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Sharp Hotbit HB-8000 Troubleshooting Guide

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Sharp Hotbit HB-8000 power supply circuit (service manual)
HB-8000 power supply: mains input, F101, transformer, the +5 V regulator with fuse F102 and crowbar D109, the LM723 for +12 V and the 7912 for −12 V (Epcom service manual)

Fault-finding on the Sharp Hotbit HB-8000 starts with the internal linear power supply, then the main board, which is built from separate chips, and the RF modulator board. Component references are from Epcom's service manual of November 1985.[1] Owners' reports are from the MSX Resource Center.

Warning: mains voltage is present inside the case on the power supply board, the power switch, the fuse and the rear AC outlet wiring. Unplug the mains cable before opening the machine.

Quick reference

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Symptoms, causes and actions
Symptom Probable cause Action
Dead, power lamp dark Voltage selector, F101, SW101, transformer, slot cover inhibit No power
Dead, F102 blown Short on +5 V, or the crowbar has fired Fuse F102 blown
Black or noisy screen, no BASIC prompt RAM, refresh or slot select logic, reset, video chip Black screen or no start
No colour, or no picture through RF RF modulator adjustment or capacitors Video and RF faults
Cassette will not load or motor does not run Relay RL1, T4, C9, cable Cassette
Accented characters wrong ROM 1.0 Software differences
Music plays fast ROM 1.2 timing Software differences

[2][3]

No power

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  1. Check the mains voltage selector under the case. It must match the local supply (120 or 220 V).[4]
  2. Unplug the machine and open it. Check F101 (0.25 A) and the power switch SW101, then the transformer primary through L101 and SW102.[2]
  3. Plug in and switch on. Measure across C112 (the +5 V reservoir), C114 (+12 V) and C116 (−12 V). No voltage on any of them, with F101 intact, points to the transformer or its wiring.
  4. If the reservoirs are charged but all outputs are missing, check the inhibit line. The supply's inhibit input acts on the regulator through T105.[2] On the main board schematic that input (pin 4 of J15) is joined to pin 3 of the keyboard connector J25 and to pin 44 of slot J1.[5] The MSX Resource Center describes this as a switch on the slot 1 cover that shuts the supply down while a cartridge is inserted, so check that the cover is closed and the switch and its wiring are good.[3]
  5. Before fitting a new F101, check the bridge D101 to D104, D107, D108 and the reservoir capacitors for a short.

The power supply manual page with its capacitor list is not in the surviving scan, so take values from the schematic; see the Sharp Hotbit HB-8000 Capacitor Replacement Guide.

Fuse F102 blown

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F102 is in the +5 V output, ahead of a crowbar: the thyristor D109 (TIC106A), fired through the zener D106 (1N754), shorts the +5 V line to ground. The MSX Resource Center describes this as crowbar protection with a fuse on +5 V.[2][3] A blown F102 therefore means either a short on the +5 V line or an overvoltage from the +5 V regulator.

  1. With the supply unplugged from the main board, check the +5 V line on the main board for a short to ground.
  2. Check the +5 V regulator: the TIP32 pass transistor T102 for a collector-emitter short, T103, T101 and the 741C op-amp CI101.[2] The 741C runs from +12 V and −12 V on the schematic, so check those rails as well.
  3. Fit a new F102 of the original rating and power the supply on a dummy load or a lamp before reconnecting the main board.

+12 V or −12 V missing

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  • +12 V comes from D107 and C114 through the LM723 (IC-102) and its TIP29 pass transistor T104. Epcom sets it with VR101, measured at pin 3 of J15.[2][6]
  • −12 V comes from D108 and C116 through the 7912 (CI-103), with C117 on its output.[2]

The +12 V rail also feeds the RF modulator board through J10.[5]

Black screen or no start

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Members of the MSX Resource Center forum discussed an HB-8000 that still gave a black screen after its power supply and modulator had been repaired. Their advice:[7]

  1. Look over the board for corrosion, cracks, loose parts and signs of overheating, and check that +5 V, +12 V and −12 V are present.
  2. Check the clock at pin 37 of CI27: Epcom's figure is 3.575611 MHz.[6]
  3. Try a game cartridge, then a 64 KB RAM cartridge or a memory mapper that works on an MSX1. The HB-8000's own RAM is in slot 3, so RAM in a cartridge slot is used first.[7][3]
  4. To rule out the on-board RAM without unsoldering eight chips, tvalenca suggests disconnecting VCC and GND from the two 74LS157 address multiplexers and booting from a RAM cartridge. If it starts, a RAM chip is the likely fault.[7] On Epcom's schematic these are CI23 and CI24.[5]
  5. tvalenca also reports that some HB-8000s were sold with a bad batch of 74LS74 and 74LS138 chips, which cause faults in RAM refresh or slot selection, and has seen a video chip with a failed interrupt output and a failed reset buffer.[7]
  6. Check the reset circuit. The reset switch SW1, C2 (22 µF) and T1 drive the reset line through CI6 and CI30.[5] A reset held active stops the machine.

tvalenca warns that the board's traces and vias are fragile and will not take much heat, so take care when removing chips.[7]

Video and RF faults

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The main board sends luminance and two colour-difference signals to the RF modulator board on J10; the modulator, built around an LM1889N (CI303), produces the composite video and RF outputs.[5][8]

  • Picture on the composite output but not through RF: check the channel switch SW301 under the case and the TV tuning, then the RF adjustment (L304 for channel 4 at 67.25 MHz, L305 for channel 3 at 61.25 MHz).[6][4]
  • Picture through RF but no sound: the sound carrier is set with L307 for 4.5 MHz.[6]
  • Colour wrong or missing: check the clock (3.575611 MHz) and the burst phase (VR301).[6] The video output is PAL-M, the Brazilian standard.[3]

The forum owner whose machine arrived dead repaired its modulator by replacing all its capacitors.[7]

Cassette

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  • The cassette motor is switched by the relay RL1, driven by T4 (BC550), with D2 (1N4001) across the coil. RL1's contacts go to REM+ and REM− on the cassette connector J35 (pins 6 and 7).[5][9] If the motor never starts, check the relay and T4.
  • Saving fails while loading works: the output passes through C9 (10 µF) to pin 4 of J35.[5]
  • The standard speed is 1200 baud, with 2400 baud available for saving.[4]

Software differences

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These are differences between the ROM versions, not faults:[3]

  • ROM 1.0 has a character table that differs slightly from the Gradiente Expert's, which caused problems with accented characters. Most 1.0 machines had the ROM replaced with version 1.1 when they were serviced.
  • ROM 1.2 counts fewer interrupt ticks than it should when timing notes, so the BASIC PLAY command plays faster than normal, at the right pitch.
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References

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  1. ↑ Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf). Hosted with a description at Sharp Hotbit HB-8000 Service Manual.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), power supply schematic and layout, PDF pages 19 and 20; also File:Sharp Hotbit HB-8000 power supply circuit (service manual).png: 120/220 V 60 Hz input, SW101, F101 0.25 A, switched AC outlet, L101, C103, SW102, TR101; bridge D101 to D104 into C112 (4700 µF 25 V), T102 TIP32 with R101 and T101, T103 BC547 and CI101 741C, F102, crowbar D109 TIC106A with zener D106 1N754, C113 (47 µF 16 V) on +5 V; D107 into C114 (1000 µF 40 V), LM723 IC-102 with T104 TIP29, VR101 and C115 (47 µF 25 V) on +12 V; D108 into C116 (1000 µF 40 V), 7912 CI-103 and C117 (47 µF 25 V) on −12 V; T105 BC547 on the inhibit input; outputs ground, −12 V, +12 V, inhibit, +5 V.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Sharp HB-8000, MSX Resource Center wiki—link(accessed 2026-10-21) Linear LM723 supply with crowbar protection and a fuse on +5 V; switch on the slot 1 cover that shuts the supply down while a cartridge is inserted; ROM 1.0 accented-character problem fixed by the 1.1 ROM; ROM 1.2 PLAY timing; RAM in slot 3; jumper P3 swaps slots 0 and 1.
  4. ↑ 4.0 4.1 4.2 Manual do Usuário - Hotbit HB-8000, Internet Archive—link(accessed 2026-10-21) Underside: RF channel selector and 120/220 V voltage selector; power lamp on the top lights when the computer is switched on; specifications appendix: cassette interface in FSK, 1200 baud standard and 2400 baud by software for recording data, 8-pin DIN connector with remote control.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), main board schematic, PDF pages 12 to 14: pin 3 of J25 to pin 4 of J15 and pin 44 of J1; reset circuit SW1 (reset switch), R13, C2 (22 µF), D1, T1, CI6 and CI30; C31 marked 4.7 µF at the Z80 reset pin with R43; CI23 and CI24 LS157 address multiplexers for the RAM CI34 to CI41; CI27 TMS9128 with X1 and trimmer C12; J10 to the RF board (Y, R−Y, B−Y, +12 V, ground, sound); cassette relay RL1 driven by T4 (BC550) with D2 (1N4001); C9 (10 µF) to CMT OUT, J35 pin 4.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), adjustments, printed page 20 (PDF page 25): VR101 for +12 V at pin 3 of J15; C12 for 3.575611 MHz at pin 37 of CI27; RF, sound carrier and burst phase adjustments on the modulator (L304, L305, L307, VR301, SW301); BASIC check program for 16 colour bars and sound.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Repair of a Sharp HB-8000 Brazilian MSX computer, MSX Resource Center forum, November 2017—link(accessed 2026-10-21) Argon: machine dead on arrival; power supply repaired by recapping and replacing the coil; modulator repaired by recapping; still a black screen, later a noisy white screen. Wierzbowsky: visual check, voltages, hot components, CPU and VDP clocks, test with external RAM or a game cartridge, logic analyser on the ROM. tvalenca: some HB-8000s sold with a bad batch of 74LS74 and 74LS138 chips, giving refresh or slot select faults; one machine with a VDP whose /INT had failed, another with a bad reset buffer; check slot select, RAS and CAS; fragile traces and vias; disable the two 74LS157 address multiplexers by disconnecting VCC and GND and boot from a 64 KB RAM cartridge to test the on-board RAM. Grauw: a memory mapper used for the test must initialise itself on an MSX1.
  8. ↑ Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), RF modulator schematic and layout, PDF pages 17 and 18, and parts list, printed page 22 (PDF page 27): CI301 4013BP, CI302 4016, CI303 LM1889N; SW301 channel 3 or 4; VR301 3K3 amplitude.
  9. ↑ Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), connector pin-outs, printed page 13 (PDF page 11): J35 pin 4 CMT OUT, pin 5 CMT IN, pin 6 REMOTE +, pin 7 REMOTE −, pins 1 to 3 and 8 ground.



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