Philips CD304 MK II - Laser Supply Troubleshooting

7-8
@ In position SEARCH the ATSB signal is “low” test
point 89; pin 22 (Attenuation Audio Signal)
— When the "uP panel” is applied, (a sub-printed circuit
board) that houses IC6451: MAB8441P/T012, test
point 89 is not connected.
IV DAC IC (Dual Digital Analog Converter)
® Check the signals between Filter-B IC and DAC IC
- See sub. "Ill Filter-B IC”:
* Check the timing signals between Filter-B IC and
DAC IC.
@ Check the output of the OP-AMP after the DAC IC
- Place a disc on the turntable.
~- In position PLAY or in SERVICE POSITION B, the ana-
log (music) signal should be present at the output of
the OP-AMP, after the lead-in track has been read.
V DEEM CIRCUIT
@® Check DEEM circuit
~ Place test disc 5 on the turntable.
- During playback of track no. 14 (recorded without
PRE-EMPHASIS) the DEEM signal at test point 84
should be "low’.
- During playback of track no. 15 (recorded with PRE-
EMPHASIS), the DEEM signal at test point 84 should
be “high”.
- During playback of track no. 14 the analogue signal
should be present at the source of 6514 (test point 91)
and 6515 (test point 92).
- During playback of track no. 15 the analog signal at
the source of 6514 (test point 91) and 6515 (test point
92) should be 0 V.
Annex I: LASER !S GIVING NO OR INSUFFICIENT
LIGHT
Together with the laser supply and the monitor diode the
laser forms a feedback system.
A defect in the laser supply might thus result in destruc-
tion of the laser. Replacement of the laser (= new light pin)
will not solve anything. The new laser will also be destro-
yed since the original fault in the laser supply is still pre-
sent.
On the other hand it is impossible to check and repair a
feedback system when one link is missing.
For this reason the so-called laser simulator 3 is supplied.
Code number 4822 395 30229.
This laser simulator consists of a PCB which contains the
laser and monitor simulation, a switch to test the On/Off
position and a number of sockets.
This PCB can be connected to the laser supply instead of
the light pin so that the feedback system is closed.
Repair procedure:
Since the light pin is very sensitive to static charges, care
should be taken that during measurements and adjust-
ments of the laser power supply the potentials of the aids
and yourself equal the potential of the CD mechanism.
Detach light pin and connect laser simulator as fol-
lows: (Connections on pre-amp PCB).
Take the flex PCB out of socket 11 and connect the simu-
lator PCB with the socket.
Remove plug 16 and insert it in the socket on the simula-
tor PCB.
Connect the plug with 4 wires to socket 16. Take out plug
17 and insert the plug with 1 wire in socket 17.
CS 5 226GB
—
Switch
on
the
mains
switch
and
ensure
that
the
drawer
is
closed
or else
that
the
tray-end-in
switch
on
the
tray
PCB
(Sk2)
is
depressed.
Now
press
the
play
key
and
check
if
the
L-line
of
the
servo
uP,
pin
17-2
on
the
pre-amplifier
PCB,
goes
‘low’.
—
In
rest
position
the
current
through
the
laser
diode
should
be
<
1:mA.
For
NEG.
VOLT.
lasers
this
can
be
checked
as
follows:
Set the switch on the simulator PCB in the OFF
position and the mains switch in the ON postion.
Turn
trimming
resistor
3180
counterclockwise
(min.
R)
and measure the voltage across resistor 3194 on the
pre-amp. PCB.
On pre-amplifier PCBs with discrete components turn
resistor
3180
clockwise
(min.
R)
and
measure
the
voltage
across resistor 3194.
The voltage should be < 15 V.
Check of laser supply control
Set the
switch
on
the
simulator
PCB
in
the
ON
position
and
measure
the
voltages
between
points
+V
and
—V
on
the
simulator
PCB.
Resistor
3180
clockwise
(max.
R):
U+y-v=225
mV+45
mV.
On
preamplifier
PCBs
with
discrete
components
resistor
3180
counterclockwise
(max.
R):
U+v-=225
mV+45
mV.
R3180
counterclockwise
(min.
R):
U+v-v=750
mV
+150
mV.
On
preamplifier
PCBs
with
discrete
components
resistor
3180
clockwise
(min.
R):
U+v-v=750
mV+150
mV
Set resistor 3180 in the mid-position.
This
is
a
preliminary
adjustment.
After
the
simulator
PCB
has
been
removed
the
laser
current
must
be
adjusted.
Fine adjustment of laser current
— Playback track 1 of test disc 4822 397 30096 (Disc
without defects). Connect a DC voltmeter across resistor
3308 on the SERVO PCB circuit diagram. Adjust the
laser power supply with resistor 3180 until the voltage
across resistor 3308 is 575 mV+50 mV.
Remark:
It is recommended to use the laser simulator for any mea-
surement in the laser supply, since incidental short-
circuits with the test pin can have inconvenient
consequences for the laser.
Annex Il: CHECKING LOCKING- IN OF PLL CIRCUIT
(Test points on decoding PCB)
First the free-running oscillator should be checked as
follows:
— Switch power on, player in pos. play.
— Connect a frequency counter between pin 22 of |C6501
(DEMOD) and _L.
— This frequency should be 4.350 MHz+30 kHz.
Attention
This adjustment should be performed immediately after
the unit is switched on.
Checking locking-in:
— Insert disc, disconnect plug 14, inject 2,5 Vg, to the
socket of plug 14 on pre-amp PCB and set player in
service loop B.
— Variation of the DC round 2,5 V should be visible on the
oscilloscope (point >) in the form of a frequency
variation. This means that the PLL is locking in.