Philips 29PT6441/85, Service Manual

Page: 136 / 338

•
VideoSupply (+200V from primary side of LOT): This supplies the RGB amplifier
and Scavem circuit at the CRT panel.
Notes:
•
The V_T voltage (to tuner) is drawn from V_batt.
•
The EHT voltage is generated by the Line Output Transformer (LOT). The Focus
and Vg2 voltages are created with two potentiometers integrated in the
transformer.
Beam Current
The beam current is adjusted with R3451 and R3452. The components R3473, R3453
and C2451 determine the EHT_info characteristic. The voltage across C2412 varies
when the beam current changes. This EHT_info is used to compensate the picture
geometry via pin 97 of the Hercules when the picture changes rapidly, and compensate
the phase 2 loop via pin 113 of the Hercules. Also from the EHT_info line, a BCL signal
is derived and sent to the Hercules for controlling the picture's contrast and brightness.
When the picture content becomes brighter, it will introduce:
•
Geometry distortion due to the impedance of the LOT causing the EHT to drop.
•
Picture blooming due to the picture characteristics
Because of the above mentioned, we will need a circuit for Beam Current Limiter (BCL)
and EHT compensation (EHT_info). These two circuits derive the signal from the picture
tube current info through LOT pin 10.