Philips MC-500 Service Manual page 47

Service Hints
4. MEASURE THE LASER CURRENT
CD DRIVE – LASER CURRENT MEASUREMENT
The laser current can be measured as a voltage drop on
resistor 3820. Typical value 170 - 190mV for CD-DA
respectively 200 - 220mV for CD-RW.
Laser power control
+5V_HF
3,9V
3,3V
7879
BC807-40
2V
LASER DIODE
5. MEASURE THE OFFSETS OF THE CD-DRIVE
CD DRIVE – OFFSET MEASUREMENT
Each photodiode of the CD-drive may have an offset.
This offset has to be compensated by the signal
processor. A high offset of the CD-drive leads to poor
playability of some CDs (skipping tracks).
Start the Service Test Program - section „Focus Test"
without a CD. Focus sensitivity = CD-RW.
Use a DC Millivoltmeter for measurement. The offsets
can be measured direct on the connector. See drawing
below.
The values from diode A-D should read 0±10mV.
Diodes E and F are less critical.
CD Drive
Sanyo DA12T3
1800
VREF
VCC
E
E
D
B
A
A
C
D
B
F
C
F
GND
If one of the offsets is higher than ±10mV the CD drive
has to be replaced.
U >250mV
->Laser damaged !
3820
+5V
4R7
4,6V
7811-A
3 0,17V
8
LM358D
3823
1
3V
1K
2
4
16
VrefCD10
15
+5V_HF
14
E
E
13
D
D
12
A
A
11
B
B
10
C
C
9 F
F
8
10-3
6. MEASURE THE OFFSETS OF THE CD10
SIGNAL PROCESSOR – OFFSET MEASUREMENT
Each ADC input of the CD10 may have an offset too.
Also this offset leads (together with the offsets of the CD
Drive) to poor playability of some CDs (skipping tracks).
Start the Service Test Program - section „Focus Test"
using a CD-RW disc.
Use a DC Millivoltmeter for measurement. The offsets
can be measured on capacitors near the signal
processor. See drawing below.
The value should read 0mV±10mV.
CD Board side A view
0,17V
V
REF
3
If one of the offsets is higher than ±10mV the signal
processor has to be replaced.
If none of the measured offsets is higher than ±10mV -
replace the part with the higher value.
Signal Processor
B A C D F E
4 5