Sharp MZ-1500 General Maintenance

MZ-1500 in the KCG Computer Museum. The Quick Disk drive is at top right, under the smoked lid (photo: うぃき野郎, CC BY-SA 4.0)

Routine work on the Sharp MZ-1500 centres on its internal 100 V power supply, the built-in Quick Disk drive, the keyboard contacts and the expansion slot. No Sharp service manual for the MZ-1500 has been found online. The drive procedures below come from Sharp's documents for the MZ-1F11, the external version of the same Mitsumi drive, and from the owner Ohishi, who has serviced his MZ-1500's supply, drive and keyboard.

Safety

Warning: the MZ-1500's power supply is inside the case and runs from the mains. Unplug the mains cable before opening the machine.

The MZ-1500 was sold only in Japan and is rated for 100 V ±10%, 50/60 Hz, 28 W. Run it from a 100 V supply; on 230 V mains that means a step-down transformer.

Opening the machine

No disassembly procedure for the MZ-1500 has been published. Ohishi's photographs show the power supply as a separate board in its own metal case, the keyboard lifted off the body for cleaning, and the expansion slot assembly lifted out of the back of the case. Note where each cable goes before unplugging it, and keep the screws from each part apart; the keyboard alone has 28.

Power supply

Ohishi describes the supply as having a single 5 V output. His machine showed what a tired supply looks like: with the kanji ROM and dictionary ROM boards fitted in the main expansion slot, kanji and kana were drawn with dots out of place. The characters were clean once the boards were moved to the MZ-1U08 expansion box, and stayed clean in the main slot after the supply was either recapped or replaced. The capacitors in that supply showed no leakage and no obvious bulging, so judge the supply with a meter: measure the 5 V output with your usual boards fitted. The 5 V reaches the main board on the two-pin connector T-1, with C50 (100 µF) across it, which makes T-1 a convenient place to measure.

Ohishi describes two ways out:

• Recap the supply. The electrolytics and suitable replacements are listed in the Sharp MZ-1500 Capacitor Replacement Guide. The new main reservoir capacitor was taller than the original, and the supply's metal case only just closed over it.
• Fit a modern 5 V switching supply in place of the original, wired to the rear power switch and to the main board's power lead. Ohishi used a 2 A unit, which ran his machine, while noting that the 28 W on the rating label suggests something nearer 5 A; a larger supply may not fit in the space.

Quick Disk drive

The MZ-1500's Quick Disk drive: lid, BUSY lamp (left) and eject button (right) (photo: うぃき野郎, CC BY-SA 4.0)

The built-in drive is the same Mitsumi drive that Sharp sold separately as the MZ-1F11. One motor turns the disk and, through a cam, moves the head, so friction at the head cam slows the disk. Ohishi advises against dismantling the drive completely, because the head cannot be realigned without a reference. For the belt and lubrication, remove the drive's outer cover and the bracket on its base; the bracket's front and rear screws are held with thread-lock.

Head cleaning

Sharp's MZ-1F11 manual tells the owner to clean the head regularly, at an interval set by how much oxide builds up, and to clean it first whenever errors become frequent.

1. Press the eject button to open the disk holder.
2. Find the head at the back of the disk table.
3. Wipe the head with an applicator moistened with head-cleaning liquid, as for a tape recorder head. Cotton buds will do; use a fresh one each time.

Drive belt

If the motor can be heard but the disk does not seem to turn, or the head never moves, the belt is slack or broken.

1. With the drive's base bracket removed, find the brass motor pulley on the underside.
2. Fit a 40 mm diameter, 1.2 mm section belt over the pulley and the flywheel, using tweezers to work it into place.
3. Check that the flywheel starts without hesitation. Ohishi treats a sluggish start as the sign that the belt needs renewing.

Ohishi once fitted a 35 mm belt when the 40 mm size was out of stock; it worked but broke later, probably from the extra tension. Belts deteriorate even in storage, so buy one when it is needed.

Lubrication

The head is driven by a follower running along the edge of a cam. With age the friction there rises and the disk turns more slowly.

1. Through the opening at the front of the drive, brush a little lubricant onto the edge of the cam.
2. Apply a little to the guide of the head unit on top of the drive.

Apply it with a brush; a spray can reach parts that must stay dry. Ohishi found that this made a large difference: Quick Disks that his drive could not read before would load afterwards.

There is a trimmer beside the motor that is often suggested as a cure for weak rotation. In Ohishi's experience it does little for torque and probably sets the motor speed. Mark its position before touching it.

Head pad

Sharp's service manual supplement for the MZ-1F11 gives the life of the head pad unit as 18,000 passes, about two and a half years at 20 passes a day, and describes how to change it:

1. Remove the two screws at the rear of the disk holder, lift its rear and push it towards the front to detach it.
2. Lift the pad arm.
3. Nip the pad unit's pin with tweezers and turn it 90 degrees to release the pad unit. Catch it so that it does not fall into the drive.
4. Fit the new pad unit by reversing the last step, with its pin between the stoppers.
5. Refit the disk holder and check that the drive works.

The supplement applies to MZ-1F11 drives from serial number SU000012, which have the assembled pad unit. Whether a given MZ-1500's drive has the pad unit or the earlier padding is not recorded, so look before ordering parts.

Disks

Quick Disks were made in two versions. The earlier ones are marked A and B in a circle; the later ones have an arrow and a square on the label, and were changed so that the disk inside turns less stiffly. Ohishi lists several remedies owners have tried on stiff disks, such as removing the liner inside the shell or warming the disk, and considers none of them a lasting cure; servicing the drive helped him more. Sharp's own advice is to store disks upright in their envelopes, never bend them, keep them away from magnets, and never open the disk holder while the BUSY lamp is lit.

Keyboard

The keyboard is made by Mitsumi. Its underside carries 28 small screws, which hold a brown circuit board with a pair of contact pads for each key and a row of U-shaped contacts on the key switches.

1. Remove the keyboard from the body and remove the 28 screws.
2. Turn the board over and clean the pads and contacts with anhydrous ethanol.
3. Refit the board. When plugging the film cable back into its connector, take care not to peel the contacts off the cable.

Expansion slot and option boards

The slot at the back holds two 50-way connectors: the outer one for general boards and the inner one for the MZ-1R18 RAM file board. The MZ-1U08 expansion box passes only I/O addresses 00h to C7h, so the MZ-1E05 floppy disk interface, and the MZ-1R12 at its standard addresses, must go in the machine's own slot. Switch off before fitting or removing a board.

The MZ-1500's clock has no battery backup. The MZ-1R12 SRAM board, an option used with it, keeps its contents with a battery; check that battery for leakage (see Battery Explosion, Capacitor or Corrosion Damage).

Capacitors

The power supply's electrolytics are listed in the Sharp MZ-1500 Capacitor Replacement Guide. General signs of failure are in Capacitor Failure Symptoms.

Maintenance checklist

1. Run the machine from a 100 V supply.
2. Unplug the mains cable before opening the case.
3. Measure the 5 V supply with your usual boards fitted, and recap or replace the supply if the display shows corrupted characters.
4. Clean the Quick Disk head.
5. Check that the drive's flywheel starts briskly; renew the belt if not.
6. Lubricate the head cam and the head guide sparingly.
7. Clean the keyboard contacts if keys are unreliable.
8. Check any battery-backed option boards for leakage.
9. Before blaming a fault, check the four system switches on the back: switch 1 sets the cassette polarity, switch 2 the printer interface, and switch 3 turns the graphics display off.

Related pages

• Sharp MZ-1500
• Sharp MZ-1500 Troubleshooting Guide
• Sharp MZ-1500 Capacitor Replacement Guide
• Recommended Tools