Toshiba HX-10 Troubleshooting Guide
Fault-finding for the Toshiba HX-10. No Toshiba service manual or schematic for the HX-10 has been found. The checks below come from the owner's manual, a teardown of a British HX-10 and repairs reported on the MSX Resource Center forum. Opening the case and the supply rails are covered in the Toshiba HX-10 General Maintenance guide.
First checks
[edit | edit source]| Symptom | Check first |
|---|---|
| Power LED off | The fuse in the mains plug, the mains cord, the transformer outputs |
| Power LED on, black screen, Caps Lock dead | All supply voltages, then the CPU clock, the ROM and the RAM |
| Power LED on, black or blank picture, machine otherwise alive | The LVA510 PAL encoder board and the video connections |
| Hard short on +5 V | RAM chips, capacitors, the cartridge slot and expansion connector |
| Supply rail too high or missing | The regulator transistors and their zener diodes |
| Cartridge does not start | Switch off, reseat the cartridge, try another |
Toshiba's own list of checks covers the obvious causes first: if the power indicator stays off, check the wiring and the fuse in the mains plug; for no picture, check the cables, the TV's input selector and that the text and background colours are not the same; for cartridge faults, switch off, reinsert the cartridge and try another.[1]
No power
[edit | edit source]The mains transformer is inside the case and its outputs are AC. With the main board unplugged from the transformer, measure the AC voltages on the transformer connector. On the British model the two windings are rated 8.5 V and 16 V AC.[5][6] One European HX-10 gave about 20 V AC on one lead but only 2 to 3 V AC on the two leads that should have carried about 9 V, which points to a failed transformer.[6] If the transformer is good, measure +5 V, +12 V and −12 V on the board.[6]
The transformer's primary winding carries mains voltage whenever the cord is plugged in, so make all connections with the cord unplugged.[7][5]
Wrong supply voltages
[edit | edit source]The rails are made by transistor and zener regulators on the main board: a B435 (TR8) for +5 V, a D880 for +12 V and an A473 for −12 V.[5][8] On one machine with a black screen the logic chips saw 6.5 to 7 V in place of 5 V, and the 4116 video RAMs had lost their −5 V supply. A new A473 (TR6) did not help; the zener diode ZD3 next to TR6 had failed.[4] A failed zener or regulator transistor can leave a rail too high or missing, so measure every rail before replacing any chips, and check the zeners when a rail is wrong.[4]
Short on +5 V
[edit | edit source]One HX-10 came out of storage with a hard short, about 3 ohms, between +5 V and ground, while the power transistors and zeners tested good.[3] Suggested checks, in order:[3]
- Look for bulging or leaking capacitors, and test each electrolytic for a short with an ohmmeter.
- Check the cartridge slot, the rear expansion connector and the joystick ports for shorts.
- Lift the capacitors on the 5 V circuit one at a time; even a ceramic capacitor can short the rails.
- Remove RAM chips one at a time. On that machine the resistance rose towards 300 ohms as the 4164s came out.
The owner fitted a replacement main board, so the original cause was not found.[3]
Black screen
[edit | edit source]With the power LED on but nothing on screen and Caps Lock not responding, the Z80 is usually not running. Check every supply voltage first, then the clock (the VDP supplies the CPU clock), then the RAM and the ROM.[2] If the supplies are right, check for activity on the address and data lines of the RAM and video RAM with a logic probe or oscilloscope, the chip select lines of the RAM and ROM, and the clock and reset on the VDP and CPU.[4]
When the machine is otherwise alive, a common cause on Toshibas is the LVA510 PAL encoder on the separate encoder board.[2][5] The encoder board is joined to the VDP by six wires, so check those connections as well.[5]
Overheating
[edit | edit source]If the machine crashes or misbehaves after running for a while, or with power-hungry cartridges fitted, the supply may be running too hot. See the heat section of the Toshiba HX-10 General Maintenance guide for the measures owners use.[8]
Cassette and keyboard
[edit | edit source]- Cassette loads fail: play back a recording to hear whether it saved, switch off any VLSS or high-speed playback setting, clean the recorder head, and use the lower transfer rate.[1]
- Keyboard locked: press CTRL and STOP together, or switch off for a few seconds.[1] The keyboard plugs into the main board on two connectors, which can work loose.[5]
Related pages
[edit | edit source]- Toshiba HX-10
- Toshiba HX-10 General Maintenance
- Toshiba HX-10 Capacitor Replacement Guide
- Toshiba HX-10 Owner's Manual
- Toshiba HX-22 Troubleshooting Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 Toshiba, Home Computer Owner's Manual, Model HX-10, 1984, chapter 2 "Getting started" and chapter 5 "In case of difficulty", "Before calling for service" (PDF pages 13 and 39 to 43). See Toshiba HX-10 Owner's Manual. Tuning: select a spare TV channel and tune to around channel 36. If the power indicator does not light, check the wiring and the fuse in the mains plug, then consult a Toshiba dealer. Cassette problems: play back a recording to hear it; switch off VLSS or high-speed playback; clean the recorder head; use computer or normal 30 or 45 minute tapes, avoid the first few inches; use the lower transfer rate. Keyboard locked up: press CTRL and STOP together, or switch off for a few seconds. Cartridge not working: switch off and reinsert it, check its leaflet, try another. No sound, joystick or display faults: check connections, the plug fuse, the TV input selector, the COLOR setting; never use an aerial mixer; disconnect the off-air aerial when using the computer.
- ↑ 2.0 2.1 2.2 My old MSX seems to be broken - any ideas?, November 2008—link(accessed 2026-10-21) An HX-10 64k after 12 years in storage: black screen, power LED on, Caps Lock not responding. Repair-Bas: if the power LED lights it is usually not the power supply; on a Toshiba it is mostly the LVA-510 PAL encoder. RetroTechie: check every supply voltage first; power LED on with a black screen and no Caps Lock usually means the Z80 is not running, from a clock, ROM or (more often) RAM fault; ageing brings bad sockets and connectors, broken tracks, and dried, leaking or shorted capacitors. PingPong: the VDP supplies the CPU clock.
- ↑ 3.0 3.1 3.2 3.3 Toshiba HX-10 Repair, October 2021—link(accessed 2026-10-21) ArkadiuszMakarenko: after storage, a hard short from +5 V to ground; the large power transistors and zeners tested good. gdx and SkalTura: check the capacitors, look for bulges or leaks, lift the ones on the 5 V circuit one at a time. Wierzbowsky: check the cartridge slot, the rear expansion connector and the joystick ports for shorts, then the transistors and regulators, then the capacitors; even a ceramic capacitor can short the rails. ArkadiuszMakarenko: removing 4164 RAMs raised the resistance from about 3 ohms towards 300 ohms. gdx, 6 October 2021: the usual causes are capacitors, RAMs and VRAMs, the video circuit chips and the microswitch in the cartridge slot, or bad contacts of socketed chips; check each electrolytic for a short with an ohmmeter.
- ↑ 4.0 4.1 4.2 4.3 Toshiba HX-10 -- unusual black screen issue!, March 2025—link(accessed 2026-10-21) WxAxNxDxExRxExR: black screen with a faulty ROM replaced; the Z80, VDP, PAL board with its LVA510, PSG and ROM all tested good in a second HX-10. sdsnatcher73: check activity on the RAM and VRAM address and data lines with a scope or logic probe, the chip selects on the RAM and ROM, then the clock and reset on the VDP and CPU. Later: about 7 V on the chips, linked to the A473 PNP power transistor TR6; with TR6 replaced the chips still saw 6.5 to 7 V; no −5 V on the 4116 video RAMs, whose −5 V supply leads back to TR6 and smaller transistors and diodes; the zener ZD3 next to TR6 was found blown.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Toshiba MSX HX-10 64K: Electrónica– Alejandro Sánchez, 25 May 2021—link(accessed 2026-10-21) In Spanish. Teardown of an HX-10 64K (HX-10P): remove the screws under the case except those holding the transformer, and lift the top with the keyboard, which connects to the main board through a 9-pin and an 11-pin connector (three pins of the 11-pin one feed the two keyboard LEDs); the transformer (marked 240 V, works on 220 V) gives 8.5 V AC and 16 V AC, rectified and regulated on the main board by transistor and zener regulators: a B435 on a heat sink for +5 V, a D880 for +12 V and an A473 for −12 V; Sharp Z80A with a 3.579545 MHz resonator; eight HM4864AP-15 RAMs; TMS9929A under a glued heat sink; separate PAL encoder board EMC PB3605E with an LVA510 and a 4.433619 MHz crystal, fed from the VDP by six wires; RF modulator in a metal box with a channel adjustment screw; eight 4116 video RAMs; MB83256 32 KB ROM; cartridge slot with a lever switch joining SW1 and SW2; four HD74LS367AP buffers; TCX-1007 gate array; cassette motor relay. Author's replies of 8 and 14 January 2024: the two transformer outputs are unregulated AC.
- ↑ 6.0 6.1 6.2 HX-10 Faulty transformer, December 2021—link(accessed 2026-10-21) jonwarby: a European HX-10 measured at the transformer connector with the main board unplugged gave about 20 V AC on the blue lead but only 2 to 3 V AC on the grey and brown leads, where about 9 V was expected; the supply is AC only, with the DC conversion on the main board. Manuel: also check for +5 V, +12 V and −12 V DC.
- ↑ Toshiba, Home Computer Owner's Manual, Model HX-10, 1984, "Safety precautions", "Controls and connections" and "Program loading from ROM cartridge" (PDF pages 4, 8, 9 and 16). See Toshiba HX-10 Owner's Manual. Mains supply 240 V 50 Hz; no plug supplied; fit a 13 A plug with a 3 A fuse (blue neutral, brown live); always switch off the computer before inserting or removing a ROM cartridge, as damage to the program on the cartridge may result. Front and right side: power indicator, cartridge slot. Rear and left side: expansion bus connector (only equipment specified by Toshiba should be connected to it), composite video output, audio output, RF output, cassette recorder socket, power switch, mains cord.
- ↑ 8.0 8.1 My Toshiba HX-10 gets mighty hot!, December 2016 – March 2019—link(accessed 2026-10-21) Colemu: the heat comes from the regulator transistor TR8, a B435 on the aluminium heat sink next to the transformer (base 8.3 V, collector 5 V, emitter 9 V); the other two transistors, without heat sinks, are a D880 and an A473. gdx (whose HX-10 is the Japanese NTSC version): added small heat sinks to the two bare regulators; with a CF interface and a memory cartridge fitted the large heat sink reached 84.4 °C, the A473 about 72 °C and the D880 about 76 °C; the two capacitors nearest the hot regulator, 2200 µF 35 V and 4700 µF 16 V, were rated 85 °C, and he replaced the four largest capacitors with 105 °C parts; the CPU is socketed, and a CMOS Z80 lowered the temperature by a little over 3 °C; the transformer reached about 56 °C. sd_snatcher: Toshiba's MSX supplies run hot with heat sinks too small or missing; use Japanese models only on 100 V and European ones only at their rated voltage; avoid slot expanders without their own supply and very power-hungry cartridges; add heat sinks or a small fan; a CMOS Z80 of 10 MHz or less (not the Z84C0020) cuts the load; beware of fake Z80s. RetroTechie: TR8 works as a linear regulator from about 9 V down to 5 V, dissipating about 4 W at 1 A; a switching 78xx-type module rated 1.5 A or more in its place would cut the heat. A member, 14 April 2017: for power circuits near heat sources, choose 105 °C capacitors with a long rated life and bend their leads to keep them away from the heat. Kacper, 2019: a PAL HX-10 has a different main board; its large heat sink reached 77 °C after 20 minutes.