Samsung CX528ZSE Service Manual page 23

(3) Operations of the PIF section.
LLD TANK AFT COIL
VIF Bt ~— VIDEO OUT AFT OUT
ie IF AGC = VIDEO AMP/ faa
B/W NOISE CAN. VIDEO DET
NOISE C.
RF AGC RF AGC
VR OUT
Pin
FILTER *
VIF AMP. NEGATIVE FEEDBACK
Fig 7. PIF block diagram of LA7520/KA2919
A. PIF amplifier :
Incoming IF signal is amplified by three IF amplifiers within IC101, all three of these amplifiers have their
gain controlled by the AGC circuit within. IC101. The input sensitivity of 1C101 is 36dB typical. The output
IF signal from the last IF amplifier is connected to the video low level detector (LLD) stage internally.
B. Video detector :
The picture carrier component is derived from the PIF signal, limited and phase shifted by a LC tank
circuit (1171, R104) connected to pin 19 and pin 20 of IC101 to produce a switching carrier for the
low level detector. This reference signal is 38.9MHz in case of B, G system and 1171 is turned to this
signal. The SIF signal is detected at the same time (intercarrier detection). Detected composite video
signal is applied to video output stage, AGC circuit and the noise inverter circuit within IC101 through
a part of the video amplifier.
C. Video amplifier :
The video amplifier consists of a buffer amplifier and an emitter follower output amplifier. Pin 22 of
IC101 is the output terminal for the composite video signal (=video signal+blanking signal+sync sig—
nal). The composite output signal from Pin 22 has 2Vp.p amplitude and sync—negative. R201 is a
bias resistor. This output is applied to the video inverter amplifier (Pin 39) of IC501 through L201
and Z201 (sound trap), and the DC component of this output signal can be a bias source of the
sequential amplifier. The peaking level can be defined by the resistance of R202.
D. Noise inverter :
The: composite video signal is also applied to the AGC circuit and the black and white noise inverter
circuit internally.
If the composite video signal includes black pulse noises which exceeds 2.5V to the minus side, they
may cause mis—operation of the AGC, unstability of the synchronization or degradation of the S/N ratio.
Also white pulse noises cause degradation of the S/N ratio.
The LA7520 provides noise inverter circuits for both noises to keep a stable operation. A noise which
exceeds the threshold level is inverted to a gray level (clamp level).
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