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Sharp X1 Turbo Troubleshooting Guide

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Sharp X1turbo Model 30 rear panel
Rear of an X1turbo Model 30, with the initial mode DIP switch beside the printer port (photo: Jun Sasaki, AKIBA PC Hotline!, copyright Impress Corporation)

Fault finding on the Sharp X1turbo Model 10, 20 and 30 (CZ-850C, CZ-851C and CZ-852C). No Sharp service manual was found on the Internet Archive or X1 Center. The checks below come from Sharp's specification and user's manual, the Oh!MZ schematic of the Model 20 and 30 that X1 Center offers for personal use, and repairs written up by the Sharp Museum and X1 Center. Inside-the-case notes are on Sharp X1 Turbo General Maintenance.

What a working X1turbo does

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  1. At power-on the IPL tries to start a system program from the drive and disk type set on the initial mode switch at the back.[1]
  2. If nothing starts, it shows "Make your device ready" and asks for F (floppy), R (ROM), C (cassette) or T (timer).[1]
  3. Disk BASIC runs a program called Startup when it starts, which sets the function keys, the year in the clock and the NEWON level.[1]

First checks

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  • Initial mode switch: Sharp ships it with all switches on. If disks that should boot do not, check the switch and the drive, then switch on again.[1]
  • Display switches: the front panel has a standard and high-resolution switch, and a WIDTH & DEFCHR switch that stops automatic 200-line switching; the Model 20 and 30 also have an interlace switch.[2] Check these before suspecting the video circuit when the picture is in the wrong mode.
  • Reset buttons: the IPL button restarts the computer as at power-on; the NMI button enters the monitor and can hang programs that do not expect it.[2][3]
  • Keyboard mode: the A/B slide switch beside the keyboard changes the kana layout.[3]

Computer completely dead

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The Sharp Museum, which owns X1-family machines from the first X1 to the X1turbo Z III, has not had a power unit fail on any of them. On the X1turbo it found a different cause of a completely dead machine: the superimpose (telopper) board.[4]

  1. Unplug the computer and open it.
  2. Find the telopper board behind floppy drive 1, in the frame with the expansion slots. Four cables run to it from the main board.
  3. Unplug the yellow and black pair, which is the board's 12 V supply.
  4. Switch on. If the computer now starts, the telopper board is the cause.

The board carries many electrolytics. As they lose capacitance they overload the 12 V supply, and the supply's protection shuts it down. On the museum's machine, removing one 1000 µF capacitor brought the resistance across the board's 12 V supply back to a sensible value. That board was an early revision with extra parts, including electrolytics, added on the back. The museum left the telopper unpowered.[4] The capacitors on the board are listed in Sharp X1 Turbo Capacitor Replacement Guide.

On the Model 10, which has no telopper board, the power unit gives 13.8 V for the cassette deck in place of the ±12 V of the other models.[5]

Clock faults

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  • Clock wrong at first power-on: Sharp says this is not a fault; set the clock.[6]
  • Clock stops after the computer has been off at the main switch for a long time: Sharp notes that this happens. Switching off the main power also resets the year to xx, and the clock does not change the year by itself or handle leap days.[6]
  • Cursor does not blink, Startup gives an error, or TIME$ and DATE$ give errors: the calendar clock is not running. If the system otherwise starts and the keyboard works, the Sharp Museum suspects the clock chip's crystal circuit before the sub-CPU.[4]
    1. The clock chip, a µPD1990, is on the main board under the sub board, so the sub board has to come off.[4]
    2. Its 32.768 kHz crystal has a trimmer capacitor (C4, 20 pF) to ground.[4][7]
    3. Turn the trimmer slightly with a fine screwdriver. The museum found that ageing of the trimmer stops the crystal, and moving it to a fresh part of its track restores it.[4]
    4. Reassemble loosely and watch the clock on the timer screen.
  • Clock does not keep time with the power off: the backup cell (2.4 V nickel-cadmium) may be exhausted or leaking; see clock backup battery.[7]

Floppy drive faults (Model 20 and 30)

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  • Loud grinding at boot or on disk access: the drive's track 0 sensor (a photo-interrupter) has probably failed. The drive cannot tell that the head has reached track 0 and keeps driving it against the stop. The Sharp Museum could not find a matching replacement sensor or a way to set it up correctly, and left its drive unrepaired.[4]
  • A replacement drive overruns on its first seek: the interface expects drives that move to track 0 at power-on, and a drive without this can overrun badly the first time it is sent to another track.[8]
  • Disks will not boot although the drive runs: check the initial mode switch.[1]

Keyboard faults

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  • Keys work only while the plug is pressed one way, or not at all: reflow the keyboard connector's solder joints in the computer.[9]
  • Cord sheath crumbling: replace the cord, because it carries +5 V.[10]

RS-232C on the Model 10

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The Model 10's board has the RS-232C circuit left unfitted. The Sharp Museum fitted the missing parts and the port still did not work until it supplied +12 V and −12 V to the 75188 driver from a DC-to-DC converter, because the Model 10's power unit does not provide them.[5]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Sharp Corporation, X1turbo CZ-850C/851C/852C ユーザーズマニュアル (user's manual, Japanese), page 56: the IPL starts the system program from drive 0 of the disk type set on the initial mode switch (DIP switch), which is set to all ON at the factory; if no system program starts, it shows "Make your device ready" and "Press selected key to start driving" with F floppy, R ROM, C CMT and T timer, and the switch and drive should be checked and the power switched on again. Front matter: when Disk BASIC starts it runs the Startup program, which sets the function keys, the year of the internal clock and NEWON. Scan on the Internet Archive, item sharp-cz-850-c-851-c-852-c-users-manual.
  2. ↑ 2.0 2.1 Oh!MZ, November 1984 (Nihon SoftBank), page 65: front panel standard/high-resolution switch, interlace/non-interlace switch (not on the Model 10), reset switches NMI (enters monitor) and IPL (enters boot), WIDTH & DEFCHR (stops 200-line automatic switching); rear initial mode switch. Scan on the Internet Archive, item oh-mz-1984-12, file Oh!MZ_1984-11, leaf 66.
  3. ↑ 3.0 3.1 X1 (コンピュータ), Japanese Wikipedia—link(accessed 2026-10-21) The NMI reset jumps to address 0066h and can hang software that does not expect it; the IPL reset added on the X1turbo behaves almost as at power-on; the keyboard A/B mode switch changes the kana layout.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 X1turboのよくある故障とその対処, シャープ博物館 (Sharp Museum)—link(accessed 2026-10-21) None of about six machines from the mania-type X1 to the X1turbo Z III has had its power supply fail; an X1turbo that stays completely dead may be caused by the telopper board behind drive 1, in the same frame as the expansion slots, with four cables from the main board; removing the yellow and black (12 V) wires lets the machine start; the board's many electrolytics lose capacitance and overload the 12 V supply, tripping its protection; removing a 1000 µF electrolytic from the board brought the resistance across the 12 V supply to a reasonable value; the board was an early revision with parts, including electrolytics, added on the back; the museum leaves the telopper unpowered. Cursor not blinking or errors from StartUp.Bas, TIME$ or DATE$: suspect the RTC rather than the sub-CPU if the system starts and keys work; the µPD1990 RTC is on the main board, completely hidden under the sub board; its 32.768 kHz crystal connects to pins 12 and 13 with C3 and C4 to ground, C4 being a trimmer; turning the trimmer slightly with a fine screwdriver restores oscillation; check on the BIOS timer screen; the cause is ageing of the trimmer. Loud grinding at boot or disk access: the drive's track 0 sensor (photo coupler) has failed; replacement parts and adjustment were not available, so the drive was left unrepaired.
  5. ↑ 5.0 5.1 X1turbo model10, シャープ博物館 (Sharp Museum)—link(accessed 2026-10-21) The Model 10's board is the same as the Model 20 and above with parts left off; fitting the RS-232C parts did not work until ±12 V was supplied to the 75188 driver from a DC-to-DC converter, because the Model 10's own power supply gives 13.8 V for the cassette in place of ±12 V.
  6. ↑ 6.0 6.1 X1turbo ユーザーズマニュアル, pages 143 to 146, clock and main power notes: when the main power is first switched on the clock shows the wrong time, which is not a fault; after a long time with the main power off the clock stops working; the clock does not change the year automatically and leap days need setting by hand; switching off the main power changes the year to xx.
  7. ↑ 7.0 7.1 CZ-851C/852C(X1turbo)回路図(Oh!MZ誌), X1 Center—link(accessed 2026-10-21) Main board sheet 1/3: µPD1990AC (IC5) with crystal X1 (32.768 kHz), trimmer C4 (20 pF) and C3 (22 pF); backup cell 2.4 V (BAT0005CE).
  8. ↑ X1/turbo/Zシリーズ搭載FDDインターフェース仕様, X1 Center—link(accessed 2026-10-21) The 150 Ω input terminators of the FDD interface assume a drive with power-on restore; a drive without it may overtrack badly on its first access to a track other than track 0.
  9. ↑ キーボードコネクタの半田剥離, X1 Center—link(accessed 2026-10-21) Keyboard connector solder cracks on the X1, turbo and Z series.
  10. ↑ X1/turbo用キーボードケーブルの劣化と交換, X1 Center—link(accessed 2026-10-21) The X1 and X1turbo keyboard cord sheath crumbles; the keyboard line carries +5.0 V.


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