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When an external center channel speaker is connected, the
link to the switch is disconnected. In this situation, the soft-
ware requests the user to change the switch position.
The left and right channels from the main board are con-
nected to connector Q3, and due to the other switch posi-
tion, the left and right channel signals are now fed via the
switches directly to the internal speakers. This is assum-
ing, of course, that external left and right speakers are not
connected.
Adjustable Audio Output module,
TA700
The adjustable audio output (line output) module TA700
makes it possible to send the TV sound to an external am-
plifier using the tone and volume controls of the TV's own
remote control.
The left and right channel audio signals are fed via a FET
pair, tta1 and tta2, to the op-amplifier icta1. Due to the nega-
tive input, the amplifier inverts the signals. Amplified sig-
nals are then fed via another FET pair, tta8 and tta9, to the
output connectors Xt7 and Xt8 (RCA connectors).
These FET pairs prevent possible unwanted switching
peaks from the audio processor from reaching the exter-
nal audio amplifier.
The first pair, tta1 and tta2, eliminate positive disturbance
peaks (internal diodes) and due to the inverting feature of
the op-amplifier, the second pair, tta8 and tta9, prevent
negative peaks coming from the audio processor.
Switch on / off damping
The FETs also operate as mute (damping) switching com-
ponents during switch on and off.
When the receiver is switched on, the +12 Vp causes (via
transistor tta5) the positive input on pin 5 of icta2 to go
high faster than the negative input on pin 6. This means
that the output pin 7 is high, and transistors tta6 and tta7
conduct holding the gates of the FETs low. A short time
later when capacitor Cta17 has charged, tta5 no longer
conducts and the voltage on pin 5 falls below that on pin 6.
The output pin 7 is now low, and transistors tta6 and tta7
do not conduct. The first FET pair gets their gate voltages
via resistor rta24. The second FET pair has a small delay
since their gate voltage is taken via transistor tta4, resistor
rta20 and capacitor Cta15.
When the receiver is switched off, pin 6 of icta2 immedi-
ately goes low, but pin 5 stays high due to the charge of
capacitor Cta18. The output pin 7 is therefore high and the
FET pairs are switched off.
Line output status
The other half of the op-amplifier icta2 operates as a line
output status switch. When the output connectors are not
used, the additional contacts (on the connectors) hold the
negative input pin 2 low via resistors rta17 and rta18. This
means the output pin 1 is high, transistor ta9 (on the main
board) conducts and holds microcontroller pin 3 low (in-
active).
When a plug is connected to the connector, the additional
contact disconnects resistor rta17 / rta18 from ground. This
causes a higher voltage level on pin 2, and output pin 1
goes low. Now transistor ta9 does not conduct and
microcontroller pin 3 is high. This enables the selection of
three different mute states:
- Line output operates, but TV speakers are muted,
- Both line output and TV speakers are muted
- Both line output and TV speakers operate (no mute).
Scart 3 + VGA-audio module, TA710
The scart 3 + VGA-audio module makes it possible to have
not only one extra scart connection, but also one extra
audio connection (VGA-audio). The audio input is via the
VGA-audio connectors, when VGA mode is selected.
The characteristics of scart 3 are identical to those of scart
1.
The RGB switch ica2 selects the signals either from scart 3
or from scart 1. The RGB signals are fed from scart 1 via
connector Xa5 which is connected to the connector Xq7
on the main board.
The audio switch ICa3 selects the signals either from Scart
3, "VGA" or from the camera connector. The connector
Xa2 is linked to the connector XA4 (on the local control)
and Xa3 to the connector Xtta (on the main board).
The voltage level on module pin 3 determines which con-
nectors the RGB and audio signals are taken from.
The control voltage is taken from the teletext circuit (pin 7,
GPO), onward via connector Xq5-3 (on the main board)
and Xa1-3 (on the module). The RGB switch ica2 is con-
trolled via pin 5 and the audio switch with the voltage lev-
els on pins 9 (=B) and 10 (=A). The logical low is about <3.5
V and logical high about >9 V with a +12 V supply voltage.
The GPO (general purpose output) operates as a three state:
high, open and low.
When the GPO is high, transistor ta2 conducts and pulls B
low, diode da2 and transistor ta4 conduct, thus A and pin 5
of ica2 are low. As a result of this combination, both the
RGB and audio signals are taken from scart 3.
When the GPO is open, ta2 does not conduct and B is high,
da2 and ta4 conduct, thus A and pin 5 of ica2 are low. RGB
signals are taken from scart 3, but audio signals are taken
from the "VGA" connector.
When GPO is low, ta2 does not conduct and B is high. Di-
ode da1 conducts and pulls the anode of da2 low. Diode
da2 does not conduct, neither does transistor ta4, and A
and pin 5 go high. Now the RGB signals are taken from
scart 1 (via Xa5) and audio signals from the camera con-
nector (via Xa2)
GPO
B
A
High
"0"
"0"
Open
"1"
"0"
Low
"1"
"1"
Scart 3 module, TA711
The scart 3 module is similar to the TA710 described above,
but without the VGA-audio function. Otherwise the func-
tions are identical.
RGB in
Audio in
Scart 3
Scart 3
Scart 3
"VGA"
Scart 1
Camera
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