Vertical Output - Panasonic EURO 4 Chassis Technical Manual

3DQDVRQLF
Chapter 3
5. Vertical Output Stage
5.1. LA7876N
On EURO 4H models the vertical output IC used is the
LA7876 which is fed a drive pulse output from the DDP
IC1504 pins 19 and 20 to the vertical output stage
IC451 pins 5 and 6.
This vertical output IC IC451 consists of an
operational amplifier to which the vertical drive pulse
in the form of a sawtooth is passed, the results of
which are output via pin 3.
The gain of the internal op-amp. is controlled by the
negative feedback pulse which is fed via R454
connected between pin 3 and pin 6.
IC451 also contains two pump-up circuits which are
used to provide a switching voltage for the vertical
flyback period. This is required as the energy
requirement of the vertical output stage is highest
during flyback, as the electron beam has to be passed
rapidly from the bottom right hand corner of the screen
to the top left corner of the screen.
This brief additional energy requirement is met by
increasing the supply voltage available to the output
stage by 3 times the supply.
+14V
-14V
Vertical
Flyback
Pulse
During vertical sweep, the bootstrap capacitors C456
and C463 are charged up to almost supply voltage via
D454 and D458. The output of the pump-up
generators at pins 8, 9 and 10 of IC451 are at this
moment ground potential.
As a result of the DC displacement at the negative
pole of capacitor C456, C463 (rising to the supply
voltage), build up of the supply voltage for the output
stage at pin 3 rises to almost 3 times the supply
voltage. At the same time, D454, D458 are reverse
biased and thus prevents discharge of C456, C463
into the supply line.
5.1.1. Vertical Protection
The output pin, pin 3 of IC451 which is directly
connected to the deflection coil is monitored by the
protection input of the DDP IC1504.
Here a vertical flyback pulse is fed to pin 11 of the DDP
IC1504, which is used to signal a fault (as described
in chapter 3 section 2.9.4. ).
When a fault is detected the RGB signals are blanked
preventing any damage of the CRT.
Vertical
Drive
40
EURO 4H Supplement