Service manual
CA-12
CA CIRCUIT DESCRIPTIONS
switching channel of this IC switches the PiP between external and internal video sources based on the PiP
Switch1 signal (pin 4, IC2902) from the microprocessor. The internal video signal, denoted TVIN, is the
same signal as the one entering pin 37 of ICX2200. Whichever signal is selected is sent as VIN PiP to the
PiP module through pin 10 of the PIPA1 connector. The second switching channel of IC2902 allows
selection between composite video input and S-Video input. While normally it defaults to composite video,
its switching action is based on the presence of S-Video. The output of this switch feeds the external side of
the PiP switch.
IC2901 is referred to as the main switching IC. Both switching channels of IC2901 are controlled by the
AuxSwitch1 input from the microprocessor. It is tempting at first glance to assume that this IC switches
between external and internal video sources. However, ICX2200 actually handles that process internally.
Rather, IC2901 switches the between the external source to be viewed (composite or S-Video) on switch-
ing channel 2. The output from this channel goes to pin 39 of ICX2200, the external video input. Channel
1 switches between the internally separated chroma signal and the S-Video chroma signal, and feed its
output back to pin 45. See below for a further description of video separation.
Video Development
For video processing, ICX2200 will switch between tuner IF, which as discussed above enters the IC at pin
37, or an external video source, which enters the Jungle chip at pin 39. Switching between these will be
directed by the microprocessor via the I
2
C bus entering ICX2200 on pins 27 and 28. The selected signal is
emitted from the switched video out pin (41) as composite video. This signal is sent to the microprocessor
and the glass comb-filter, DL2200.
The purpose of the comb-filter is to separate chroma from luma. Its superiority to conventional filtering and
separation schemes is that it reduces cross talk between chroma and high-frequency luma. Historically, sets
without this feature were prone to have rainbow effects on objects whose luminance frequency approached
the 3.58 MHz color subcarrier. (Note that this frequency is established by CR2201, the color crystal in CA
chassis.) The result of the comb-filters operation is that cross color distortion is minimized for much more
highly detailed objects.
Composite video enters the comb-filter on pin 2. Luma is produced on pin 5 and is fed to pin 43. A
3.58MHz trap (C2229M, L2201, & R2245M) removes any residue chroma. Separated chroma exits the
comb-filter at pin 3. In sets that utilize IC2901 for jack-pack switching purposes, chroma from the comb-
filter is sent to pin 5 and exits at pin 8. This signal is then denoted by CM, and is passed back to the video
processor via pin 45. (In sets that do not use IC2901, chroma passes directly to pin 45.) A band-pass
filter (C2232M, R2253M, & L2202) eliminates any remaining luma.
Once inside the Jungle IC, luma is processed with numerous factors including contrast and brightness.
Brightness and contrast information from the beam current passes into pins 36 and 38 from the circuitry
consisting of Q2204, 2205, and 2209. ICX2200 responds to this information by limiting (when necessary)
contrast and brightness to the RGB outputs. Basically, the ABL responds to current from pin 8 of the
sweep transformer, TX3204. D2204, R2235, and C2220 make this signal positive. It is then inverted by
Q2205 and smoothed as it passes to Q2204. As beam current approaches 1.5 mA (1.3 mA on 25, 0.9
mA on 19/20) Q2209 starts to turn on and shunts excess current to ground. In addition to controlling the