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TROUBLESHOOTING METHOD
When setting up the error-finding method for Compact Disc, it
turned out that a different approach than the usual approach
was necessary.
It is no longer possible to assume the method in which a
number of possible faults in the device form the starting point
for the fault finding method.
A certain error with an associated symptom can have a large
number of causes. The reason for this is that a number of
closed-loop circuits occur in the Compact Disc, which can also
influence each other, making obvious measurements
impossible.
In the following method, the device is schematically divided into
nine clearly recognizable subgroups. The defective subgroup
can be located more clearly by a few adjustments. After this,
the circuit can be metered according to the indicated method.
HINTS
Test CDs
It is important that the test CDs are handled with great care.
The distortions on the CD (black splashes, fingerprints, etc.) are
exclusive and are unambiguously positioned.
Damage can cause extra drop-outs, etc., making the wanted
error on the CD just that little bit more exclusive.
Testing the proper functioning of the track detector is then no
longer possible.
Measurements using op-amps
Op-amps are frequently used in the servo circuits.
These can be used as amplifiers, fillers, inverters and buffers.
In those cases where feedback looping has been applied in
some way, the voltage difference at the differential inputs
converges to zero. This applies to both DC and AC signals.
The cause of this can be traced back to the properties of an
= ꝏ G = ꝏ Z
ideal op-amp (Z
= 0). When an input of an op-
i
0
amp is connected directly to ground, it is virtually impossible to
measure the inverting and non-inverting inputs. In such a case
only the output signal is measurable.
Therefore, in most cases the AC voltage at the inputs will not be
given. The DC voltages at the inputs are equal to each other.
Simulate with "O" and "1"
During troubleshooting, certain points must sometimes be
connected to ground or to the supply voltage. As a result,
certain circuits can be brought into a desired state, which
shortens the diagnosis time. In some cases, the points in
question are op-amp outputs. These outputs are short-circuit
proof. i.e., they may be brought to "0" or ground with impunity.
However, the output of an op-amp should never be
connected directly to the supply voltage.
Measurements of microprocessors
Microprocessor inputs and outputs must not be connected
directly to the power supply ring. The inputs and outputs may
only be set to 0 or ground when this is explicitly stated.
Selection of the ground potential
It is very important to choose a ground point as close as
possible to the test point.
Conditions for Injection
• Injection of levels or signals from an external source should
never be done if the circuit in question has no supply voltage.
• The projected levels or signalers may never exceed the
supply voltage of the relevant circuit.
Short burning of the laser
After removing plug A17 and bridging the lid switch, the laser
will continue to burn when the mains voltage is switched on.
The focus loop and the radial loop are then also interrupted:
at points A171 (FE = Focus Error), A174 (RE1 = Radial Error 1)
and A173 (RE2 = Radial Error 2).
When the unit is in service loop A, the laser will burn
indefinitely, even if there is no CD on the turntable.
Irregular operation of the display
Erratic display behavior when the device is open and running
may be caused by hand effect near the crystal oscillators.
Switching the reset switch off and on cancels this effect.
Adherence of the test points
In the drawings of the schematics and the printed circuit boards,
the test points are indicated with a number (e.g. <12> to which
the fault finding method refers.
For oscillograms, amplitudes, time bases and position of the
device see the list of test points.
GENERAL CHECK POINTS
In the following detailed troubleshooting method, a number of
general conditions, which are necessary for a well-functioning
device, will not be mentioned.
Before starting the detailed troubleshooting method, these
general points should be checked first.
a. Make sure that the lid is closed or the tilt switch is bridged
during measurement.
b. Make sure that the CD and objective are clean (dissolved
dust, fingerprints, etc.) and work with undamaged CDs.
c. Check the presence of the necessary clock frequencies:
• 4.433619 MHz for decoding µP
• 6 MHz for servo µP
• 4.233600 MHz for CIM-IC
• 4.35 MHz for free running PLL circuits on DEMOD IC.
d. Check whether all supply voltages are present and have the
correct value.
e. Check that the two "mutes" (KILL and NOT(SMSE)) are
inactive so that the information flow is never interrupted.
f.
Check the proper functioning of both microprocessors by
means of their built-in test program and any peripheral test
program.
Method:
Self-test decode µP 6506
• Take the servo µP 6201 out of its socket.
• From decoder µP 6506 connect the points 18 and 21 with 14.
• When switching on the mains voltage, connect the points 6
and 14 together.
• If the µP works properly, point 22 within 1 sec. go from "1" to
"0".
Self-test servo µP 6201
• Take the decode µP out of its socket.
• Connect points 18 and 21 with 14 of servo µP 6201.
• When switching on the mains voltage, connect points 6 and 14
together.
• If the µP is functioning properly, measure point 22 within 1 sec.
to go from "1" to "0".
Peripheral test servo µP 6201
1. Place a CD on the turntable and switch off the mains power.
Hold down the stop key while the mains voltage is switched
on.
Release the stop button after 1 sec.
The device is now in the so-called service loop A. In this
mode, the laser and the focus control are working and the
motor is running. The light pen remains against the inner
stop (i.e. the light pen remains continuously below the run-in
tracks).
The radial servo system is disabled.
In this service loop, all LEDs and operating keys can be
checked as follows:
• All program LEDs must light up and can go out one by
one in a rhythm of 1Hz.
When LED no. 15 is off, the process repeats. In the
trackbar, only the LED that corresponds to the lowest
program LED at that moment lights up.
• When any of the keys, pause, select, store, cancel,
repeat or reverse is pressed, the "pause" LED and
"repeat" LED will cycle from on to off or vice versa.
The "error" LED will also light up. It goes out again when
a track LED lights up.
2. The player can be moved from service loop A to service
loop B by pressing the FWD key until a whistling sound is
heard.
Now, independent of the state of the P bit and the subcode
(via the bus), the radial servo system is switched on.
The display remains in the service loop.
3. The player can be returned to normal operating mode from
service loop A or B by pressing the PLAY key.
4. Eye pattern.
Check with an oscilloscope the RF signal (called "eye
pattern") on the output of the preamplifier (measurement
point <65>). Set the time base to 0.5 µsec.
The oscilloscope should show a fairly stable signal when the
PLL circuit is captured and the turntable motor's servo circuit
is properly regulated. A shaky or jittery picture can be
caused by a bad motor or because the device is in service
loop A
DETAILED TROUBLESHOOTING METHOD
A number of quick and effective checks provide an immediate
answer to malfunctioning parts of the device. Two service loops
(A and B) are built into µP 6201 for short-rolling the servo
systems. Before placing the device in service loop, A or B, it
must be checked whether the bus (clock, data or connection
points 3 and 2 of µP 6201) is free of ground or supply voltage
(level "low" or "high").
If the lines are free of ground or supply voltage, then all keys
must be operable when the mains voltage is switched on.
For troubleshooting, the step-by-step method given below
must be followed.
First step (with a CD on the turntable).
Put the player in service loop A (method: Press and hold
the stop button while switching on the mains voltage). In
this mode, the laser, focus control, and turntable motor
control should operate. The light pen must rest against
the inner side (= under the run-in tracks).
If one of the above conditions does not occur, the following
questions must be answered positively in the order given.
In practice, this means that if a certain question is answered
positively, this means that all previous circuits to which the
questions refer are working properly.
Example: if the "eye pattern" is present then it can be
concluded that the laser is working, the laser is in focus and
the turntable motor is working
Remark:
In some circumstances, errors in the radial servo system can
affect the focus servo system.
(e.g. When supply voltage +1 for IC 6214 fails in the radial
circuit, the focus coil starts to oscillate).
In order to be able to determine whether this situation occurs,
measuring point <36> (FS) must be grounded. In this way, the
influence of the radial servo system on the focus servo system
is eliminated.
A. Does the laser light up? (Measuring method: see Sub A)
B. Is the plate light pen angle within tolerance, i.e. equal to
90° ± 0.5° ?
(Measuring method: see chapter 6.)
C. Does the laser provide enough light?
(Measuring method: see Sub C).
D. Does the lens come into focus?
(Measuring method: see Sub D),
E. Is turntable motor running and if so, is it running at the
correct speed?
(Measuring method: see Sub E).
If the answers from A to E are positive, the device should be
able to be brought into service loop A.
Second step (with a CD on the turntable)
Bring the player into service loop B. (Method: Bring the
device into service loop A by pressing the stop button
and the power switch simultaneously. Then press and
hold the FWD button until a whistle is heard.)
Now the radial servo system is switched on but the servo
µP 6201 ignores the information on the P-line (P-bit) or
bus (clock and information for the subcode). This means
that the light pen DOES NOT SKIP to the beginning of the
first track, so it will take some time before music is heard.
(This depends on the length of the lead-in track). By
placing the light pen under the music track by hand,
music is immediately audible.
In this position, the eye pattern at the measuring point <65>
should be stable, while the MCES signal at measuring point
<17> should also be stable.
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