Technics SJMD-100 Service manual

$ This document is supplementary to the

following Service Manual:

ORDER NO. AD9903069S2

Minidisc Deck

SJ-MD100

Colour

(K) ........ Black Type

Areas

(E) ........ Europe.

(EB) ...... Great Britain.

(EG)...... Germany.

Model No.

SJ-MD100

Area Code

(E) (EB) (EG)

$ Purpose

Supplement for technical information about MD.

$ Contents

1. Measuring instruments and special tools

Test Disc

Extension Cable Kit

Laser Power Meter

O ROM Part No.: RFKV0006
O ROM Part No.: RFKV0014

O Extension Cable Kit

Part No.: RFKZJMD100EK

O Laser Power Meter Model No.: LE8010

Order No.

MB9902001C2

Made by Laser Electric

2. Basic Knowledge of MD

3. Operating Procedures

4. Troubleshooting Guide for MD Servo Circuit

This service information is designed for experienced repair technicians only and is not designed for use by the general public. It does not
contain warnings or cautions to advise non-technical individuals of potential dangers in attempting to service a product.
Products powered by electricity should be serviced or repaired only by experienced professional technicians. Any attempt to service or
repair the product or products dealt with in this service information by anyone else could result in serious injury or death.

WARNING

1999 Matsushita Electric Industrial Co., Ltd.

©

All rights reserved. Unauthorized copying
and distribution is a violation of law.

SJ-MD100

$ Basic Knowledge of MD

T Definition of an MD and the types of MDs

U What is an MD?

U Two types of MDs

U Playback-only MD is same

as a CD.

U Recordable MD uses

magnet optic recording.

U Playback of a Recordable

MD

U 74-minute recording time

U Can also be used on a

computer.

Y "MD" stands for "mini disc".
Y Similar to a music CD, an MD is also a small disc capable of recording and playing back digital sound.

Y There are 2 types of MDs, a optical disc for playback-only MD and a magnet optic disc for recordable MD that is

capable of both recording and playback.

Y A playback-only MD is merely a smaller-diameter version of a CD. Just like a CD, the signals are read by light

striking pits on the surface of the disc.

Y With a magnet optic disc MD(Recordable MD) that is capable of both recording and playback, recording is per-

formed by a vertical magnetization system in which a magnetic thin film on the surface of the disc is heated by a
laser beam, and magnetism is applied in accordance with the data (audio signal) being recorded.

Y When a recordable MD is played back, a laser beam weaker than that used during recording strikes the disc and

is reflected back, and the reflected light is twisted (polarized) in accordance with whether the magnetized direction
is upward or downward, causing the reflected light to rotate very slightly clockwise or counterclockwise. Those
subtle differences in the reflected light are picked up by two light-receiving elements and detected as either a "1" or
a "0" by reading whether there is electrical current or no electrical current.

Y If an MD were recorded in the same way as a CD, it would only have about 15 minutes of recording time. However,

by using a new signal compression technology called ATRAC that was specifically developed for MDs, the signals
are compressed to approximately one-fifth, making it possible to record for an extended time of 74 minutes, the
same as with a CD. (Blank MDs are currently marketed in two recording times, 74 minutes and 60 minutes.)

Y Although MDs were originally developed for use in recording and playing back music, in July 1993 the "MD data"

standard was established. By using an MD data+music player, MDs can be used as external memory storage
media for computers, and a single MD has a storage capacity of 140 MB, equivalent to about 100 floppy discs.

T Construction of an MD

U Construction of an MD Y The playback-only MD and the recordable MD are exactly identical in size and shape.

U Materials used in an MD Y The MD disc is made of polycarbonate, the same material that is used for a CD. Polycarbonate is a type of

Construction of

magnet optic disc

MD disc form

Y The figure below is a cross-sectional diagram of an MD disc. The diameter of an MD disc is 6.4 cm, approximately

half that of a CD, and the thickness is 1.2 mm, the same as a CD. Similar to a CD, only one side of an MD disc is
used to store data.

engineering plastic that is highly resistant to temperature and humidity, as well as having excellent wear and
impact resistant.

Y A clamping plate is mounted in the center of the MD disc, and when the MD is loaded into a player, a magnet in the

player attracts that metal plate to secure the disc in place. If the MD disc were to be secured by clamping it from
above and below similar to a CD, it would be necessary to have a hole pass through the center of the MD cartridge,
which would reduce the amount of space available for attaching a label. By using this magnetic method of securing
the disc, the entire front side of the MD cartridge can be used as a label area.

Y Because of the metal plate mounted at the center of the MD disc, the center of the cartridge is 2 mm thick, slightly

thicker than the rest of the cartridge.

Y To protect the MD disc from dust, fingerprints, and other things that might hinder the reading of the recorded

signals, the disc is stored inside a cartridge similar to that of a floppy disc. When the MD is loaded into a player, the
shutter on the cartridge is opened and the disc is ready to be played.

Y For a recordable MD, because there is no need for a recording head and it is only necessary for a laser beam to be

directed at the underside of the disc, the shutter is located only on the back of the cartridge.

Y For a playback/record magnet optic disc MD, because it is necessary for the recording head and the laser beam to

be able to access both sides of the disc, the shutter is located on both sides (upper shell and lower shell) of the
cartridge.

– 2 –

D

Recording on a magnet optic disc

SJ-MD100

U Can be recorded and played

back repeatedly.

U Recording principle

U Vertical magnetization

system

U Number of recordings

possible

Y By using a magnet optic disc, digital signals can be recorded and played back over and over again.

Y To record on a magnet optic disc, a laser beams momentarily heats "pin spots" on the magnetic film on the back of

the disc and a magnetic field is applied from the other side of the disc. Thus, both sides of the disc must be
accessed in order to record.

Y To explain the recording principle, we will assume that the directions of the magnetism on an unrecorded disc are

all facing downward (south-north = "0 0 0 0 ..."). (Actually on an unrecorded disc the directions are random.)

Y Thus, to record the signals "1 0 1 1 0 1 0", the direction of the magnetism at the locations where "1" is to be

recorded must be changed to face upward (north-south). Because the magnetic film is strongly magnetic, once a
downward-facing magnetism is recorded, it is not easy to change it to an upward-facing magnetism.

Y By directing a laser beam at the magnetic film, the temperature of the location that the laser beam strikes rises to

the Curie temperature (recordable MD; about 180pC), eliminating the magnetic force (retention force). (Because
the magnetic film is strongly magnetic, similar to a permanent magnet, once it is magnetized it has a strong
retention force. In order to eliminate that retention force, it is irradiated with a laser beam so that the temperature
rises to the Curie temperature.)

Y After the magnetism of the specific location is eliminated, an external magnetic field with an upward direction

(north-south) is applied, thus changing the direction of the magnetism at that location to face upward (north-south).

Y Conversely, if a downward-facing (south-north) external magnetic field is applied, the direction of the magnetism at

that location is changed to face downward (south-north).

Y Then, when the disc rotates and the location which has been changed to upward-facing magnetism leaves the

laser spot, the temperature of the magnetic film drops, and the upward-facing magnetism recorded at that location
is retained.

Y In this way, digital signals of "1" (upward-facing magnetism) and "0" (downward-facing magnetism) are recorded

on the tracks on the disc.

Y With a conventional magnetic recording tape, the magnetic material is magnetized parallel (horizontal) to the

surface of the tape. A magnet optic disc, however, uses a vertical magnetization system in which the magnetic
poles are recorded perpendicular (vertical) to the disc surface. Because the magnetism is recorded vertically
rather than horizontally, much more data can be recorded in a smaller area.

Y A magnet optic disc can be recorded more than 1 million times, so it can virtually last forever.

Magnet optic disc recording principle

A

Assuming that the directions of

the magnetism on the unrecorded

disc are all facing downward

(they may also all face upward).

D

Recorded signals.

Directions of magnetism differ

according to whether a "1" or

a "0" digital signal is recorded.

Surface of the disk is a magnetic thin film made of terbium-cobalt alloy.

Disk rotation direction

wwwwwwwwwwwwwww

S

Disk rotation direction

wwwqwtwwqqqwwq

00010100111001

Upword : "1"
Downword : "0"

BA

i

CD

Laser spot

Disk rotation

direction

wwwwwwwwwwwwwww

N

Laser spot

Disk rotation

i

direction

wwwwwt wwwwwww

N

External magnetic field is applied.

S

Heated

Vertical magnetizationHorizontal magnetization

i

B

Only locations struck by a

powerful laser beam

(reach the Curie temperature)

lose their magnetism

retention force.

C

An external magnetic
field is applied to the

demagnetized location,

creating upword-facing

magnetism.

– 3 –

SJ-MD100

D

Playback of a recordable MD

U Playback of a recordable

MD

U Reading of magnetic signals

Y Because the signals on a magnet optic disc are recorded vertically as north-south and south-north magnetism, the

north and south magnetic poles appear on the surface of the disc's magnetic film. These signals are played back
utilizing a phenomenon called the "Kerr effect" which occurs when a weak laser beam strikes the magnetic poles.

Y Light has wave vibration directions called "planes of polarization". ("Polarization" refers to a light wave which

vibrates only in a fixed direction.)

Y With normal light, because the wave vibration directions are all mixed, no planes of polarization appear.
Y Because a laser beam is artificially generated light, it is possible to align the planes of polarization.
Y When a laser beam strikes something that has a magnetic field, the direction of the plane of polarization of the

reflected light varies very slightly in accordance with whether the magnetism is north polarity or south polarity.
When playing back a magnet optic disc MD, these slight changes in the direction of the plane of polarization are
read.

Y To further explain the principle used to read the signals recorded on a magnet optic disc, first a laser beam is

directed at the disc. If the direction of the magnetism recorded on the disc is upward (north polarity), the plane of
polarization of the light reflected from the disc rotates very slightly clockwise as a result of the Kerr effect. Conversely,
if the direction of the magnetism is downward (south polarity), the plane of polarization rotates very slightly
counterclockwise.

Y When the reflected laser light is passed through a Wollaston prism, the light is distributed to photo detector 1 if the

direction of rotation is clockwise or to photo detector 2 if the direction of rotation is counterclockwise.

Y The light striking the two light receiving elements is converted into electrical current and a subtraction is performed.

If the result of A-B is plus, a "1" is detected, and if the result of A-B is minus, a "0" is detected.

Y An MD player is compatible with both optical recording and magnetic recording, changing the reading system in

accordance with the type of disc that is loaded.

Changes in the polarization axis
due to the Kerr effect

Playback of recordable MD

– 4 –

D

Rewriting action of a magnet optic disc

SJ-MD100

U MD rewriting process

U No need for a erasing head

Y The signals recorded on an MD are rewritten using a new process called “magnetic field modulation overwriting”*.
Y In this process, a laser beam spot of about 5 mW is focused on the location on the disc to be rewritten, heating that

location to the Curie temperature (180pC) and thus canceling the magnetization.

Y At the same time, current flows to the optical pickup and to the magnetic head opposite it, between the two of which

the disc is held, thus generating a magnetic field.

Y When the disc revolves so the laser spot moves from the location to be rewritten, the temperature drops below the

Curie temperature and the magnetic field generated by the magnetic head re-magnetizes that location.

Y At this time, if the direction of the current flowing to the magnetic head is reversed in accordance with whether the

data being recorded is “1” or “0”, the direction of the magnetic field also changes between north and south, and
accordingly, the direction of the magnetization of the recording film changes between upward-facing and downward­facing. Thus, it is possible to directly magnetize the recording film on the disc in accordance with the “0” and “1”
digital signals.

Y Thus, the new recording data is overwritten regardless of the direction of the previously recorded magnetization,

eliminating the need for an erasing head.

Y This process is called “magnetic field modulation overwriting”.
Y Because this “magnetic field modulation overwriting” makes it possible to directly overwrite the new signals on top

of the old signals in a single process, re-recording on a MD is just as easy as with a magnetic tape, making the MD
ideally suited for use in personal audio equipment.

Magnetic field modulation overwriting

*Overwrite means to write new data while erasing the old data.

– 5 –