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Sharp Hotbit HB-8000 General Maintenance

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Sharp Hotbit HB-8000 rear panel
Rear of a white HB-8000: audio, video and TV (RF) sockets and the switched AC outlet (Sintegrity, CC BY-SA 4.0)

Routine work on the Sharp Hotbit HB-8000 covers the mains supply and its settings, the power supply voltages, Epcom's adjustments for the clock and the RF modulator, a video and sound check from the service manual, and care of the cartridge slots and the main board. Component references are from Epcom's service manual of November 1985, which is in Portuguese.[1]

Safety

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Warning: the HB-8000 has its mains transformer, power switch and fuse inside the case, and a mains outlet on the back panel. Unplug the mains cable before opening the case.

The power supply schematic shows the mains input through the power switch SW101 and the fuse F101 (0.25 A) to the line choke L101, the voltage selector SW102 and the transformer TR101. The rear AC outlet is wired to the mains input and is switched by the computer's power switch.[2][3]

The mains voltage selector is under the case. Epcom's user manual says to check the local mains voltage before use and gives the machine as working on 120 or 220 V, at 24 W.[4] Set it before plugging in a machine of unknown history.

Opening the case

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The service manual has no written disassembly procedure. Its exploded views, on PDF pages 21 to 23, show how the case, keyboard, boards and power supply fit together.[5] Work on a soft surface, note where each cable goes, and keep the screws from each part separate.

Handling the main board

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A member of the MSX Resource Center forum, tvalenca, warns that the HB-8000's main board cannot take much heat, because its traces and vias are fragile, and advises extra care when removing chips.[6] Use a temperature-controlled iron and a desoldering tool, and keep the heat on each joint short.

Power supply checks

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Epcom's power supply adjustment is a single step: with a DC voltmeter on pin 3 of J15, the power supply connector on the main board, set +12 V with VR101.[7] The other rails have no adjustment. The schematic shows how each is made:[2]

HB-8000 supply rails (power supply schematic)
Rail Rectifier and reservoir Regulator Output capacitor Protection
+5 V Bridge D101 to D104; C112 (4700 µF 25 V) TIP32 pass transistor T102, driven by T103 and the 741C op-amp CI101; current sensing through R101 and T101 C113 (47 µF 16 V) Fuse F102 and crowbar D109 (TIC106A) with zener D106
+12 V D107; C114 (1000 µF 40 V) LM723 (IC-102) with TIP29 pass transistor T104; set with VR101 C115 (47 µF 25 V) –
−12 V D108; C116 (1000 µF 40 V) 7912 (CI-103) C117 (47 µF 25 V) –

[2] The five output wires are labelled, in order, ground, −12 V, +12 V, inhibit and +5 V.[2] The inhibit line switches the supply off through T105. On the main board schematic, pin 4 of J15 is joined to pin 3 of the keyboard connector J25 and to pin 44 of slot J1.[8] The MSX Resource Center describes a switch on the slot 1 cover that shuts the supply down while a cartridge is inserted.[3]

Clock adjustment

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Epcom sets the clock with C12, a 6.8 to 45 pF trimmer: connect a frequency counter to pin 37 of CI27 (the TMS9128NL video chip) and adjust C12 for 3.575611 MHz.[7][9] The service manual's circuit description gives the video chip's crystal as 10.726834 MHz, three times the target frequency, but the parts list gives X1 as 10.738635 MHz.[10][9] Read the marking on the crystal before ordering a replacement.

RF modulator adjustments

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Epcom's manual gives three adjustments for the RF modulator board:[7]

  1. RF: ground the video and audio inputs of the modulator (pins 1 to 4 of J10) and connect a frequency counter to pin 11 of CI303. With SW301 on channel 4, set L304 for 67.25 MHz; with SW301 on channel 3, set L305 for 61.25 MHz.
  2. Sound carrier: with the inputs still grounded, put the counter on pin 12 of CI303 and set L307 for 4.5 MHz.
  3. Phase: with the machine on and an oscilloscope on the video output, set VR301 so that the burst levels are equal.

Use a plastic trimming tool on the coil cores.

Video and sound check

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Epcom's check after repair: switch on, press F2 and RETURN, then type and run this program.[7] 

10 SCREEN 3
20 READ C
30 LINE (X,0)-((X+16),191),C,BF
40 X = X + 16
50 DATA 0,15,14,7,5,4,13,9,8,6,11,10,3,2,12,1
60 IF X > 255 GOTO 70 ELSE 20
70 FOR I = 1 TO 96 STEP 6
80 PLAY "N=I;"
90 NEXT I
100 GOTO 70

The screen should show 16 coloured bars, and the machine should produce sound.[7]

Cartridge slots

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The MSX Resource Center describes a switch on the slot 1 cover that shuts the supply down while a cartridge is inserted, and notes that the second slot has no protection against a cartridge being inserted with the power on.[3] Switch the machine off before inserting or removing a cartridge in either slot. Clean the slot contacts with isopropyl alcohol on a cartridge-shaped card or a cotton bud, with the power off.

Capacitors

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The electrolytic capacitors on the power supply, main board and RF modulator board are listed in the Sharp Hotbit HB-8000 Capacitor Replacement Guide. General signs of failure are in Capacitor Failure Symptoms.

Maintenance checklist

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  1. Check the voltage selector under the case before plugging in.
  2. Unplug the mains cable before opening the case.
  3. Measure +5 V, +12 V and −12 V; set +12 V at pin 3 of J15 with VR101 if needed.
  4. Check the clock at pin 37 of CI27 (3.575611 MHz).
  5. Check the RF output and the colour, then run the video and sound check program.
  6. Clean the slot contacts, and switch off before changing cartridges.
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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 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 ("tomada AC"), L101, C103 (0.1 µF 400 V), SW102, TR101; +5 V from bridge D101 to D104, C112, TIP32 pass transistor T102 and 741C CI101, fuse F102, crowbar D109 (TIC106A) and zener D106 (1N754); +12 V from D107, C114, LM723 IC-102, TIP29 T104 and VR101; −12 V from D108, C116 and 7912 CI-103; inhibit input through T105; output wires labelled ground, −12 V, +12 V, inhibit and +5 V.
  3. ↑ 3.0 3.1 3.2 Sharp HB-8000, MSX Resource Center wiki—link(accessed 2026-10-21) Switched AC outlet; reset button at the bottom left; mains voltage selector; 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; second slot not protected against insertion with the power on; internal supply not strong enough for several devices; jumpers P1 to P3.
  4. ↑ Manual do Usuário - Hotbit HB-8000, Internet Archive—link(accessed 2026-10-21) Underside: RF channel selector (3 or 4) and voltage selector (120 or 220 V); "check the local mains voltage before starting work"; 120 or 220 V, 24 W.
  5. ↑ Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), exploded views, PDF pages 21 to 23.
  6. ↑ Repair of a Sharp HB-8000 Brazilian MSX computer, MSX Resource Center forum, November 2017—link(accessed 2026-10-21) tvalenca: some HB-8000s were sold with a bad batch of 74LS74 and 74LS138 TTL chips, giving faults in refresh generation or slot selection; one machine had a VDP with a failed /INT signal and another a bad reset buffer; test with a 64 KB RAM cartridge, then check RAS and CAS; take extra care removing chips because the board's traces and vias are fragile; to test the on-board RAM, disconnect VCC and GND from the two 74LS157 address multiplexers and boot from a RAM cartridge. Argon: a dead machine was brought back by recapping the power supply, replacing its coil and recapping the modulator. Grauw: a memory mapper used for testing must be MSX1-compatible.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), adjustments ("ajustes"), printed page 20 (PDF page 25): power supply (DC voltmeter on pin 3 of J15, VR101 for +12 V); oscillator (frequency counter on pin 37 of CI27, C12 for 3.575611 MHz); RF (inputs on J10 pins 1 to 4 grounded, counter on pin 11 of CI303, SW301 on channel 4 and L304 for 67.25 MHz, SW301 on channel 3 and L305 for 61.25 MHz); audio (counter on pin 12 of CI303, L307 for 4.5 MHz); phase (oscilloscope on the video output, VR301 for equal burst levels); video and audio check with a BASIC program giving 16 colour bars and sound.
  8. ↑ 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, R13, C2, T1, CI6, CI30; cassette relay RL1 and T4; C9 to CMT OUT on J35; jumpers P1 and P2.
  9. ↑ 9.0 9.1 Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), main board parts list, printed page 21 (PDF page 26): C12 trimmer 6.8 to 45 pF; X1 crystal 10.738635 MHz; CI27 TMS9128NL.
  10. ↑ Epcom, HB-8000 service manual (File:Sharp Hotbit HB-8000 Service Manual.pdf), video processor pin functions, printed page 7 (PDF page 6): XTAL pins 39 and 40, oscillator at 10.726834 MHz; pin 37 clock output at XTAL/3 for the CPU.



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