Hitachi C1422R/RS Wartungshandbuch Seite 17

TUBE BASE PANEL
The picture tube amplifiers produce high voltage CRT electrode drives from the low voltage RGB outputs of
the TDA884x (I501). They also produce black current information to maintain the grey-scale characteristics
which is returned to the colour decoder (I501). Also included is circuitry for picture blanking during switch on.
The picture tube aquadag, (P901), is used to provide beam current information for the colour decoder for the
purposes of beam current limiting. The tubebase socket (P902) is fitted with internal spark gaps. D910
provides tube base protection when the spark gaps flash-over.
The panel includes supplies to the tube electrodes and the video amplifiers. Focus and A1 supplies are
connected directly from the flying leads of T401.
Video Drives
All video drive adjustments are done via the I ² C bus (See adjustments section) with the exception of the
black level set point (140V) which is set by the A1 control on the line output transformer (T401). An additional
feature for diagnostic purposes is the ability to disable the auto black level circuit by pressing '2' on the
remote control hand set when in service parameter 02 (See adjustments section)
Tubebase Video Amplifier Circuit
Each video amplifier channel includes a bipolar cascode amplifier. A R, G or B signal from the colour
decoder (I501) is fed to each video amplifier input.
The low frequency gain of each video amplifier is approximately 51, this being determined by the ratio of the
feedback resistors to the input resistors. The gain of each video amplifier channel above 2.2 MHz is
increased by including a small capacitor across one of the two input resistors of each channel.
Video Amplifier Switch-On Blanking
The emitters of Q906, Q907 and Q908 are biased from the +200V video HT via R917, R905 and Q910. The
+13V supply to bias the base connections of Q902, Q905 and Q908 is switched on under control from
microcontroller I701 pin 35 (via R435, Q408 and R434 on the main panel) during power up or from standby
after emitter bias of Q906, Q907 and Q908 is established.
Thus picture blanking during power up or from standby is ensured.
Auto Grey Scale
During the field blanking period the colour decoder (I501) measures the total cathode circuit leakage currents
at the video amplifier outputs via Q901, Q902 and Q903: - During field blanking, the colour decoder applies
monitoring pulses to the video amplifier inputs and thence current outputs from Q901, Q902, Q903 are
applied to the colour decoder pin 18 via R904 and R516. (During normal unblanked picture operation, the
colour decoder ignores the information on its pin 18.) . The current outputs from Q901, Q902 and Q903 are
used by I501 to adjust the black current and video gain of each video channel automatically. Thus black level
picture beam current and black to white video drives are maintained. Diodes D903, D905 and D906 prevent
horizontal smearing/ shadowing of the OSD from spreading across the screen.
17