Philips FIMI Field Support Manual page 26

3.2 HORIZONTAL SYNCHRONIZATION AND DEFLECTION
The horizontal synchronization is achieved by a dual retriggerable one-shot
monostable in conjunction with several discrete components. The deflection
amplifi~r and transformer switchin g circuit formed by transistors V51, V53 and
~54, drives the horizontal yoke winding whilst also switching CRT power supplies
input transformer T52.
3.2 . 1 HORIZONTAL SYNCHRONIZATION TIMING
Refer to the truth table shown in table 3.1. The monostable U51 (74LS123)
produces the horizontal deflection timing, operating as follows:
Input at pin 1 is set low, a condition established by zener diode
V50 with the +12V supply at the correct level .
• Input at pin 2 goes high on the leading positive edge of each HOSY
pulse (horizontal sync. pulse of 2,5 to 5V Amplitude and 4 to 2luS
width) .
• Input at pin 3 is set at a permanent high via the +5V rail (derived
from the op amp regulation of the +12V line) .
• A short duration pulse appears at pin 13 when a rising edge appears
at pin 2. The duration of th i s pulse and therefore the timing of i t s
falling edge is dependent upon the fast RC time constant established
by C53 , R54 and R53. Potentiometer R53 allows adjustment for picture
centering purposes,
. Pin 13 output is linked directly to pin 9 .
. Inputs at pins 10 and 11 are set permanently high via the +5V rail •
. The falling edge of the input pulse presented at pin 9 triggers the
second monostable producing an output at pin 5 of a single pulse.
The pulse parameters 4, 2V amp
1
i tude and 18uS width, are determined
by the RC network C54 and R56.
Pin 1 Pin 2 Pin 3 Pin
13 Pin
9
Pin
10
Pin 11 Pin 5
H
x
H L H
x
H L
x
L H L
x
L H L
L
t
H
p
~
H
H
p
x x
L L
x
x
L L
t= rising edge,
i
= falling edge, P= pulse X= not significant
Table 3.1 TRUTH TABLE FOR IC U51
3.2.2 +5V LOGIC SUPPLY
The +5V required for TTL logic levels and VCC for the U51, is derived from t he
+12V supply line. The +12V is dropped across resistors R55 and R52, and t he
resultant voltage is series regulated by USO (an op amp regulator) to produce a
stabilized +5V.
The +5V supply is also utilized as a pre-heat supply for the CRT heater circuit,
the +5V is routed to the CRT via diode V52 acting as a block to the +6V derived
from the CRT supplies circuit.
3-4
FIMI MON 12"/15"
8305