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Sony HB-F900 General Maintenance

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Sony HB-F900 main board
Main board of a black HB-F900, with the Ni-Cd backup cell at the left

The Sony HB-F900 is a Japanese MSX2 from 1986 with two 3.5-inch drives, a separate keyboard and its power supply built into the case.[1][2] No service manual for it is known to be available, so the procedures here come from owners' repair reports, Sony's catalogue and photographs of the inside.[3][4]

Mains supply

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  • The HB-F900 is built for 100 V AC, 50/60 Hz.[4] On 220 to 240 V mains, run it from a step-down transformer with a 100 V output. Sony's catalogue gives no power consumption for the HB-F900; it gives 27 W for the HBI-F900 AV Creator.[4]
  • The rear panel has a 100 V AC outlet as well as the mains lead.[2]
  • The power supply sits at the back left of the case, with its transformer and a large reservoir capacitor.[5] Unplug the machine before opening it; the supply's primary side carries mains voltage.

Opening the case

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  • The bottom panel comes off on its own and exposes the underside of the main board, so joints can be checked and parts changed without taking the board out.[6]
  • To take the main board out, unplug its connectors and remove its six screws. A grey coaxial cable to the video output board is soldered in place and has to be unsoldered; note where each conductor goes before doing so.[6]
  • Photograph every connector before unplugging it, as no wiring diagram has been published.

Floppy drives

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The two drives are held by four screws at the back. The bar that supports them is often found bent, so that the drives sit tilted in the front panel. Take the drives out, remove and straighten the bar, and refit.[6] The drives have no belt.[6] Clean the heads while they are out; see Floppy Drive Repair.

repairBas (Bas Kornalijnslijper) lists the HB-F900 among the machines that use the same double-sided drive as the Philips NMS 8250 and the Sony HB-F700. His preventive modification, for drives that still work, stiffens the eject mechanism so that it does not spring back while a disk is being withdrawn, which can catch the head on the disk's shutter. The change is to remove a small white plastic part and cut away its small nub with a knife.[7]

A dead drive can be replaced with a PC 3.5-inch drive, with two points to watch, from one owner's attempt and the advice he was given:[8]

  1. Fit the cable the right way round. On one HB-F900 a new Mitsumi drive needed the cable turned round compared with the original drives. A drive LED that stays lit means the cable is upside down.
  2. The drive must supply the READY signal on pin 34. Bridging pins 33 and 34 on the drive or the cable provides it; do not touch pin 32. Another method is to rework the cable so that lines 10 and 12 swap and lines 33 and 34 are joined, which makes the PC drive drive A with a constant READY.

One drive is enough for the machine to work.[8]

Backup battery

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The clock and the machine's settings are kept by a GS sealed Ni-Cd cell on the main board, labelled 50 mAh, 2.4 V.[9]

  1. Inspect it whenever the case is open, looking for crust on the cell and on the board round it.
  2. A cell that is running low after a long time unused corrupts the stored settings. The start-up screen may change colour, or the machine may not reach BASIC; a red screen with "@ D" under the MSX logo has been put down to a dying cell.[8]
  3. To restore the settings, press SHIFT+F1, type SET TITLE "" and SET SCREEN, then reset. SET TITLE "",1 brings back the usual blue start-up screen.[8]
  4. If the cell has leaked, remove it and clean the board, following Battery Explosion, Capacitor or Corrosion Damage. Replace it with a rechargeable Ni-Cd cell, preferably in a holder so that a later leak stays off the board.[10]

Cartridge slots

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Slot 1 is on the front panel and slot 2 on the back.[2] The slot lines are buffered. One repairer who replaced two failed SN74LS367 buffers on a machine that recognised no cartridge put the damage down to a faulty cartridge, so check unknown cartridges before trusting them in this machine.[6]

HBI-F900 AV Creator

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The HBI-F900 connects to the 50-pin video interface socket on the back of the HB-F900 through a Centronics cable wired pin to pin. A Centronics SCSI cable has been used in its place.[6] Keep the contacts of both connectors clean.

Electrolytic capacitors

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No capacitor list for the HB-F900 has been published. The Sony HB-F900 Capacitor Replacement Guide describes where the capacitors are and how to record them.

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References

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  1. ↑ Sony HB-F900, MSX Resource Center wiki, 7 May 2026—link(accessed 2026-10-21) Community page: an MSX2 for the Japanese professional market that can superimpose its picture on a video signal (genlock) through a video interface connector on the back and a DSP board; used with the HBI-F900 AV Creator; black (B) or cream (W); separate KBD-13 or KBD-13(B) keyboard of HB-F500 design; supplied with a DIN-8 to RGB21 cable, MOS-1 mouse and the Wordship word processor on two disks; 1986; ¥148,000; 256 KB of RAM in slot 3-1; 128 KB VRAM; Yamaha V9938; PSG in the Yamaha S1985 MSX-Engine; two 720 KB 3.5-inch drives, internal speaker, reset button, JIS level 1 kanji ROM; Z80A from Sharp (LH0080A) or NEC (μPD780C); Western Digital WD2793A-PL floppy controller; an 8 KB "Video Utility" ROM with SonyVideo1 BASIC; connections: analogue sound and composite video, RGB (8-pin DIN), keyboard, video interface for the HBI-F900, data recorder, Centronics printer port, two general purpose ports, two cartridge slots, AC power socket and AC plug.
  2. ↑ 2.0 2.1 2.2 HB-F900, USB Secret Base (up-c)—link(accessed 2026-10-21) In Japanese. Specification list (Z80A at 3.57 MHz, 256 KB RAM, 128 KB VRAM with the V9938, JIS level 1 kanji ROM, MSX-JE not fitted, PSG equivalent to the AY-3-8910, two 3.5-inch 2DD drives, two cartridge slots, MSX-BASIC 2.0 and Disk BASIC 1.0) and annotated photographs. Front panel: power switch, power lamp, slot 1, reset switch, drives A and B, volume slider, joystick ports A and B, keyboard connector. Rear panel: RF channel switch, RF output, video output, audio output, RGB output, HBI-F900 interface connector, slot 2, earth terminal, printer connector, data recorder interface, AC outlet (AC 100 V), mains lead. Inside: the power supply section, the speaker, the DSP board (interface to the HBI-F900 and video and audio output), the main board and drives A and B.
  3. ↑ HB-F900 + HBI-F900 troubleshooting, May 2024—link(accessed 2026-10-21) Post by drone, 11 May 2024: an HB-F900 with an HBI-F900; cartridges work but the drives do not; the HB-F900 can change HBI-F900 settings from BASIC, but video from the HBI-F900 input does not come through for superimposing; he asks for any service documents or schematics, including Japanese ones, and none are posted.
  4. ↑ 4.0 4.1 4.2 Sony, HB-F900 / HBI-F900 catalogue, Japan, dated '86.10 (October 1986), Internet Archive item sonyhbf900hbif900, page 12, "HB-F900/HBI-F900の主な仕様" (main specifications), and page 3. In Japanese. HB-F900: Z-80A; ROM 192 KB (MSX2 BASIC, MSX2 Disk BASIC and kanji ROM); main RAM 256 KB; video RAM 128 KB; interfaces: two cartridge slots, video, RGB (8-pin), RF, audio, cassette tape, two joystick ports, printer, AC outlet and 50-pin video I/F; accessories include the mouse, a video cable, a plug adapter, an antenna connector and the Wordship software; main unit 355 mm wide × 325 mm deep × 76 mm high, 5.7 kg; keyboard 409 × 183 × 32 mm, 1.2 kg; power 100 V, 50/60 Hz; price ¥148,000. HBI-F900 AV Creator ¥64,800, 100 V, 27 W. Page 3 shows the front panel: power switch, cartridge slot 1, drives A and B with IN USE lamps, reset button, computer sound slider, controller ports A and B, keyboard socket.
  5. ↑ USB Secret Base, "HB-F900", annotated photograph of the inside (http://usbsecretbase.michikusa.jp/hb-f900/index.html, retrieved 2026-10-21). Read from it: the power supply at the back left with its transformer, heatsinks and a large reservoir capacitor; the speaker at the front left; the DSP board along the back right, with many small electrolytics; the two drives side by side at the front, over the main board.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 O MSX Sony HB-F900 que não aceitava cartuchos...– Alexandre Souza, 15 August 2018—link(accessed 2026-10-21) In Portuguese. Repair of an HB-F900 that recognised no cartridge in either the front or the rear slot. The HB-F900 and the HBI-F900 are joined by a 50-pin Centronics cable wired pin to pin, so a Centronics SCSI cable works. The bottom panel comes off to expose the underside of the main board without removing it. The main board is held by six screws plus connectors; removing it completely also means unsoldering a grey coaxial cable to the video output board, so note its connections. The drives are held by four screws at the back; the bar that supports them is commonly bent so the drives sit tilted at the front, and can be removed and straightened; the drives do not use a belt. In his experience the power supply does not normally need recapping. All address, data and control signals to the slots are buffered; with an oscilloscope he found two signals present on the inputs of the buffers and missing on the outputs, and replaced a pair of SN74LS367 chips, fitting them in sockets; he blames a faulty cartridge.
  7. ↑ repairBas (Bas Kornalijnslijper), Modificatie dubbelzijdige diskdrive (preventive modification for the double-sided disk drive), PDF, 2010, in Dutch, https://repairbas.file-hunter.com/docs/drives/Preventieve%20modificatie%20dubbelzijdige%20diskdrive.pdf (retrieved 2026-10-21). Lists the drive as used in the Philips NMS 8250, 8255 and 8280, Sony HB-F500P/D/F, HB-T600, HB-F700P/D/F/S, HB-F900, HB-D20W and HBD-30W, Panasonic FS-FD1 and Sanyo MPC-25. For drives that work: tighten the eject mechanism so it does not spring back by itself while a disk is being removed, so the head does not catch on the disk shutter; remove the white part shown and cut away its small nub with a knife, as in the photographs.
  8. ↑ 8.0 8.1 8.2 8.3 Instructions to repair or replace HB-F900 Floppy Drive(s)?, May 2021—link(accessed 2026-10-21) Thread by BitSquad: the HB-F900's drive answers "Disk Offline" to FILES. sdsnatcher73, 12 May 2021: a PC drive converted for MSX with the READY signal works; without it, on an HB-F700, the machine freezes when booting into BASIC until a formatted disk is inserted, while games and DOS boot normally. Manuel, 12 May 2021: Sony machines use the WD2793; if the HB-F900's drive is like the HB-G900P's, it has a standard 34-pin Shugart connector. BitSquad, 13 May 2021: the machine then started with a red screen and did not reach BASIC; Manuel: the stored settings were corrupt, cured with SHIFT+F1, SET TITLE "", SET SCREEN and a reset, which worked; Grauw: the start-up colour is a setting, SET TITLE "",1 gives the blue screen; sdsnatcher73: this happens when the backup battery is low after long disuse and the clock memory is corrupted. BitSquad, 15 May 2021: a new Mitsumi drive lit its LED continuously until the cable was turned round, then answered "Disk Offline"; Manuel: a continuously lit LED means the cable is upside down; meits: bridge pins 33 and 34 to provide READY, without touching pin 32, and one drive is enough; jltursan: on the cable, swap lines 10 and 12 and join 33 and 34 to make a PC drive into drive A with a constant READY. sdsnatcher73, 16 May 2021: a red screen with "@ D" under the logo means the backup battery is dying; remove it. Accumulator, 10 March 2023: a Mitsumi D359T5 worked in an HB-F700.
  9. ↑ File:Sony HB-F900 main board.jpg, photograph of the main board of a black HB-F900, uploaded to the MSX Resource Center wiki by Gdx on 20 March 2016. Read from it: Yamaha V9938, Sharp LH0080 Z80, WD2793A-PL floppy controller, and at the left a GS sealed Ni-Cd battery labelled 50 mAh, 2.4 V (Japan Storage Battery Co.). The values of the electrolytic capacitors cannot be read in the photograph.
  10. ↑ Diagnostic on dead Sony HB-F700F, 2015–2017—link(accessed 2026-10-21) gdx, 8 November 2016: replace a rechargeable backup cell with the same type (Ni-Cd) and fit it in a holder rather than soldering to it, so a later leak stays in the holder; l_oliveira, 6 November 2016: a dead battery will not stop an MSX booting unless the clock's power circuit is faulty; sd_snatcher, 28 October 2016: a battery fault stopped an NMS 8245 booting.


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