Teac CTM6845, Service Manual

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一 . Introduction to IC’s
1. TDA8842 (OM8838) multi-system small signal processing IC
TDA8842 (OM8838) is the new type Video/Chroma/Deflection (V/C/D) processing chip
developed by the Phillips in 1998, i.e., the Video/Chroma/Deflection small signal processing
IC. By fully adopting the I
2
C bus control technique and integrating a number of unique
circuits into the chip, it has greatly simplified the elements external to the IC and achieved
overall improvement in the picture and sound quality of the popular model color TV sets.
TDA8842 (OM8838) was developed on the basis of its TDA8362/8361 single-chip IC,
therefore it has retained the main functional circuits in the ordinary single-chip IC
TDA8362/836 1, and it mainly includes: phase-locked loop (PLL) synchronized detection and
picture intermediate frequency (PIF) signal processing module; the second sound intermediate
frequency (SIF2) signal processing module centered with PLL frequency discrimination; the
commissioning free scanning deflection indirect triggering pulse forming module with the
dual PLL as the core; the brightness signal processing module with a number of measures to
improve picture quality; the chroma signal decoding module suiting the selection of a number
of systems; and the matrix circuit module satisfying the conversion and transfer of
multi-channel basic color levels.
TDA8842 (OM8838) series IC’s have introduced the I
2
C bus “Read/Write” technique,
the frequency synthesis technique (FS), charge coupling switch capacitance network
technique (CCD), and digital signal processing technique (DSP). Also inside the chip, circuits
such as bus controlled matrix switch, 1H delay horizontal and comb filter are integrated. All
functions traditionally accomplished by cooperation with external circuits can all be
performed inside the chip, thus greatly increasing the signal processing ability of the chip,
realizing the self-adapting adjustment for the control objects. Especially, the “Read” technique
of I
2
C bus is used, so that the TDA8842 (OM8838) series IC’s have advanced functions such
as automatic fault diagnosis. The internal block diagram of TDA8842 (OM8838) series IC is
as shown in Fig. 1.
(I) It mainly consists of the following function sections:
1. Video intermediate frequency (VIF) unit circuit
This circuit consists of the video intermediate amplification, adjustment free VCO, PLL
demodulator, video buffer, automatic frequency control circuit AFC, automatic gain control
circuit AGC and video signal identification circuit, as detailed in Fig. 2.
2.
Sound intermediate amplification unit circuit
The sound intermediate amplifier consists of the PLL demodulator, preamplifier and
muting circuit, volume control circuit and audio switch, as detailed in Fig. 3.
3.
Horizontal and vertical synchronization unit circuit
The horizontal and vertical synchronization unit circuit consists of the horizontal
synchronization separator, vertical synchronization separator, horizontal oscillation circuit and
correction system, PHI-1 and PHI-2 detectors, sandcastle pulse generator, the horizontal
output circuit with slow start and slow stop devices, noise detector and vertical frequency
divider, as detailed in Fig. 4.
4.
Horizontal and vertical geometric correction and deflection driving unit circuit