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Sharp X68000 ACE Troubleshooting Guide

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Sharp X68000 ACE black (Darklanlan)
A black X68000 ACE with its keyboard and a Sharp monitor (photo: Darklanlan, CC BY 4.0)

Fault-finding on the Sharp X68000 ACE (CZ-601C) and ACE-HD (CZ-611C) is less well documented than on the later models: their SH2 and SH3 power supplies and their boards differ from the SH4 machines, and no service manual for them has been published.[1] The faults below come from Poe, a Japanese repairer, and from general X68000 sources.[2]

Warning: the supplies run from 100 V mains. On the SH4 of the later models the primary side holds more than 100 V DC after the plug is pulled; treat the SH2 and SH3 the same way.[3]

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, leaking supply capacitors No power
Settings lost; corrosion near the backup cell Leaking nickel-cadmium cell, corroded MC68901P or tracks Backup cell
Disc ejected without being read Drive fault after servicing Floppy drives
Settings corrupt SRAM contents Clearing the SRAM

No power

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  1. Check the supply voltage: the X68000 series is built for 100 V mains.[4]
  2. Check the main switch on the rear, which the tower models have as well as the front switch.[4]
  3. Open the supply and look for leaked electrolyte; Poe found leakage on an SH2 board.[2]
  4. Recap it, recording each value from the part; only C9 (390 µF 180 V) and C40 and C41 (3900 µF 10 V) of the SH2 are on record.[5] See the Sharp X68000 ACE Capacitor Replacement Guide.
  5. If a recapped supply still fails, compare in-circuit resistances with kunichiko's readings from an ACE's SH3; open transformer windings and failed transistors do not show on inspection.[1]

Backup cell and control board

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On Poe's ACE the nickel-cadmium cell on the control board (K5710DE) had leaked, corroding its terminals almost through, leaving green corrosion on the contacts and rusting pins 1 to 4 of the MC68901P beside it, which he replaced.[2] The MC68901P is the MFP that, on the XVI at least, also handles the keyboard data and interrupts.[6] After a leak:

  1. Remove the cell and see how far the electrolyte has gone.[2]
  2. Look for broken tracks.[2]
  3. Replace corroded chips and capacitors in the leak area.[2]

Without a working cell the 16 KB SRAM loses the memory switch settings.[4]

Clearing the SRAM

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Poe gives this method with the debugger DB.X under Human68k:[7]

  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 reset.

Some versions of DB.X did not accept the commands for Poe; SRAM_CLR.R clears the SRAM as well.[7]

Floppy drive faults

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  • From the ACE onwards a stuck disc is forced out from the front of the machine.[4]
  • After Poe replaced the capacitors and re-greased the drives of an ACE-HD (K61432-72, version 02), drive 0 ejected discs without reading them. He had not traced the cause when he wrote.[2]
  • Capacitors leak under the spindle motor and on the motor driver board of these drives.[8]
  • Except where the spindle motor is designed to come out easily, as on version 02, leave the spindle and stepping motors in place; the head position is set against the index timing.[2][8]

See also Floppy Drive Repair.

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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) ACE SH2, ACE-HD SH3, later tower models SH4. Open transformer windings and failed transistors do not show on inspection; in-circuit resistance readings from an ACE's SH3 are given for comparison.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 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) SH2 board with leakage; control board K5710DE with the leaked nickel-cadmium cell, terminals corroded almost to breaking, contacts with green corrosion, rust on pins 1 to 4 of the MC68901P (replaced); check the leak's extent and look for broken tracks. ACE-HD drive version 02: spindle motor easy to remove; capacitors and grease; afterwards drive 0 ejected discs without reading them, cause not found.
  3. ↑ 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.
  4. ↑ 4.0 4.1 4.2 4.3 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.
  5. ↑ SHARP X68000 修理– Poe—link(accessed 2026-10-21) Power supply table, ACE SH2: C9 180 V 390 µF, C40/C41 10 V 3900 µF.
  6. ↑ Sharp, service manual CZ-134 for the X68000 XVI (File:Sharp X68000 XVI Service Manual.pdf), 1-2 specifications, printed page 4 (PDF page 4): MFP MC68901, receiving key data and handling interrupts.
  7. ↑ 7.0 7.1 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) X68000 SRAM initialisation with DB.X: db; -mes e8e00d 31; -mes ed0000 00; -mes e8e00d 00; -q; some versions of DB.X did not accept the commands; SRAM_CLR.R as an alternative.
  8. ↑ 8.0 8.1 SHARP X68000 FDD K61432-71 K61432-72 K-61435-72 修理– Poe, 2020-02-23—link(accessed 2026-10-21) Capacitors under the spindle motor and on the motor driver board found leaking; do not remove the spindle or stepping motor just to change capacitors.


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