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DICITAL Al/DIQ
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, Z
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
"O"
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 "0" 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 "0" or
ground if this is stated 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 levels or signals from an external source
should never take place if the related circuit has no
supply voltage.
The injected levels or signals should never be greater
than the supply voltage of the related circuit.
cs
9 984
4-1
GENERAL CHECKPOINTS
In the detailed measuring method below a number of
general conditions, 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. ~ ) to
which the measuring method refers. In the measuring
method below, the symbol
<◊l
has been omitted for the
testpoints indicated.
Initiation of the servicing programme of the
µP
- Servicing position "0"
Simultaneously depress the PREVIOUS, NEXT and
TIME/TRACK buttons. Keep these three buttons
depressed while the mains voltage is switched on.
This is the STAND-BY mode, "O" appears on the display.
(Tray switch SK2 must be activated)
In this state as in service position "1", "2" and "3", it is
possible to move the arm by means of the SEARCH
FORW and SEARCH REV keys with a minimum torque
to the outside and the inside resp.
This enables a check to the free motion of the arm
across the disc.
After depressing one of those keys (SEARCH FORW
or SEARCH REV) the laser will emit light.
- Servicing position
"1"
From servicing position "0" the player can be brought
in servicing position 1" by depressing the NEXT key.
In this state the laser emits light and the objective
starts to focus. When the focal point has been
reached, "1" appears on the display.
When no disc has been inserted the objective goes
16x to and fro. Then the player reassumes servicing
position "0".
- Servicing position "2"
To be reached by depressing the NEXT key after
servicing position "1" has been reached.
The tumtable motor starts to run
On the display appears "2".
When in service position "1" the arm is moved or the disk
is moved roughly (disturbance of focus), the motor will
start running at maximum speed in position -,,2".
By pressing the NEXT key, the player can be brought
back, via position "3", into position "0".
- Servicing position "3"
To be reached by depressing the NEXT key after
servicing position ·2· has been reached.
The radial control is switched on. The subcode
information is ignored. MUSB is high so that the music
information Is released.
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