Philips CD304 MK II - Faultfinding Method

|COMPACT
DIGITAL AUDIO
FAULTFINDING METHOD
Preface
In course of the development of the troubleshooting guide
for the Compact Disc it has become clear that a different
approach from the one applied so far was required.
For, it isno longer possible to use the classic strategy, i.e.
basing the troubleshooting method on a number of
possible faults in the unit.
Practice has shown that a certain fault, with the associated
symptom, can have a wide variety of causes.
The reason is that this player incorporates a number of
feedback loop configurations — which, moreover, might
affect each other — and this impedes the obvious
measurements.
The method below divides the player from diagram point
of view into nine clearly distinguishable sub-groups and
by performing some measurements, the sub-group being
in failure can be isolated. Later the defective circuit can be
further examined according to the method given.
PRACTICAL HINTS
Test discs
It is important to handle the test discs with great care.
For, the troubles (black dots, fingerprints, etc.) are exclu-
sively and unambiguously positioned.
Damage can cause additional drop-outs etc. and as a
result the conscious fault on this disc is no longer
exclusive.
In that case it is no longer possible to check e.g. whether
the track detector is working correctly.
Measurements on op-amps
In the electronic circuits of the servo systems op-amps are
frequently being applied. These op-amps can be used as
amplifiers, as filters, as invertors, as buffers, etc.
In those cases where feedback is applied in one way or the
other, the voltage difference at the differential inputs
inclines to zero. This applies botr. to DC and to AC.
The cause can be traced back to the properties of an ideal
Op-amp (Z,; = ©, G= «©; Z, =0).
In practice this means that it is nearly impossible to per-
form measurements on the inverting and non-inverting
inputs of op-amps if one input is directly connected to
ground.
In those cases only the output signal will be measurable.
That is why in most cases no AC voltages can be given to
the inputs.
The DC voltages at the inputs are equal.
Simulating with ,,0” and ,,1”
in the troubleshooting method certain pins should ina
number of cases be connected to ground or be connected
to the power supply voltage.
This way of acting offers the possibility to overrule certain
Circuits and to stimulate others.
In this way the diagnose time can be reduced.
In a number of cases the relevant pins appear to be op-
amp outputs.
In this respect it should be mentioned that the outputs of
the used op-amps are short-circuit protected.
This implies that the output of an op-amp can be made low
(= usually ground potential) without consequences.
On the other hand should be pointed out that it is not
allowed to connect the output of an op-amp directly to the
power sunpolv voltage.
7-1
1/0s of microprocessors should not be connected directly
to power supply voltage.
These I/Os are allowed to be brought to ,,0” in case this is
mentioned explicitly.
Selection of ground point
It is very important to select a ground point as close as
possible to the test point.
Conditions for injecting
— It should be pointed out that injection of levels or
signals from a strange source is never allowed to
occur when the power supply voltage is lacking in the
circuit in question.
— Naturally, the injected level is never allowed to exceed
the power supply voltage of the circuit in question.
Continuous burning of the laser
— Disconnect plug 24 on the servo PCB and connect
pin 24-3 (laser) of the cable connector to ground.
Now the focus loop and the radial loop are interrupted
as well:
24-1 (RE1 = Radial Error 1), 24-2 (RE2 = Radial
Error 2) and 24-4 (FE = Focus Error).
The taser also burns continuously when the set is in
service loop A.
Irregular working of the display
Irregular working of the display when the set is opened
and playing, might have been caused by incidental body
effect in the region of the crystal oscillators.
Switching ,,off” and ,,on” of the mains voltage will elimi-
nate this effect.
Indication of checkpoint
In the circuit diagram and PCB diagrams the checkpoints
have been given a serial number (e.g. ), to which the
troubleshooting method will refer.
For oscillograms, amplitudes, time bases and position of
set, see tables of checkpoints.
GENERAL CHECKPOINTS
In the detailed troubleshooting method following
below a number of general conditions, required for
proper functioning of the player, will not be repeated.
Before starting the detailed troubleshooting method these
general points should be checked.
a. Ensure that disc and objective are clean (remove dust,
fingerprints, etc.) and use undamaged discs.
b. Convince yourself of the presence of the clock frequen-
cies, viz.:
. 6 Mrz for uP servo (pin 16)
° 6 MHz for control and display uP (pin 15) _
c. Check whether all power supply voltages are present
and have the correct level.
See PCB drawings.
d. KILL CIRCUIT
ev
0.85
-8V
1
!
1
POWER ON POWER ofF = MDA.00134
728
- During switching on and off the mains voltage the sig-
nal on the collector of 6327 (to be measured on a jum-
per, tp93) should be as indicated in the figure below.
CS 5 2191GB