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EN 118
9.
EM5A P/M
The RGB amplifiers on the CRT-panel are supplied with 200 V
from the LOT.
The Auto-SCAVEM circuit modulates transitions of the
Luminance (Y) signal on the horizontal deflection current,
giving a sharper picture.
The sound part is built around the MSP34xx (Multi-channel
Sound Processor) for IF sound detection, sound control and
source selection. Dolby decoding is also done by the MSP.
Amplification is done via a class D integrated power amplifier
IC, the TDA7490.
The microprocessor, called OTC (OSD, Teletext and Control)
takes care of the analogue TXT input- and output processing.
The OTC, ROM, and RAM are supplied with 3.3 V, which is
derived from the +5V2.
The NVM (Non Volatile Memory) is used to store the settings,
the Flash-RAM contains the set software, and the DRAM is
used for storing the Teletext pages.
There is a separate Standby Supply, in order to reduce the
Standby power consumption. During Standby, the Main Supply
is switched "off" (via TS7529).
A relay (1550) is used to switch the Degaussing circuit. It is
switched "on" after set start-up and switched "off" by the P after
12 s.
The Main Supply, an SMPS based on the "buck-converter" or
"buck-boost-convertor principle (dependant on the set type),
generates the 141 V (V
Note: Voltage V
is not mains isolated ("hot"), but is
BAT
alignment free.
9.3
Power Supply (Diagram A1 & A2)
The power supply has a number of main functions:
•
Mains harmonic filter.
•
Degaussing picture tube.
•
Standby power supply.
•
Main supply.
9.3.1
Mains Harmonic Filter (Diagram A1)
3501
1590
3551
1501
2
3
2507
3521
V
mains
4M7
5503 or
1
4
5504
Figure 9-2 Mains harmonic filter circuitry
The mains harmonic filter has two functions: to prevent that
high frequency signals (harmonics) are transferred into the
mains and to protect the set from lightning damage.
C2507 prevents that the high frequency signals, generated by
the set, are conveyed into the mains (it forms a short-circuit).
In case of a lightning surge between the 2 phases (differential
mode), the energy is immediately bled away through the VDR
(R3509) to the other phase.
In case of a lightning surge on both phases of the mains in
relation to the aerial earth (common mode), the filter acts as a
high resistance (U
= L * dI/dt), as a result of which the
EMK
voltage across coil L5503/04 increases. A spark gap (1590)
prevents that the voltage increases too much, which would lead
Circuit Descriptions and Abbreviation List
) and the +/- 16 V for the audio part.
BAT
Mains harmonic
diversity
3
9503
9504
1
3500
2537
5511 or 5516
-
+
9504
2
2
3
1
4
5502
CL 16532044_020.eps
to a damaged coil. When ignited, the current will be discharged
via this spark gap.
Resistor R3500 is used for limiting the inrush-current.
9.3.2
Degaussing (Diagram A1)
As soon as the set is switched 'on' via the mains switch, the
5V2 is present. As the 'DEGAUSSING' signal from the
processor (OTC) is 'low', transistor 7528 will conduct and relay
1550 is activated. Initially a considerable current will flow, via
PTC 3516, through the degaussing coil. The PTC will heat up,
resistance will rise, and the current will decay rapidly. The OTC
makes the 'DEGAUSSING' signal 'high' after 12 s, which will
switch 'off' the relay.
9.3.3
Standby Power Supply (Diagram A2)
Principle
This power supply is delivering the standby voltage, but also
the main voltages for the small signal part. It is a SOPS type
(Self-Oscillating Power Supply) and is regulated by the
controlled switching of an oscillator. It uses the so-called
'Flyback' principle:
375V
375V
After closing switch 'S', the current I
The magnetic energy in the primary coil is directly proportional
with the self-inductance of the coil and current I
time the switch is closed).
The voltage polarity at the secondary winding is negative (due
to different winding direction), meaning that diode D will block.
Capacitor C will discharge via R
Opening switch 'S' will generate a counter-e.m.f. in the primary
+375V
winding, trying to maintain current I
2516
of the secondary voltage will inverse. The magnetic energy,
stored in the coil, will now be transformed to the secondary
-
side. Diode D will now conduct, capacitor C will be charged and
U
will increase.
OUT
140501
S
Id
D
+
-
C
+
-
S
D
-
+
Isec
C
-
+
Figure 9-3 Flyback principle
will increase linear in time.
D
, U
will decrease.
L
OUT
. Through this the polarity
D
U out
+
RL
U out
+
RL
96532156_020.eps
210100
(thus with the
D
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