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Sony HB-F700 Troubleshooting Guide

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Sony HB-F700 PS-133 power supply schematic
The PS-133 power supply from Sony's service manual: three linear regulators fed from one transformer

Sony's Sony HB-F700 Service Manual for the Sony HB-F700 MSX2 has full schematics and an adjustment procedure but no fault-finding charts. The checks below combine the manual with repairs that owners have reported on the MSX Resource Center forum.[1][2] For opening the case and the board layout, see the Sony HB-F700 General Maintenance guide.

First checks

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Quick symptom table
Symptom Likely cause Go to
No power light, nothing happens Mains fuse F901, power switch, transformer No power
Drive light and Caps Lock work, no picture or sound on any output 9 V regulator IC307 on the video board No picture
Black screen, Caps Lock does not respond Clock, video processor, ROM, RAM or a supply rail Dead machine
Picture in black and white only Normal on the composite output Not faults
Corrupted characters and graphics; boot screen shows 64 KB of VRAM Video RAM or its wiring to the V9938 Corrupted graphics
Some keys need hard presses Dirt in the keyboard Keyboard

Before taking anything apart, try a known good RGB cable or monitor, and take every cartridge out.[2][3]

Things that are not faults

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  • A television on the 6-pin DIN AUDIO/VIDEO socket shows a black-and-white picture. That socket gives black-and-white composite video only. Colour is on the 8-pin DIN RGB socket.[4][1]
  • The supplied disks will not stop at MSX-DOS. The two disks that came with the machine load their intro or menu even with CTRL+STOP held at start-up. To get MSX-DOS, copy MSXDOS.SYS and COMMAND.COM to a newly formatted disk.[5]
  • A program can still be in memory after a reset. Sony states that a reset erases memory, but the MSX Resource Center reports badly written software surviving a reset, and even a short time switched off. Switch off for longer, or clear memory from BASIC, before deciding that the reset circuit is faulty.[4][6]
  • Some software will not run as it stands because the RAM is in secondary slot 3-3; it needs the POKE -1 workaround.[6]

No power

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The supply is linear. The mains passes through the power switch, the T630 mA fuse F901 and the line coil L901 to the transformer T901.[7][8]

  1. Unplug the machine. Check the mains lead and the fuse F901 with a meter.
  2. If F901 has blown, look for the cause before fitting a new one: a shorted bridge D904 or diodes D901 to D903, a shorted reservoir capacitor (C909, C905 or C912), or a short on one of the rails. Fit only a T630 mA time-delay fuse; Sony marks F901 as a safety part.[7][8]
  3. With the fuse good, measure the transformer secondaries at the rectifiers with the power on, then the regulator inputs and outputs below.
Regulator checks (service manual, page 40)
Rail Regulator and pins Input marked Output Reservoir, output capacitor
+5 V IC902 SI3052V: 3 in, 1 ground, 2 out 9.3 V 5 V C909 22000 µF 16 V; C904 100 µF 10 V
+12 V IC901 SI3122V: 3 in, 1 ground, 2 out 18.2 V 12 V C905 10000 µF 25 V; C903 100 µF 16 V
−12 V IC903 NJM79L12A: 2 in, 3 ground, 1 out 19.1, negative side −12 V C912 470 µF 35 V; C907 100 µF 16 V

[7]

  • Input present and output missing: unplug, and measure the output to ground for a short before condemning the regulator.
  • Input low or missing: check the rectifier diodes and the reservoir capacitor for that rail, then the transformer winding.
  • The power light D905 is on the LE-50 board. If every rail is good and only the light is out, look there.[7]

Machine runs but no picture

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If the drive light flashes at power-on and reset and the Caps Lock light can be switched on and off, the processor is running and the fault is in the video output.[3]

  1. Measure IC307, the 9 V regulator (NJM78M09A) on the video board near the RGB socket. Its input should be about 12 V and its output about 9 V. It feeds the R, G, B and sync output circuits, and when it fails both the RGB and the audio/video outputs go dead.[9][8]
  2. If the output is near 0 V, switch off and measure the output to ground. On one HB-F700D it read 2.5 or 4.5 kΩ, not a short, and replacing the regulator with an LM7809 brought the picture back.[3] On an HB-F700S the same fault was cured the same way.[9]
  3. If 9 V is present and there is still no picture, check the RGB levels with an oscilloscope at CN309 on a white screen: 0.7 V ± 0.02 V on pins 6 (red), 7 (green) and 8 (blue), set by RV302, RV303 and RV301.[10]

The repairBas channel has published a video of this repair, "No RGB and Video Signal on a Sony HB-F700P/D MSX 2 Computer".[3]

Dead machine

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A black screen with no Caps Lock response usually means the processor is not running: no clock (a dead V9938 can cause this), or a fault stopping the Z80, the ROM or the RAM.[2] Work in this order:[2]

  1. Check the three supply rails as above.
  2. Check the clock. The CPU clock is adjusted at IC301 pin 8 on the DUS-135 board, where it should be 3,579,540 to 3,579,549 Hz.[10] With only a multimeter, a forum member suggests measuring the Z80's pin 6 (clock) and pin 26 (reset) and reporting the voltages.[2]
  3. Look over the tracks with a continuity tester, especially round any corrosion.
  4. Test with a known good RAM cartridge, and with a cartridge that plays sound as soon as it starts, with a speaker on the audio output. Sound means the processor and ROM are running and the fault is in the video circuits or the RAM. No sound points to the ROM, the processor, the clock or the supply.[2]
  5. The main ROM, IC5, is a 32 KB 28-pin chip soldered to the board. It can be removed, socketed and replaced with an EPROM or flash chip holding an MSX1 BIOS. If the machine starts on that, suspect the clock circuit and its backup supply.[2]

One HB-F700F stayed dead after its owner replaced every electrolytic on both boards and removed a leaking backup battery. The board was passed to another member, and the thread ends without a diagnosis.[2]

Backup battery

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The clock is backed up by BT1, a rechargeable Ni-Cd cell, type GB50H.[8] Members disagree on whether a leaking or dead cell can stop the machine starting: one has seen it on an NMS 8245, and another says it should not unless the clock's power circuit is damaged.[2] Remove a leaking cell and clean the board straight away whatever the symptom, using Battery Explosion, Capacitor or Corrosion Damage.

Corrupted graphics

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On an HB-F700S, the start-up screen reported 64 KB of VRAM instead of 128 KB, characters were garbled and the cursor looked wrong. A RAM test passed and the video RAM test restarted the machine. Replacing all the RAM chips did not help; a complete replacement video board did.[9]

  1. A 64 KB VRAM report means a fault in the video RAM chips or their connection to the V9938. Trace the CAS0 and CAS1 lines from the V9938 to each pair of video RAM chips.[9]
  2. Reflow the joints round the video RAM. The video board is not glass-epoxy and cracks easily.[9]
  3. If none of that finds it, the V9938 itself is the remaining suspect. The owner who swapped the whole board thought the video processor might have failed.[9]

A video RAM fault spoils the picture but does not stop the machine starting.[2]

Keyboard faults

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On one HB-F700D, the space bar, X, M, CTRL, the cursor keys and sometimes 3 worked only when pressed hard. They share no row or column of the matrix, so the owner ruled out the wiring. The keyboard was full of dirt, and brushing it out and cleaning each conductive track and key dome cured it.[3] See the Sony HB-F700 General Maintenance guide.

Cartridge faults

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Never plug a cartridge in with the power on. A long-standing MSX Resource Center member warns that the HB-F700's video processor does not tolerate it.[2] If a cartridge does not start, clean its contacts and try the other slot before suspecting the machine.

Floppy drive faults

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  • Do not remove a disk while the IN USE lamp is lit.[4]
  • If disks catch or the head snags as a disk is withdrawn, see repairBas's eject modification in the Sony HB-F700 General Maintenance guide.[11]
  • Read errors on every disk, with a clean head and a drive that turns, can come from the floppy controller's data separator. Sony's procedure sets its VCO to 250 kHz ± 1 kHz at TP3 and the read pulse width to 0.5 µs ± 10 ns at TP2, both with TP1 grounded, using RV2 and RV1 on the PU-53 board. Do this only with a frequency counter and an oscilloscope.[10]
  • A replacement PC drive needs the READY signal. An MSX Resource Center member who tried drives converted for MSX in an HB-F700 found that one without READY froze the machine when it booted into BASIC until a formatted disk was put in, while games and MSX-DOS booted normally. Another owner reports that a Mitsumi D359T5 worked as a replacement in an HB-F700.[12]
  • A coloured start-up screen, or a machine that will not reach BASIC, can come from settings corrupted by a low backup cell. Press SHIFT+F1, type SET TITLE "" and SET SCREEN, and reset; SET TITLE "",1 restores the blue start-up screen. This was reported on an HB-F900; the commands are part of MSX2 BASIC.[12]
  • See Floppy Drive Repair for head cleaning and general drive work.
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References

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  1. ↑ 1.0 1.1 Sony, Service Manual HB-F700P/F700S/F700F/F700D, pages 1 and 2, "Specifications". See Sony HB-F700 Service Manual. Z80A at 3.58 MHz; ROM 64 KB; RAM 256 KB; VRAM 128 KB; V9938; 90-key KBD-12P keyboard; composite video on the 6-pin DIN socket 1 V p-p black and white; RGB 0 to 0.7 V into 75 ohms on an 8-pin DIN; 3.5-inch double-sided drive, 720 KB formatted, 300 rpm; 220 V AC 50/60 Hz (HB-F700P), 240 V AC 50/60 Hz (HB-F700B); 42 W for the main unit; resetting automatic at power-on or manual, "memory contents are not maintained".
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Diagnostic on dead Sony HB-F700F, 2015–2017—link(accessed 2026-10-21) Thread by Louthrax about an HB-F700F that showed nothing and gave no Caps Lock response. gdx, 1 March 2015: failed electrolytics are often swollen on top, replace any that is even slightly concave; old solder joints age badly, resolder them with flux; "The VDP of HB-700 doesn't like" a cartridge inserted with the power on. RetroTechie, 2 March 2015: no Caps Lock response usually means the CPU gets no clock (a dead VDP can cause that) or something stops the Z80, ROM or RAM; test with a known good RAM cartridge and a cartridge that plays sound at power-on, with a speaker on the audio output. l_oliveira, 3 March 2015: check power, then clocks, then the tracks, then the VDP, ROM, RAM and CPU; a VRAM fault spoils the picture but does not stop the machine starting. Louthrax, 28 October 2016: the top board had been recapped without effect; the backup battery had started to leak and was removed and the tracks cleaned; he describes it as a sealed Ni-Cd battery, 50 mAh, 2.4 V. sd_snatcher, 28 October and 6 November 2016: a battery fault stopped an NMS 8245 booting; the main ROM is IC5, a 32 KB 28-pin part that can be socketed and replaced with an MSX1 BIOS for a test. l_oliveira, 6 November 2016: a dead battery will not stop the machine booting unless the clock's power circuit is faulty. Eugeny_Brychkov, 6 November 2016: with a multimeter, measure the voltage on the CPU's pin 26 (reset) and pin 6 (clock) with the machine switched on. gdx, 8 November 2016: a turbo R would not boot with drained batteries; replace a rechargeable cell with the same type (Ni-Cd) and fit it in a holder rather than soldering to it. Louthrax, 15 June 2017: all motherboard capacitors changed and the board still dead; it was passed to another member for repair.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 on repairing a Sony HB-F700 to checkout FD50, 13 October 2025—link(accessed 2026-10-21) Post by Pat about an HB-F700D after about 25 years in storage: no video or sound on two cables; the drive light came on at switch-on and reset and the Caps Lock light could be toggled; the supply lines from the power supply to the main board were normal; on the video board, which sits on top, the 9 V regulator IC307 had 12 V on its input and almost nothing on its output, and measured 2.5 or 4.5 kΩ from output to ground with the power off; replaced with an LM7809, after which the MSX logo appeared. Several keys (space, X, M, CTRL, the cursor keys and sometimes 3) were unreliable and are not on one row or column of the matrix; the keyboard was full of dirt, and brushing the board and cleaning each conductive track and key dome cured it without any fluid. Links repairBas's video "No RGB and Video Signal on a Sony HB-F700P/D MSX 2 Computer".
  4. ↑ 4.0 4.1 4.2 Sony, Service Manual HB-F700P/F700S/F700F/F700D, pages 3 and 4, "Location and Function of Parts". See Sony HB-F700 Service Manual. Front: ON/STANDBY switch and indicator, cartridge slots 1 and 2, disk eject button, RESET button (resets the computer; "the internal memory will then be erased"), controller connectors A and B, keyboard connector, IN USE lamp (do not remove the disk or switch to standby while it is lit). Rear: TAPE (8-pin DIN), AUDIO/VIDEO output (6-pin DIN; "Video signals from this connector are black-and-white"), RGB (8-pin DIN, for a colour monitor with a 21-pin RGB input), ground terminal, PRINTER (14-pin), EXT DRIVE (34-pin, for a second drive such as the Sony HBD-30W).
  5. ↑ Sony HB-F700P, MSX Resource Center wiki, 7 March 2026—link(accessed 2026-10-21) The two supplied disks (MSX-DOS 1 with HiBrid, and MSX-DOS 1 with Halos and a demo) load their intro or menu even with CTRL+STOP held at boot; copy MSXDOS.SYS and COMMAND.COM to a freshly formatted disk to get MSX-DOS.
  6. ↑ 6.0 6.1 Sony HB-F700, MSX Resource Center wiki, 7 March 2026—link(accessed 2026-10-21) Community page for the series: B, D, F, P and S models; 256 KB memory mapper in slot 3-3, so some software needs the POKE -1 workaround; WD2793A-PL floppy controller; "badly programmed" software could survive a reset in memory, even after the machine had been off for some time; connections include a screw terminal for connecting the metal housing to protective earth.
  7. ↑ 7.0 7.1 7.2 7.3 Sony, Service Manual HB-F700P/F700S/F700F/F700D, page 40, schematic diagram, PS-133 board. See Sony HB-F700 Service Manual and File:Sony HB-F700 PS-133 power supply schematic.png. Transformer T901; primary through the power switch, fuse F901 and line coil L901, with C911 and C915 (0.022 µF 250 V) and C906 (2200 pF 400 V). +5 V: bridge D904 (D5FB20F), reservoir C909 22000 µF 16 V, regulator IC902 SI3052V (input pin 3, 9.3 V marked; ground pin 1; output pin 2, 5 V), output capacitor C904 100 µF 10 V, C902 0.33 µF. +12 V: D901 and D903 (U05G), reservoir C905 10000 µF 25 V, IC901 SI3122V (input pin 3, 18.2 V marked; ground pin 1; output pin 2, 12 V), C903 100 µF 16 V, C901 0.33 µF. −12 V: D902 (10E2N), reservoir C912 470 µF 35 V, IC903 NJM79L12A (input pin 2, 19.1 marked; ground pin 3; output pin 1, −12), C907 100 µF 16 V, C908 0.33 µF. Power LED D905 (TLG124A) on the LE-50 board.
  8. ↑ 8.0 8.1 8.2 8.3 Sony, Service Manual HB-F700P/F700S/F700F/F700D, pages 76 to 80, section 8, "Electrical Parts List". See Sony HB-F700 Service Manual. Boards PU-53 (PR-53 on the F and S models), DUS-135, CN-149, CN-150, CN-151, PS-133, LE-50 and EX-119; BT1 1-528-171-11, battery, storage, Ni-Cd (GB50H); IC307 NJM78M09A; IC901 SI3122V; IC902 SI3052V; IC903 NJM79L12A; F901 fuse T630 mA (1-532-284-11, marked as safety-critical); S901 push switch (AC power); T901 power transformer 1-448-626-11.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Sony HB-F700S with problems, March 2016—link(accessed 2026-10-21) Thread by Pablibiris about an HB-F700S with no picture. RetroTechie, 13 March 2016: IC307 (78M09) near the RGB connector should have about 12 V on its input pin, ground on the middle pin and about 9 V on its output, which feeds the R, G, B and sync output circuits; if the 9 V fails, RGB and A/V output are lost. Pablibiris, 14 March 2016: two boards, an upper one with the RGB and tape connections and a lower motherboard; replacing IC307 brought the picture back, after which graphics were corrupted and the boot screen reported 64 KB of VRAM. Wierzbowsky, 15 March 2016: most MSX2 machines have 128 KB of VRAM, so a 64 KB report points to the VRAM chips or their connection to the VDP; trace CAS0 and CAS1 from the VDP to the pairs of VRAM chips. Pablibiris, 15 March 2016: photographs of a strange cursor and garbled characters and graphics; the TESTRAM program passed the RAM but restarted the machine during the VRAM test; changing all the memory did not help; fitting a complete replacement video board did, and he suggests the VDP may have been broken. Jipe, 15 March 2016: check the solder joints on the VRAM; the board in the HB-700 breaks easily because it is not epoxy.
  10. ↑ 10.0 10.1 10.2 Sony, Service Manual HB-F700P/F700S/F700F/F700D, pages 25 and 26, section 5, "Adjustments". See Sony HB-F700 Service Manual. BASIC adjusting program headed "CHECK PROGRAM for HB-F5/F500": W key, white screen for the RGB level adjustment; S key sets the timer clock output (lines 200 and 210: OUT &HB4,14:OUT &HB5,0 and OUT &HB4,15:OUT &HB5,8); R key cancels it (line 240: OUT &HB5,12). Connect a KX-14CP1 monitor to CN309 or terminate pins 6, 7 and 8 of CN309 in 75 ohms. CPU clock: IC301 pin 8 on the DUS-135 board, 3,579,540 to 3,579,549 Hz, adjust CV301. Timer clock: 100 kΩ pull-up from TP4 to +5 V, TP4 on the PU-53 board, 15.9999 to 16.0001 Hz, adjust CV1. RGB levels: 0.7 ± 0.02 V from sync level to colour level at CN309 pin 6 (red, RV302), pin 7 (green, RV303) and pin 8 (blue, RV301). Floppy controller, three minutes after switch-on with TP1 grounded by a jumper: VCO frequency 250 ± 1 kHz at TP3, adjust RV2; read pulse width 0.5 µs ± 10 ns at TP2, adjust RV1; remove the jumper afterwards.
  11. ↑ 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.
  12. ↑ 12.0 12.1 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.


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