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C
4. ELECTRICAL MEASUREMENTS AND
ADJUSTMENTS
HINTS
Test disc
It is important to treat the test discs with great care. The
disorders on the discs (black spots, fingerprints, etc.) are
exclusive and unambiguously positioned. Damage may
cause additional drop-outs, etc. rendering the intentional
errors no longer exclusive. In that case it will no longer
be possible to check e.g. the good working of the track
detectors.
Measurements on op-amps
In the electronic circuit op-amps have been used
frequently. Some of the applications are amplifiers,
filters, inverters and buffers.
In those cases where in one way or the other, feedback
has been applied the voltage difference at the differential
inputs converges to zero. This applies to both DC and
AC signals. The cause can be traced to the properties of
an ideal op-amp (Z
1
=
oo, G
=
oo, 2
0
=
0). If one input of
an op-amp is directly connected to ground it will be
virtually impossible to measure at the inverting and the
non-inverting inputs. In such cases only the output signal
will be measurable.
That is why in most cases the AC voltages at the inputs
will not be given. The DC voltages at the inputs are equal.
Stimulation with
"0"
and
"1"
During troubleshooting sometimes certain points should
be connected to ground or supply voltage. As a result
certain circuit can be brought in a desired state thus
shortening the diagnosis time. In a number of cases the
related points are outputs of op-amps. These outputs are
short-circuit-resistant, i.e. they can be brought to
"O"
or
ground without problems.
The output of an op-amp, however, should never
be
connected directly to the power supply voltage.
Measurements on microprocessors
Inputs and outputs of microprocessors should never be
connected directly to the power supply voltage. The
inputs and outputs should only be brought to
"O"
or
ground if this is staed explicity.
Measurements with an oscilloscope
During measurements with an oscilloscope it is
recommended to measure with a 1: 10 test probe, since a
1 :10 probe has a considerably smaller input capacitance
than a 1 :1 probe.
Selection of ground potential
It is very important to select a ground point that is as
close as possible to the test point.
Conditions for injection
- Injection .of le\lels 0r si_gnals from an extemal source
should never take place
'if
ihe related circuit has no
supply voltage.
T<he ,injected ,levels or signals ·Shol!lld never be greater
than the supply voltage of the ·r.elated circuit.
4-1
GENERAL CHECKPOINTS
In the detalled measuring method below a number of
general conditioAs, required for a properly functioning
set, will not be mentioned. Before the detailed measuring
method is started, these general points should first be
checked.
a. Ensure that disc and objective are clean (remove dust,
fingerprints, etc.) and work with undamaged discs.
b. Check if all supply voltages are present and
if
they
have the correct values.
c. Check the good working of the CD mechanism by
means of the servicing programme.
Indication of test points
In the drawings of the diagrams and the panel~e test
points have been indicated by a number (e.g.
'(.3/')
to
which the measuring method refers. In the measuring
method below, the symbol
(◊J
has been omitted for the
testpoints indicated.
Initiation of the servicing programme of the µP
- Service position "0"
Press the STOP/CM, NEXT and PAUSE keys. Keep
these three keys depressed while switching on the
mains voltage. This is the 1
st
standby position. The
number of the internal ROM program of the µP (e.g.
P101) appears on the display. The "TRACK" Led lights
up as well. In this position the radial arm of the COM
can be driven Inwards (with SEARCH
> >)
and
outwards (with
< <
SEARCH) with minimum torque.
When one of these search keys is actuated, laser
control and focus control become active as well. If a
disc is put on the turntable, the working of the focus
control can already be observed in this position. (It the
arm moves outwards and inwards, you can hear the
optical pick-up unit jump across the tracks).
Attention: After one of these keys (SEARCH
> >,
<<
SEARCH) is pressed, the laser remains controlled
up to service position 3. Avoid, therefore, direct
exposure to the laser beam in service positions 0, 1
and 2).
-
Service position "1"
From service position
"O"
the player can be brought
into service position "1" by pressing the "NEXr key.
The indication "0001" appears on the display. In this
position the focus start procedure is repeated over
and over again. The laser control is active.
However, this start procedure does not release the
focus ,control. So we cannot ascertain that the focal
point is found. This position is very suitable, however,
tor measurements of the focus circuit.
In service positions 1 and 2 the arm with the rafoc
unit can be moved inwards and outwards with the
SEARCH
> >
key and the
< <
SEARCH key
respectively. In this way the free movement of the
arm under the disc can be checked.
CS 11 949 GB
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