Sony HB-55 General Maintenance

HB-55P with the upper case off: main board and the power board on its bracket (MSX Resource Center wiki photo)

Routine work on the Sony HB-55 covers opening the case, removing the keyboard without losing keytops, checking the internal power supply and its rails, and the three video adjustments in Sony's service manual. The steps below are from Sony's service manual for the HB-55P, HB-75P and HB-75B. The Japanese HB-55 has a different main board, marked 1-611-737-13, which the manual does not cover.

Safety

Warning: the HB-55 has its mains transformer, fuse and power switch inside the case. Unplug the mains cable before removing any screws.

The HB-55P runs on 220 V. Its transformer has a second tap for 240 V, used on the British HB-75B, and a thermal fuse in the 0 V side of the primary. The mains fuse F401 (time-lag, 160 mA) and the power switch S401 are on the fuse board, with C9 (4700 pF, 400 V) across the switch. Sony marks these parts as critical to safety.

Opening the case

Sony's procedure:

1. Unplug the mains cable and remove any cartridges and cables.
2. Remove the three set screws (B2.6 × 3) from the lower cabinet.
3. Lift the upper cabinet enough to disconnect CN11 (13-pin) and CN12 (9-pin) on the main board and CN2 (2-pin) on the power board.
4. Release the three hooks and remove the upper cabinet in the direction of the arrow in Sony's drawing.

To remove the main board:

1. Remove the four set screws of the heat sink. The reinforcing plate shown in the drawing is not fitted to the HB-55P.
2. Remove the three set screws of the main board.
3. Remove the two set screws of the blind plate.
4. Disconnect CN10 (8-pin) on the main board and lift the board out.

Removing the keyboard

Sony's manual warns that none of the HB-55P's keytops are locked in place. Before turning the keyboard over or removing it, hold the keytops in with tape so that none fall out. Sony's drawing shows P2.6 × 8 tapping screws and smaller 2 × 4 screws holding the keyboard and its frame.

Cleaning

With the boards out, wash the plastic cabinet parts in warm water with a little washing-up liquid and dry them fully. Clean the cartridge slot contacts with isopropyl alcohol. On the main board, look for corrosion around the sixteen 4116-type RAM chips and the electrolytic capacitors, and on the power board around the large block capacitors C1 and C2.

Power supply checks

The transformer has a 5 V winding and a centre-tapped 12 V winding. The power board turns them into three rails:

HB-55P supply rails (Sony service manual, page 5-24)

Rail	Rectifier and reservoir	Regulator	Output capacitors
+5 V	D6 (RB-402 bridge), C1 10000 µF 16 V	IC1, SI-3052V, on the 5 V board	C6 (5 V board), then L1 and C4
+12 V	D1 and D2 (10E-2), C2 3300 µF 25 V	IC2, SI-3122V, on the 12 V board	C7 (12 V board)
−12 V	D3 and D4 (10E-2), C3 330 µF 35 V	IC3, NJM79L12	C8

Sony gives no tolerances. Sony's board drawing shows pads marked GND, 12V and 5V on the component side of the power board.

The 4116-type RAMs also need −5 V on pin 1. Measure it at pin 1 of any of them, alongside +12 V at pin 8.

The 6-pin audio/video socket (CN1 on the power board) carries +12 V on pins 1 and 5, as well as video on pin 2 and audio on pins 4 and 6. Check the wiring of any cable before connecting a monitor.

Video adjustments

Sony's manual gives three adjustments for the HB-55P, HB-75P and HB-75B, to be done in this order:

1. VCO phase: connect a digital voltmeter to TP1, the junction of R37 and R38 on the main board, and adjust VC1 for 7.0 V ±0.1 V.
2. Sub-carrier frequency: connect a frequency counter to TP2, the junction of C28 and C47, and adjust VC2 for 4.433618 MHz ±10 Hz.
3. Video output level: terminate pin 2 of CN1 (the video output on the power board) with 75 Ω, or connect a monitor with a 75 Ω input. The manual also says to leave the RGB output, CN3, unconnected; only the HB-75P and HB-75B have one. Run 10 COLOR 15,15 and 20 END in BASIC to get a white screen, connect an oscilloscope to TP3 (pin 2 of CN1) and adjust VR4 on the main board for 1 V ±0.1 V peak to peak from sync level to white level.

The RGB level adjustments on page 6-2 apply only to the HB-75P and HB-75B.

Capacitors

The electrolytic capacitors on the main board and power board are listed in the Sony HB-55 Capacitor Replacement Guide. General signs of failure are in Capacitor Failure Symptoms.

Things that are not faults

• The Personal Databank starts at power-on. With a disk drive connected it is skipped, and with a disk drive and an HBI-55 cartridge both fitted the disk drive is disabled.
• Cartridge slot B on the HB-55P needs its receptacle fitted before use.
• The RESET button clears memory.

Maintenance checklist

1. Unplug the mains cable before opening the case.
2. Tape the keytops before turning the keyboard over.
3. Look for leaked electrolyte on the power board and around the RAM chips.
4. Check +5 V, +12 V and −12 V on the power board, and −5 V at pin 1 of a 4116 RAM.
5. Clean the cartridge slot contacts.
6. If you have the instruments, check the three video adjustments in order.

Related pages

• Sony HB-55
• Sony HB-55 Troubleshooting Guide
• Sony HB-55 Capacitor Replacement Guide
• Sony HB-75
• Sony HB-55P, HB-75P and HB-75B Service Manual
• Recommended Tools