection; in-circuit resistance readings from a working SH4 board are given for comparison.
  2. ↑ 2.0 2.1 2.2 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, fuse F1 125 V 4 A, output connector pin-outs. See Sharp X68000 XVI Service Manual.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 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) EXPERT drive K61432-72, version 05: head motor shaft lubricant hardened, head did not move; cleaned, re-greased, all capacitors replaced; version 05 spindle motor capacitors easy to change. SRAM initialisation with DB.X: db; -mes e8e00d 31; -mes ed0000 00; -mes e8e00d 00; -q; reset. DB.X from XC before version 2.0 did not accept the commands; use 2.0 or later. SRAM_CLR.R from http://retropc.net/x68000/software/system/sram/sram_clr_r/ as an alternative. Reset button replaced with an Alps SKHCBFA010 tactile switch, 12 × 12 × 4.3 mm, RS 119-2015.
  4. ↑ 4.0 4.1 4.2 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 unplugging. Leaked electrolyte shorted the zener diode, R33/R34 and a transistor; the power transistor below them burst; the fuse had blown. The supply turns its outputs on when its own main switch is on.
  5. ↑ 5.0 5.1 5.2 5.3 X68000, Wikipedia (Japanese)—link(accessed 2026-10-21) Power 100 V AC; the tower models have a main power switch on the rear; 16 KB battery-backed SRAM keeps the memory switch settings; the ACE (March 1988) added forced ejection of floppy discs from a front switch.
  6. ↑ 【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.
  7. ↑ SHARP X68000 FDD K61432-71 K61432-72 K-61435-72 修理– Poe, 2020-02-23—link(accessed 2026-10-21) 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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