Bose AV28 Manual page 21

The 3 video signals are sent to the internal/external video MUX, U602, a NJM2285 chip. This chip
will select either the internal or external set of video signals to be sent on further through the video
chain. The selection is made by the control pins 2, 7, and 12, driven by transistor Q601, a level
shifter driven by the main processor. No matter which mode the user selects, Composite and S-
Video, or Component video, the same circuitry is used in the video chain.
After internal/external selection, the three video signals are sent to the output video drivers, U603
and U604. These are NJM2267 chips, with dual video channel capability. Each channel contains a
clamp circuit on the input, which does a DC restore on the video signal by clamping the negative
sync tip of the video signal to a DC voltage. The output driver drives the video signal through a
330uF capacitor (100-220uF on early units), and a 75-ohm resistor. This ensures equipment
compatibility even if there are DC differences between the AV28 and the driven equipment. The 75-
ohm resistor provides reverse cable termination for best signal integrity. Video sent through a
330uF capacitor would have low frequency drop-off, causing sag in the video signal. This is cor-
rected by the sag compensation pins of the NJM2267s, by sampling the video signal past the
capacitors, and correcting of any sag that occurs. This prevents synchronization problems with
video monitors that may otherwise occur.
For the Cr and Cb signal in component mode, and the C signal of the S-video signal, there is no
synchronization pulse for the input clamp of the 2267s to clamp to, which could distort the negative
tips of these chrominance signals. For these signals, the transistors Q601 and Q602 are turned on
by the port pins of U601, supplying enough DC voltage to the signal to raise them above the clamp
level, preventing any clamping action on them.
The CS4955 must receive a 27 MHz signal from the clock oscillator, with an accuracy of +- 1350-
hertz. This frequency is phase lock-looped inside the CS4955 to generate the 3.579545 MHz color
subcarrier for the composite video signal, and must be of high accuracy so that television monitors
can lock onto it and be able to decode color information to display.
6.0 Tuner Electronics
6.1 FM Tuner
The FM RF signal is provided by the F connector, J102, and goes to the FM front-end module. The
antenna supplied with the media center is the standard Bose
the FM front-end is a tuned RF amplifier, FM local oscillator and mixer. The 10.7 MHz IF output
signal (pin 7 of the module) passes through a 10.7 MHz ceramic filter, CF101, to a FM IF amplifier,
and then back through a second ceramic filter, CF102. Transistor Q301 and related circuitry form
the FM IF amplifier that produces about 15 dB of voltage gain and provides the proper impedance
matching for ceramic filters CF101 and CF102. These FM IF filter stages reject unwanted FM
stations and noise.
The output signal from CF102 is fed to the LA1837 AM/FM detector IC, U101. This device contains
the FM IF limiter, FM detector, FM stereo MPX decoder, S-meter circuitry which is used for seek
processing, and most of the AM circuitry. The FM IF input signal to the LA1837 goes through
several gain/limiter stages and then to a single-tuned, coil-based discriminator circuit. The discrimi-
nator coil, T103, is adjusted for minimum second harmonic audio distortion. The recovered FM
composite signal appears on pin 23 of U101.
Theory of Operation
21
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FM dipole antenna. Contained within