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Sharp X68000 XVI Troubleshooting Guide

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Sharp X68000 XVI CZ-634C front (htomari)
Front of an X68000 XVI. The 16MHz and 10MHz lamps on the right-hand tower also serve as the power lamp (photo: htomari, CC BY-SA 2.0)

Fault-finding on the Sharp X68000 XVI (CZ-634C and CZ-644C) starts with the lamps, the two power switches and the SH4 power supply. Sharp's owner's manual explains what the lamps mean, and Sharp's service manual gives the circuit of the supply and the voltage on every pin of its output connectors.[1][2] The fault reports below come from Fram and Poe, Japanese repairers who have worked on XVI machines, and from kunichiko's notes on the X68000 supplies.[3][4][5]

Warning: the supply runs from 100 V mains and its primary side holds more than 100 V DC after the plug is pulled. The service outlet on the rear is live whenever the machine is plugged in, whatever the switches are set to.[3][6]

Quick reference

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Symptoms, likely causes and where to look
Symptom Likely cause Section
No lamps at all with the rear switch on No 100 V supply, blown fuse F1, failed supply No power
16MHz or 10MHz lamp red, front switch does nothing Supply's main outputs missing Lamp red
Lamp flashes green and stays that way Timer or remote start in progress, or switching off Reading the lamps
Settings lost every time the machine is unplugged Flat backup battery Settings
Machine misbehaves after the keyboard was plugged in Keyboard connected with the rear switch on Keyboard
Disc will not eject Eject button lamp off, label, damaged drive Floppy drives

Reading the lamps

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The 16MHz and 10MHz lamps show the clock speed and also act as the power lamp. Sharp's manual gives these states:[1]

  • Red: only the rear main switch is on (standby).
  • Green: the front switch is on as well and the machine is running.
  • Flashing green: the machine is being switched on by the timer or a signal on the rear REMOTE connector, or the front switch has been turned off and the power has not yet gone off. After a timer start the lamp stays steady green for one minute and then flashes.

The TIMER lamp is red while the timer is set. The HD BUSY lamp lights red while the internal hard disk is working; on the CZ-634C it works only if the optional CZ-68H hard disk has been fitted.[1] Turning the front switch off while a drive is working does not cut the power at once; the switch is designed to wait.[1]

No power

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  1. Check that the machine is on 100 V AC. Sharp says a voltage far too high or too low causes faults.[7]
  2. Check that the rear main switch is on. Sharp's order is the rear switch first, then the front switch.[7]
  3. The power lamp board takes both the standby +5 V rail (VCC2) and the main +5 V rail (VCC1).[8] If no lamp lights with the rear switch on and mains present, start with the supply's input and standby side.
  4. Take the supply out and check its fuse, F1, rated 125 V 4 A.[2] If it has blown, find out why before fitting another. In Fram's XVI-HD the leaking capacitors had shorted the zener diode, R33 and R34 and a transistor on the standby side, a power transistor had burst, and the fuse had blown.[3]
  5. Replace the supply's electrolytic capacitors and clean off any electrolyte; see the Sharp X68000 XVI Capacitor Replacement Guide. Fit C27, C28 and C31 by the silkscreen on the board, not by the service manual's diagram, which has their polarity reversed.[9]
  6. If the supply still does not work, kunichiko notes that new capacitors and fixed solder joints are often not enough, because open transformer windings and failed transistors do not show on inspection; he has published in-circuit resistance readings from a working SH4 for comparison.[5]

Lamp red but the machine will not start

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  1. Measure the supply's outputs. The SH4 switches its outputs on when its own main switch is on, so it can be tested out of the machine.[3] The connector pins are:
SH4 output connectors (service manual, printed pages 97 and 98)
Connector Pins
A (control board) A1 +5 V, A2 ground, A3 +12 V, A4 −12 V, A5 +5 V standby (VCC2), A6 power control
B (main board) B1 +5 V, B2 ground
P and Q (floppy drives) +12 V, ground, +5 V
D (hard disk) D1 +5 V, D2 ground, D3 +12 V

[2]

  1. A missing +12 V points at IC22 (SI3122V) and its capacitors C24 and C25; a missing −12 V at IC23 (µPC78M12HF) with C27 and C28; a missing +5 V at the rectifier D27, C40 to C42 and the switching stage on the primary side; a missing standby rail at IC31 (µPC78M05HF), C33, C34 and the T2 converter.[2]
  2. Fram found the outputs of his repaired supply unsteady when first measured, and steady when he measured again after the machine had run.[3]
  3. If the supply is good, check the cable to the control board, connector A, which also carries the power control line on pin A6.[2]

Settings lost or corrupt

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  • The 16 KB SRAM is backed up by a 3.0 V battery on the control board.[8][10] Fram found it completely flat in his XVI-HD, fitted a CR2032 in a holder, initialised the SRAM and set the machine up again with SWITCH.X.[3]
  • To clear corrupt SRAM contents, Poe gives this method with the debugger DB.X under Human68k:[11]
  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.

Poe found that some versions of DB.X did not accept these commands. The program SRAM_CLR.R clears the SRAM as well.[11]

Keyboard

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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 lamp to turn red, switch off at the rear, wait about 10 seconds and switch on again.[7] The keyboard's own controller is an 80C51 with a reset capacitor C3 (2.2 µF 50 V) and a supply capacitor C8 (100 µF 10 V), the only electrolytics in the keyboard.[8]

Floppy drive faults

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  • Drive lamps: green when a disc is in, red while the drive is working, flashing green when the machine wants a disc.[1]
  • A disc inserted the wrong way round is ejected automatically.[12]
  • The eject button works only while the green lamp in the button is lit. While a disc is being written the lamp goes out and the button does nothing, but holding it for about three seconds or more ejects the disc anyway.[12]
  • Thick labels, or labels with lifting corners, can stop a disc ejecting.[12]
  • Poe replaced a faulty write-protect sensor in drive 0 of an XVI, as well as all the drive capacitors.[4] He notes that version 07 drives, fitted to some late machines, use their own protect sensor and other parts with no substitute; if a 07 drive will not lock a disc after insertion, the wheel board or the index sensor is at fault and he replaces the drive.[4]
  • Do not remove the spindle or stepping motor to reach a capacitor; the head position depends on them.[4]

See also Floppy Drive Repair.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Sharp, X68000 XVI・XVI HD 取扱説明書 (File:Sharp X68000 XVI Owner's Manual.pdf), 3.1, front of the computer, printed pages 13 to 15 (PDF pages 29 to 31): floppy drive lamps green with a disc in, red while working, flashing green to ask for a disc; eject buttons work while their lamps are lit; power switch, with a function that stops the power going off at once if it is switched off while a drive is working; 16MHz and 10MHz lamps double as the power lamp, red with only the rear main switch on, green with the front switch on, flashing green while the power is being switched on by the timer or the REMOTE connector and between switching off and the power going off, steady green for one minute after a timer start; TIMER lamp red while the timer is set; HD BUSY lamp red while the internal hard disk works, on the CZ-634C only with the CZ-68H hard disk fitted. Described at Sharp X68000 XVI Owner's Manual.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 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); see Sharp X68000 XVI Service Manual. Board UADP-0069CEZZ; fuse F1 125 V 4 A; +12 V from IC22 (SI3122V) with C24 and C25; −12 V from IC23 (µPC78M12HF) with C27 and C28; +5 V from D27 with C40, C41 and C42 after L21; standby +5 V (VCC2) from IC31 (µPC78M05HF) with C33 and C34, fed by the T2 converter. Connector A: A1 +5 V, A2 GND, A3 +12 V, A4 −12 V, A5 +5 V (VCC2), A6 P.C; B: B1 +5 V, B2 GND; P and Q: +12 V, GND, +5 V; D: D1 +5 V, D2 GND, D3 +12 V.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 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. Leaking capacitors; 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, so it can be tested alone; outputs unsteady at first, steady when rechecked after running. Backup battery completely flat; replaced with a CR2032 in a holder; SRAM initialised and settings restored with SWITCH.X.
  4. ↑ 4.0 4.1 4.2 4.3 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: write-protect sensor replaced in drive 0, all capacitors replaced. Version 07, the last version, fitted to late machines: its parts, including the protect sensor, differ from 06 and have no substitute; if a disc is not locked after insertion, the wheel board or index sensor is faulty and the drive must be replaced. Do not remove the spindle or stepping motor just to change capacitors.
  5. ↑ 5.0 5.1 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) Repairs usually start with replacing leaking electrolytics, fixing solder joints and replacing burnt or cracked parts, but often that does not fix the supply; open transformer windings and failed transistors do not show on inspection. In-circuit resistance readings measured from the solder side of a working SH4 board are given for comparison.
  6. ↑ 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): the service outlet AC100V OUT (up to 300 W) is live whenever the mains plug is in, regardless of the front and rear switches.
  7. ↑ 7.0 7.1 7.2 Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 4.3, power, printed pages 24 and 25 (PDF pages 40 and 41): 100 V AC; a voltage far too high or too low causes faults; rear main switch on before the front switch, front switch off and lamp red before the rear switch is turned off; 10 seconds or more between switching off and on. Plugging in the keyboard with the rear main switch on can cause misbehaviour; then, with the keyboard plugged in, switch off at the front (lamp red), turn the main switch off, wait about 10 seconds and turn it on again.
  8. ↑ 8.0 8.1 8.2 Sharp, service manual CZ-134 (File:Sharp X68000 XVI Service Manual.pdf): control board diagram with the 3.0 V backup battery, printed pages 85 and 86 (PDF page 78); I/O and LED diagrams, printed pages 89 and 90 (PDF page 80), showing the power lamp board DUNTK7249DE01 fed with VCC1 and VCC2; keyboard diagram, printed pages 101 and 102 (PDF page 87), with the 80C51 controller, C3 2.2 µF 50 V on its reset line and C8 100 µF 10 V on its supply.
  9. ↑ 【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 service manual diagram draws C31, C27 and C28 with reversed polarity; the board silkscreen is correct.
  10. ↑ 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.
  11. ↑ 11.0 11.1 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 (allow SRAM writes); -mes ed0000 00 (clear the SRAM program area); -mes e8e00d 00 (protect again); -q; some versions of DB.X in XC did not accept the commands; SRAM_CLR.R as an alternative.
  12. ↑ 12.0 12.1 12.2 Sharp, XVI owner's manual (File:Sharp X68000 XVI Owner's Manual.pdf), 6.3, setting a floppy disc and drive precautions, printed pages 46 and 47 (PDF pages 62 and 63): a disc inserted the wrong way is detected and ejected; the auto-eject drives also eject with the button below each slot; the button works only while its green lamp is lit; while writing the lamp goes out and the button does not eject, but holding it for about three seconds or more does; thick or lifting labels can stop a disc ejecting.


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