(3) Short-circuit protection: an over-current sampling triode is connected to output load resistor of L,
R channel, sampling triode of L channel is V114 (on power amplifier board), load resistor is R126,
R127(on power amplifier board). C, SR, SL channel power amplifying IC has been with over-current
protection inside. Only over-current appears in L channel, the pressure drop produced on R126, R127
will increase, once the pressure drop of R129 (on power amplifier board) is more than 0.7, V114 will be
on, V103 will also be on, relay disconnects finally, and short-circuit protection completes. The working
process is shown in the figure 3.2.5.3:
Over-current
sampling triode
is connected to
each channel
Similarly, when R channel has over-current, voltage on R159 (on power amplifier board) will be
more than 0.7V to make (on power amplifier board) V129 on, also to make V103 on, and relay
disconnect finally to achieve the purpose of protecting loudspeaker.
AV321T (RU) is with two-channel composite video input (VCD, DVD or 5.1CH); two-channel S-video
video input (VCD, DVD or 5.1CH); 1-channel composite video output (VCD, DVD or 5.1CH); 1-channel
S-video video output (VCD, DVD or 5.1CH).
3.2.6 Video input/output board
Input/output switch of video signal is realized through two electronic switch Cd4052 (N100,N101).
When VCD channel is selected, pin 1 and 2 of N104 (M62446) of signal processing board send out A and
B high level control signal, according to working principle of signal processing board in section 2, now A
and B also control gating of CD4052 of audio part, that is, audio channel also selects VCD channel, thus,
the synchronous switch of audio and video of VCD channel is realized.
Similarly, for synchronous switch of audio and video of DVD channel, pin 1 and 2 of N104 (M62446)
of signal processing board send out 1 and 0 control signal of A and B to finish the corresponding switch.
When 5.1CH is selected, it is equal to selecting DVD channel.
Composite video signal is outputted through triode V100 (1015), V101 (1015) and V102 (1015)
output S-video signal.
Working principle of microphone part has been shown in section 2 of chapter 2, so we will not
repeat here.
Headphone part: L and R channel signal is outputted from pin 31 and 32 of signal processing board
3.2.7 MIC, headphone board
L channel
V114
C electrode
voltage
decreases
V103 is on
Relay
disconnects
and protection
starts up
When over-current,
pressure drop on
R126, R127
increases
V114 is o n
Figure 3.2.5.3 Short-circuit protection flow chart
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