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Sony HB-75 Troubleshooting Guide

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Sony HB-75P inside
HB-75P with the upper cabinet off: transformer at the rear left, power board at the rear, main board 1-613-622-12 in the lower case.

Sony's service manual for the HB-55P, HB-75P and HB-75B has schematics, parts lists and alignment figures but no fault-finding chart. The checks below are built from those, for the European models; the Japanese Sony HB-75 runs on 100 V and is not covered by the manual.[1][2] Opening the case is described in the Sony HB-75 General Maintenance guide.

Start with the supply

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Measure the rails at a cartridge slot, against pin 41:[3]

Supply rails and where they come from[4][5]
Slot pin Rail Regulator Rectifier and reservoir
45, 47 +5 V IC1 SI-3052V on the +5 V board D6 (RB402), C1 10,000 µF
48 +12 V IC2 SI-3122V on the +12 V board D1 and D2 (10E2), C2 3300 µF
50 −12 V IC3 NJM79L12A on the power board D3 and D4 (10E2), C3 330 µF

All three rails reach the main board through CN10 (8-pin).[4]

Symptoms

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Symptom Check first Notes
Nothing at all, power lamp off Fuse F401, the power switch, the transformer primary F401 is a 160 mA time-lag fuse on the fuse board. The transformer has a thermal fuse in its primary. The power lamp is an SLP171D on the LED board, fed from the power board.[4][5] Before fitting a new fuse, check the rectifiers and reservoir capacitors for a short.
+5 V missing or low The +5 V board Check C1, the bridge D6, IC1 (SI-3052V) and the 47 µF output capacitor C6 on the +5 V board. One HB-75P had burst capacitors on both regulator boards.[4][6]
+12 V missing The +12 V board Check D1, D2, C2 (3300 µF), IC2 (SI-3122V) and C7.[4]
−12 V missing IC3 on the power board Check D3, D4, C3 (330 µF), IC3 (NJM79L12A) and C8.[4]
Rails correct, black screen, no start The reset circuit, then the ROMs At switch-on R106 and C49 hold pin 3 of IC26, a Schmitt-trigger inverter, low for several hundred milliseconds to reset the PPI (IC16) and CPU (IC41). C49 is a 47 µF 25 V electrolytic. The BASIC ROM is IC42 and IC43, or one 32 KB MSM38256 mask ROM at IC42 with jumpers JW5 and JW7 to JW10 set to MASK.[7][8][9]
Machine starts but the picture has no colour on composite or RF The PAL subcarrier At start-up the firmware ROM (IC44) loads the PLL chip IC31, which generates the 4.43 MHz subcarrier. Measure TP2 (junction of C28 and C47): it should read 4.433618 MHz ±10 Hz, set with VC2 after the VCO phase at TP1 (7.0 V ±0.1 V, VC1).[7][10]
Composite picture too dark or too bright Video level 1 V ±0.1 V p-p at TP3 (pin 2 of CN1 on the power board) into 75 Ω, set with VR4.[10]
One RGB colour wrong or missing The RGB driver and its level control Each output is 0.7 V ±0.1 V p-p into 75 Ω: blue at pin 3 of CN3 (Q1, VR1), red at pin 15 (Q3, VR2), green at pin 11 (Q5, VR3).[10]
Function or cursor keytops fall out Not a fault On the HB-75P and HB-75B these keytops are not locked in; tape them before handling the keyboard on its own.[9]
Picture or sound breaks up when a lead is moved The connector's solder joints Reflow the joints on the SCART, DIN and joystick connectors.[11]
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References

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  1. ↑ Sony, HB-55P/75P/75B Service Manual, PDF page 3 (printed pages 1-1 and 1-2), features and specifications: Z80A at 3.579545 MHz with one wait state in each M1 cycle; HB-75P/75B 64 KB RAM, HB-55P 16 KB; 48 KB ROM (MSX-BASIC 32 KB, utility program 16 KB); TMS9929ANL video; PAL composite video 1 V p-p, analogue RGB 0 to 0.7 V on the HB-75P/75B, RF on UHF channel 36; audio −5 dBs; 74 keys on the HB-75P/75B; cassette 1200/2400 baud; 14-pin printer port; two 9-pin joystick ports; two MSX cartridge slots; 220 V ±10 %, 50/60 Hz (HB-55P/75P) or 240 V ±10 %, 50/60 Hz (HB-75B); 24 W; about 405 × 67 × 245 mm and 3.5 kg. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  2. ↑ Category:Sony HB-75, MSX Resource Center wiki—link(accessed 2026-10-21) The HB-75 series: HB-75 (Japan, 100 V, one cartridge slot plus an expansion bus, RF and composite outputs, black or white, ¥69,800), HB-75AS (Australia, 240 V), HB-75B (UK, 240 V, £299), HB-75D (Germany, 220 V, QWERTZ), HB-75F (France, 220 V, AZERTY, RGB SCART only) and HB-75P (Europe, 220 V); the export models have two cartridge slots, RGB on SCART and composite video and audio on a 6-pin DIN; 1984; 64 KB RAM in slot 2, 16 KB VRAM; TMS9918ANL (HB-75) or TMS9929A (others); PSG AY-3-8910A or YM2149F; NEC D780C-1 Z80A; built-in Personal Databank; the Japanese HB-75 came with the HBI-55 data cartridge; links to Sony's HB-55P/75P/75B service manual on the MSX Archive.
  3. ↑ MSX Cartridge slot, MSX Resource Center wiki—link(accessed 2026-10-21) Pin-out of the MSX cartridge slot: pins 41 and 43 ground, 45 and 47 +5 V, 48 +12 V, 50 −12 V, 49 sound input, 42 CPU clock 3.579545 MHz; +5 V must supply up to 300 mA per slot and ±12 V up to 50 mA for all slots.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Sony, HB-55P/75P/75B Service Manual, PDF pages 54 and 55 (printed 5-23 and 5-24), schematic of the fuse, LED, power, transformer, +5 V and +12 V boards: mains through fuse F401 (T160mA) and power switch S401, with C9 (4700 pF) across the switch, to transformer T401, which has a thermal fuse in its primary and taps for 220 V (AE) and 240 V (UK); +5 V from bridge D6 (RB-402), reservoir C1 (10,000 µF 16 V) and regulator IC1 (SI-3052V) on the +5 V board with C10 (0.33 µF) and C6 (47 µF 16 V), then L1 (22 µH) and C4 (47 µF 16 V); +12 V from D1 and D2 (10E-2), C2 (3300 µF 25 V) and IC2 (SI-3122V) on the +12 V board with C11 (0.33 µF) and C7 (47 µF 16 V); −12 V from D3 and D4 (10E-2), C3 (330 µF 35 V) and IC3 (NJM79L12) with C5 (0.33 µF) and C8 (47 µF 16 V); +5 V, +12 V and −12 V go to main board connector CN10; the power board also carries the RF unit and the 6-pin audio/video DIN socket. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  5. ↑ 5.0 5.1 Sony, HB-55P/75P/75B Service Manual, PDF page 76 (printed 7-14), electrical parts list for the fuse, LED, power, +5 V and +12 V boards: C1 1-125-375-11 electrolytic (block) 10,000 µF 16 V; C2 1-125-376-11 electrolytic (block) 3300 µF 25 V; C3 1-123-362-00 electrolytic 330 µF 50 V; C4, C6, C7 and C8 1-123-332-00 electrolytic 47 µF 25 V; C5, C10 and C11 1-136-171-00 film 0.33 µF 50 V; C9 1-161-953-00 ceramic 0.0047 µF 400 V (safety-critical); F401 1-532-612-00 time-lag 160 mA; D1 to D4 10E2; D6 RB402; IC1 SI-3052V; IC2 SI-3122V; IC3 NJM79L12A; L1 22 µH micro inductor; LED D1 SLP171D; RF modulator 1-464-384-11 (55P/75P) or 1-464-383-11 (75B); +5 V board 1-613-624-11; +12 V board 1-613-625-11. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  6. ↑ Photograph posted to msx.pics on 25 March 2019 (https://www.msx.pics/image/Ud0TS), linked from the MSX Resource Center's vulnerable hardware page: "These capacitors had exploded on the Sony HB-75P when I got it", showing two small regulator boards marked RE-19 (1-615-726-11) and RE-20 (1-615-727-11) with replacement electrolytics and film capacitors marked 330nK beside each regulator.
  7. ↑ 7.0 7.1 Sony, HB-55P/75P/75B Service Manual, PDF pages 13 and 14 (sections 3-2 and 3-3), theory of operation: at switch-on R106 and C49 hold pin 3 of IC26, a Schmitt-trigger inverter, low for several hundred milliseconds to reset the PPI (IC16) and the CPU (IC41); the firmware ROM (IC44) then sends PLL setting data to IC31, which starts the 4.43 MHz PAL subcarrier, before BASIC (IC42 and IC43) starts; slot 0 holds the ROMs, slot 1 is the top cartridge slot, slot 3 the rear cartridge slot, and on the HB-75 the 64 KB main RAM is in slot 2. The contents list (PDF page 2) gives section 3-5-3 as "Dynamic RAM (3764) (HB-75P/75B)" and the HB-55P's as the 4116. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  8. ↑ Sony, HB-55P/75P/75B Service Manual, PDF page 72 (printed 7-9), electrical parts list, main board (mounted board A-8050-098-A for the HB-75P/75B, A-8050-100-A for the HB-55P): electrolytics C1, C3, C5, C6, C7, C34, C44, C49, C54, C55, C59, C60 and C68 47 µF 25 V; C66 and C67 47 µF 25 V (HB-55P only); C2 and C022 0.47 µF 100 V; C4 and C73 470 µF 6.3 V; C8, C9, C14 and C018 10 µF 50 V (HB-75P/75B only); C019 10 µF 50 V; C10 470 µF 10 V; C027 100 µF 10 V; C030, C41, C42, C45, C52 and C61 to C64 4.7 µF 63 V; C031 22 µF 25 V; C57 220 µF 10 V. The list prints some designators with a leading zero (C016 to C035). Page 7-10 lists the remaining main board capacitors, all 0.01 µF ceramic. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  9. ↑ 9.0 9.1 Sony, HB-55P/75P/75B Service Manual, PDF pages 9 and 10 (printed 2-1 to 2-3): upper cabinet removal (three screws in the lower cabinet; unplug CN11 (13-pin) and CN12 (9-pin) on the main board and CN2 (2-pin) on the power board; release three hooks); main board removal (four heat sink screws, three main board screws, two screws in the blind plate, unplug CN10 (8-pin)); on the HB-75P/75B the function and cursor keytops are not locked in, so take the keyboard out and tape them before handling it; the BASIC ROM can be a single MSM38256-70RS mask ROM at IC42 with jumpers JW5 and JW7 to JW10 set to MASK and IC43 removed; the firmware ROM IC44 is an MSM38128A-77RS, with JW6 removed for an EPROM; parts marked with a warning triangle are safety-critical. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  10. ↑ 10.0 10.1 10.2 Sony, HB-55P/75P/75B Service Manual, PDF pages 62 and 63 (printed 6-1 and 6-2), alignment, in this order: VCO phase, TP1 (junction of R37 and R38) 7.0 V ±0.1 V, set with VC1; subcarrier frequency, TP2 (junction of C28 and C47) 4.433618 MHz ±10 Hz, set with VC2; video output level, TP3 (pin 2 of CN1 on the power board) 1 V ±0.1 V p-p into 75 Ω, set with VR4; on the HB-75P/75B only, RGB levels 0.7 V ±0.1 V p-p into 75 Ω at TP4 (blue, pin 3 of CN3 or the collector of Q1, VR1), TP5 (red, pin 15 of CN3 or the collector of Q3, VR2) and TP6 (green, pin 11 of CN3 or the collector of Q5, VR3), with a white screen from COLOR 15,15. Hosted here as Sony HB-55P, HB-75P and HB-75B Service Manual.
  11. ↑ Vulnerable MSX hardware, MSX Resource Center wiki—link(accessed 2026-10-21) All MSX2 and later machines have a battery for the real-time clock; drained batteries, alkaline ones especially, leak; take removable batteries out before storage; lithium AA cells as an alternative. Lists the Sony HB-75P ("not as serious, as the problem caps are on separate small PCBs"), HB-F1II, HB-F1XD, HB-F1XDJ ("the ELNA caps are risky, the Nichicon ones are ok"), HB-F1XDmk2 and HB-F1XV among models with leaky capacitors. Fractured solder joints on DIN, RCA and DE-9 connectors cause flickering video and crackling sound; reflow them.


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