Philips L9.2A Service Manual Download

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Circuit description new circuits

GB 61

L9.2A

9.6.6

Second RGB insertion

Pins 23, 24, 25 are used as the inputs for the second R, G, B
signals insertion. Pin 26 of the TV-processor is the input for the
insertion control signal which is called "FBL". When the FBL
signal level becomes higher than 0.9V (but less than 3V) the R,
G, B signals at pins 23,24,25 are inserted into the picture by
using the internal switches incorporated in the TV-processor.
This second insertion possibility is used for insertion of the on
screen display signals , TXT or R. G. B signals from the CINCH
socket..

9.6.7

RGB processing

The RGB processing circuit enables the picture parameters to
be adjusted by using a combination of the user menus and the
remote control. Additionally automatic gain control for the RGB
signals via cut-off stabilisation is achieved in this functional
block..
The block also inserts the cut off point "measuring pulses" into
the RGB signals during vertical retrace period.. From outputs
19,20 and 21 the RGB signals are then applied to the output
amplifiers on the CRT panel.

9.6.8

Black current calibration loop

The black current calibration loop ensures that the white
balance at low signal levels and low light white balance is
skipped. By means of the inserted measuring pulses, the black
current calibration loop, tracks the beam current feed back of
the RGB signals at the cathodes of the picture tube. As a result
of this calibration, the individual black level of the RGB output
signals is shifted to a level which allocates around 10uAof
beam current to each of the RGB signals. Pin 18 (BC_info) of
the BIMOS is used as the feed back input from the CRT base
panel.

9.6.9

Beam current limiting

A beam current limiting circuit inside the BiMOS handles the
contrast and brightness control for the RGB signals. This
prevents the CRT tube being over driven, which may cause
serious damage in the line output stage. The reference used for
this purpose is the DC voltage on Pin 22 (BLCIN) of the TV-
processor. Contrast and brightness reduction of the RGB
output signals is therefore proportional to the voltage present
on this pin. Contrast reduction starts when the voltage on pin
22 is lower than 3.0 V. Brightness reduction starts when the
voltage on pin 22 is less than 2.0 V.
The voltage on pin 22 is normally 3.3V (limitor not active). To
enable correct operation however, an external adaptation to
the circuit is required for the correct functioning of the limiting
function. This is connected to Pin 22, the circuit therefore
ensures that correct peak white limiting and the average beam
current limiting takes place. Components 6212, 2227, 3253,
3246 are for the average beam current limiting and the items
connected to 7263 are for the peak white limiting. As a
reference for the average beam current control the signal
EHT_info is used. This signal is a measurement of the picture
contents. It is filtered by 3253, 2227. As the time constant of
the filter is much bigger than the frame period time, the DC at
the anode of 6212 represents the average value of the picture
content. Via 6212 and 2226 the DC voltage at pin 22 is slowly
'clamped'. For peak white limiting transistor 7263 is utilised.
When peak white occurs, the DC voltage at the base of 7263
drops rapidly. 7263 starts conducting, which provides a path to
discharge the capacitor 2226 very fast. The voltage bias at the
base of 7263 is fixed via voltage divider 3251 and 3249. The
RGB output signals are applied to the CRT panel via connector
0243. Via diodes 6263, 6264 and 6265 and series resistor
3253, the RGB signals are also connected to the

CRT_discharge signal. The level of this signal is only high
during the time the set is switched off. And id due to the
cathodes of the CRT are driven fully negative. That means that
the beam current is increased. and consequently the CRT
quickly discharged.

9.6.10 CRT panel (see circuit diagram B)

On the CRT panel the output amplifiers for the RGB signals (
IC T7330, DA6107Q) are located. Via the outputs 9, 8 and 7 of
the IC the cathodes of the CRT are driven. The supply voltage
for the IC is +200VA and is derived from the line output stage.

9.7

List of abbreviations

2CS

2 Carrier Stereo

A/P

Asia Pacific; schematic/PCB
information (only) applicable for Asia
Pacific sets

AFC

Automatic Frequency Control

AQUADAG

Aquadag coating on the (outside of
the) picture tube

AudioOutR

Audio signal at Right output channel.