Samsung MM-28 Service Manual page 18

Special Circuit Descriptions
5-1-7 Defect Circuit
The RFO signal bottom, after being inverted, is held with two time constant of long and short.
The short time-constant bottom hold is done for a disc mirror defect more than 0.1msec, the long
time-constant bottom hold is done with the mirror level prior to the defect. By differentiating this with a
capacitor coupling and shifting the level, both signals are compared to generate the mirror defect detection
signal.
RFO
5-1-8 APC Circuit
As the laser diode has large negative temperature characteristic in its optical output when driven with a
constant current on laser diode. Therefore, the output on processing monitor photo diode, must be a
controlled current for getting regularly output power, thus the APC (Auto Power Control) circuit is composed.
5-4
75K
37.5K
75
-
28K
+
75K
BOTTOM
VC
HOLD
2
DCB
PD 69
43.5K
150K
5.15K
DCC1 DCC2
5 4
BOTTOM
HOLD
-
DFCT
43K
+
PN
From Micom
150K
0.75K
150K
300K
LDON
SSTOP/DFCT
41
70
LD
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