SERVICING
3828VD0171G
3-5
DIGITAL FCS - SERVICING
VIDEO OUTPUT AMP IC
IC5121, IC5141 and IC5161 consists of
one monolithic video output amplifier.
I t i s c a l l e d a t r a n s a d m i t t a n c e -
transimpedance amplifier because it can
be split into two amplifier stages in
series. The first stage is a voltage to
current stage (trans admittance) and
the second stage is a current to
voltage stage (trans impedance). At the
second stage voltage to current feed-
back is applied by resistor (R5125)
connected from out (pin 13) to in (pin
5). The trans admittance of the first
stage can be chosen by means of a re-
sistor between pin 1 and pin 3, repre-
sented on this circuit by RM5122. The
signal input is through a differential
pair of transistors. The signal is con-
nected on pin 2 (Vin). The input con-
figuration consists of a bipolar npn
transistor in a long tailed pair con-
figuration. Input current is 30uA type.
Black current data pin (7) is not used.
Video output current is delivered by a
quasi-complementary class A/B push-pull
stage designed in DMOS technology which
can source and sink a current of 100mA
for video output voltages of 125Vp-p.
A feature of this output stage is the
low saturation voltage (typ. 4V) and
the low voltage drop at high level (typ.
Vdd- 6V). Frequency response is im-
proved by means of a capacitor
(CM5121) in series with a resistor
(RM5123) connected between pins 1 and
3 of the IC. Value of those elements is
compromise between bandwidth, overshoot,
and ringing.
AUDIO PROCESSING
STEREO MODULE
There are six different Audio inputs:
Antenna/Cable, Aux video, S-Video, Rear
Computer, and Front Computer/ Front
Video. The sixth input come from a pro-
vision for one scan card that can be
plugged in to the rear jack pack. The
Tuner/IF section processes the Antenna/
Cable signal. It then passes as Sound
IF (4.5MHz) to the audio processor to
be demodulated.
At the front jack pack one pair of
audio jacks is used with both the Com-
puter and Aux. Video inputs. (If both
video inputs are in use the Aux. Video
is selected as default input.) The rear
jack pack includes one set of audio
jacks for the S-Video, another set of
audio jacks for the Aux. Video input,
and a third pair for the RGB Computer
input. The Audio processor IC1400
switches between the Sounf IF input on
pin 47 () from Tuner/IF section
and pins 41 & 42 (SC1_IN_L & SC1_IN_R,
the L/R channels from the A/V Switch
ICM2900). The A/V Switch chooses from
among five inputs from external audio
sources (Rear and Front jack packs).
The I²C bus controls both IC’s. The
power amplifier IC801 takes the Left
and Right channels from the audio pro-
cessor DACM_R (pin 24) and DACM_L (pin
25) outputs. Variable output monitor
is taken from the audio processor
SC1_OUT_R (pin 30) and SC1_OUT_L (pin
31) outputs.
SWITCHING THEORY
Audio source selection for Digital FCS
chassis sets is made through the Source
Select internal switch inside the IC1400
audio processor. This switch is con-
trolled by Loudspeaker Source Select
register to select Tuner /IF
signal or SC1_IN (Audio/Video switch),
or SC2_IN (Not used). The A/V Switch
chooses one of the Audio/Video input
External sources: Video Aux., S-Video,
Rear Computer, Front Computer/Video or
from Scan Card (when this is plugged in
to the rear jack pack and it has L/R
audio inputs).
The A/V switch ICM2900 selects which
auxiliary input goes to the audio pro-
cessor.
After the A/V switch makes the audio
source selection the L/R channels
(LOUT1/ROUT1) are fed to the audio pro-
cessor. The output signals of the au-
dio processor (DACM_L and DACM_R) are
applied to the Audio power amplifier
IC800. The coupling between the Audio
processor and the Audio power ampli-
fier is made through the electrolytic
capacitors C811 and C821. This is done
in order to block the DC level of the
DACM_R and DACM_L pins respectively.
