Philips EM5E AA Service Manual page 118

EN 118 9.
Beam-current Correction
The 'EHT-info' signal at point 10 of the LOT depends on the
value of the beam-current and the voltage from divider R3450,
R3451, and C2450. This signal is fed to the HOP to trim the
contrast, and to compensate for the changes in picture-width
as a function of the EHT-info, when EHT is decreased.
The 'EHT-info' is also used to correct the EW-current.
The 'DYN-FASE-CORR' signal, derived from the 'EHT-info'
signal, is fed to the HOP via C2455 and drives a dynamic phase
correction necessary because of beam-current variations. This
is done by regulating T
9.10.4 Secondary Line Voltages
During the blocking time of TS7421, the magnetic energy of coil
1-5 of the LOT is transferred to electrical energy in the
secondary winding. Via rectifying and smoothing, the several
secondary supply voltages are generated, like:
• EHT, Focus and Vg2-voltage.
• +200V for the CRT panel (pin 8 LOT).
• +11D for the line deflection (pin 12 LOT).
• +13V-LOT for the frame deflection (pin 6 LOT).
• -15V-LOT for the frame deflection (pin 3 LOT).
• Filament voltage (pin 9 LOT).
9.11 Vertical (Frame) Deflection (Diagram A4)
9.11.1 Frame Stage Drive
+11V
HOP
TDA 7052
1620
TILT 25 ROTATION COIL
+8V
+8V
FD- 7440-A
2
FRAME 7450-A
FD+
1
+8V
HFB-X-RAY-PROT 7455
7450-B
E/W DRIVE
E/W 3
Figure 9-19 Frame deflection circuitry
The HOP drives the frame output stage with a symmetrical
saw-tooth voltage. As the HOP is 'cold' and the frame output
stage is 'hot', they are galvanic isolated. This is done via a
transformer (5621). As in the MG-chassis, the HOP generates
three signals needed for the frame output stage:
FRAMEDRIVE+, FRAMEDRIVE- and TILT (for rotation).
The rotation circuit is kept at the 'cold' side of the chassis, to
avoid the costs an extra optocoupler.
The circuit around IC7440 will amplify this signal and the output
current will flow through the rotation coil.
9.11.2 Flyback Generator
The frame output stage is supplied via the +13 V and -15 V
coming from the LOT. The output of the amplifier is 0 V
coupling capacitor is not required.
During the (forward) scan, a supply of +13 and -15 V is
sufficient to respond to the slow changing current. The internal
flyback generator puts a voltage of -15 V on pin 3. Because of
the voltage drop over zener diode D6622 (8.2 V), C2622 will be
charged to 19 V: being 13 + (15 - 8.2 - 0.7) V.
During the flyback scan, the change in current-per-time is much
larger, so a higher voltage is required. The flyback generator
will now generate a voltage of +13 V on pin 3. Added to the
EM5E Circuit Descriptions and Abbreviation List
of the line transistor TS7421.
ON
+8V
COLD HOT
3628
V-SHIFT
3642
3640
7611
5621
3631
+13V LOT
3633
7620
3639 2603
7641
-15V LOT
7612
3623
2653
1 5
E/W E/W
7482
2 4
CL 26532041_078.eps
charge on C2622 this will give a flyback voltage of 32 V
(depending on the CRT size, this value can differ).
The amplifier IC (IC7620, pin 5) supplies the saw-tooth current
to the frame deflection coil. The current through this coil is
measured via R3620//R3621//R3622 and fed back to the
inverting input of the amplifier.
R3624 and C2624 on the output of the amplifier, form a filter for
high frequencies and in that way also prevents oscillations.
Peak voltages on the output, e.g. as a result of a possible flash,
are damped by the clamp circuit consisting of D6619, C2627,
and R3627. The network consisting of R3625, R3626, R3629,
and C2629 form an extra damping circuit.
9.11.3 Anti-moiré circuit (Diagram B9)
Introduction
By shifting two following frames w.r.t. each other, it is possible
to decrease the 'Moiré' effect. This is achieved by adding an
extra current to the frame drive sawtooth.
For 50/75 Hz frame rates, the sequence is: one normal frame
followed by one shifted frame.
For 100 Hz frame rates, the sequence is: two normal frames
followed by two shifted frames.
Implementation
The circuit around TS7020 and 7019 (one shot timer) detects
which frame rate is used.
The circuit around TS7017 and TS7018 (Flip-Flop) generates
the extra current for the frame drive.
9.11.4 Protection circuits
Bridge Coil Protection
The secondary voltage of the bridge coil L5422 is guarded at
the diode modulator (D6421/6422) via a 10 V zener diode
FRAME
(6499 on diagram A4). When the bridge-coil is working
DEFL.
1625
COIL
properly, the average voltage on D6422 is such that this zener
diode will conduct. It will drive TS7652 into saturation.
When, for any reason, the secondary side of the bridge coil is
3620
shorted, the average voltage on D6422 will drop below the
zener-voltage, and TS7652 will block. Now capacitor C2642 is
charged. Transistor TS7407 starts conducting and the SUP-
170402
ENABLE signal is grounded via R3403. This will switch 'off' the
main supply (see diagram A1).
Frame Output Protection
Via the circuit built around TS7641, the frame output stage is
guarded. If the frame output stage is working properly, TS7641
and TS7652 will both conduct, and thereby discharge C2642.
TS7443 is blocked, so the SUP-ENABLE signal is 'high'.
If there are frame pulses missing, TS7641 will block and
capacitor C2642 is charged. Transistor TS7443 starts
conducting and the SUP-ENABLE signal is grounded via
R3403. This will switch 'off' the main supply (see diagram A1).
ARC Protection
If there are 'open' connections (e.g. bad solder joints) in the
high-energy deflection circuitry, this can lead to damaging
effects (read: fire). For that reason, the E/W current is sensed
(via 3479//3480). If this current becomes too high, the 'thyristor'
, so a
DC
circuit (TS7653 and TS7654) is triggered. TS7442 is switched
'on' and TS7443 is forced into conduction. The 'SUP-ENABLE'
signal is shorted now to ground level, which will force the Main
Power Supply to Standby mode