Philips EM5A NTSC Service Manual Download Page 136

Page: 136 / 153

Circuit Descriptions and Abbreviation List

EN 136

Parabola Generation(optional)

Figure 9-30 Auto Scavem: Stage 5

Optional (depending on the set design): the Scavem signal is
modulated with a parabola. This is necessary because the
picture tube is less sensitive for Scavem in the center than on
the sides.
The 'heater' pulses on the CRT-panel are used to create a
parabola on line frequency. This parabola is made with
transistors 7204 and 7206. The DC-level of these transistors is
controlled via the DAC3 line. In this way, you can control the
parabola via software. The level is dependent on the
'sharpness meter' reading in the Auto TV program, the picture
tube size, and the sharpness setting by the customer.

Parabola Modulation and Pre-amplifier

Figure 9-31 Auto Scavem: Stage 6 and 7

The Scavem signal enters at the base of transistor 7030.
Transistors 7028 and 7032 are complementary. Normally, the
same current goes through both components, but because
(7028) the parabola signal is offered on one of them, the
Scavem signal is now modulated and inverted.
After buffer 7038, the parabola signal is again added to the
(now inverted) signal, so that only the modulated signal is
available on the input of the pre-amplifier (items 7040 and
7042.)

Pre-amplification is done via a voltage amplifier (7040) and an
emitter follower (7042.)

Final Amplifier (Diagram F2)

Figure 9-32 Auto Scavem: Stage 8

Note: The figure above is taken from a previous chassis
manual, and so may deviate slightly from the EM5 execution.
However, the principle is the same.

Finally, the Auto Scavem signal arrives, via emitter follower
7422, at stage [A] on the CRT panel. Transistors 7427 and
7428, together with power transistors 7414 and 7415, form the
power amplifier, which delivers the high Scavem currents.
From the emitter of 7422, a signal is taken for stage [B]. This
signal is inverted by transistor 7425 and buffered by 7431 and
7424. The next stage is identical to that of stage [A].
At a positive alteration of the Scavem signal, transistors 7427
and 7415 in stage [A] will conduct, while 7428 and 7414 are
blocked. In stage 'B,' the positive pulse is inverted, so
transistors 7426 and 7412 will conduct, while both 7423 and
7413 are blocked. The Scavem current will now flow, via
transistor 7412, through the Scavem coil and then through
7415.
During negative alterations, the current will flow through
transistor 7414, the Scavem coil, and transistor 7413.

9.18 Double Window (DW, diagram C)

9.18.1 Introduction

The Double Window (DW) panel provides the option for the
viewer to see two pictures or programs on the displayed area
of a TV screen. The displayed pictures can be in PIP mode or
DW mode. The viewer can also select the size and position of
the "second" picture.
Key components:
•

Tuner (7201.)

•

SAW Filter (1352.)

•

IF + Video processor (7301.)

•

PIP processor (7801.)

•

Switching ICs: TDA8601 (7803) and HEF4053 (7401 and
7402.)

•

IO expander (7403.)

•