Barco GRAPHICS 1209S, Service Manual

Page: 213 / 378

Horizontal Deflection
15
Date : 10/02/98 R7621085
R7621085
Technical description
DEFL board. With this relay a correction on the linearity coil is obtained between
the high and low frequency ranges.
b) To switch on and off Q58. This transistor permits a correction on the efficiency of the
HTHD voltage for the second range of the input protection circuit (transistor Q9 - IC1)
that will be discussed under the paragraph "Protections".
c) To switch on and off Q60. When this transistor is 'on' the LIN REF voltage is dropped
by the divider R20 / R63 (=on the subunit) as a compensation for the drop of the
HTHD voltage.
d) To switch on and off Q59 . When "on", P64 is added in parallel to the gate of Q1and
allows a correction on the linearity tracking
Protection circuits. a) Overcurrent protection.
If the sum of the currents of the three scan coils exceeds a pre-determined level,
the drive is inhibited as follows :
The wire J2-J3 in series with the three scan coils , acts as a low value resistor and
is connected across the base-emitter of Q16. When a 0.6 volt or greater voltage is
dropped across the wire, Q16 starts to conduct and triggers the monoflop Q10/
Q14.As long Q10 is blocked, the drive transistor Q13 remains "on", inhibiting the
drive. BY re-applying the drive pulse to the base of Q10 via D9 (a kind of feedback),
a faster reaction on the overcurrent can be obtained.
b) Overvoltage protection.
The sum of flyback pulses on each of the series connected Mosfets are checked
by a rectifier network consisting of diodes D30, D31 and D32 and common
decoupling capacitors. The pulses at the node of the two Mosfets (HDM ) are
rectified with D24 . This voltage must be half of the total flyback voltage in order to
protect the mosfets against overvoltage. This is realised with the circuit R73/C46/
D24/SR5/ R56/C32.
The rectified voltage is dropped with R52 / P3 / R51 and sent to two level detectors.
The threshold level is set by a zener at 6.2 volt with Z5. At the moment pin 6 of IC2
exceeds the threshold, the horizontal amplitude is reduced with Q11. This will avoid
the action of the "Hold Down Deflection" protection circuit.
If for any reason, the 1950V level is reached the HOLD DOWN DEFLECTION circuit
is activated.
1) The drive is inhibited through the DEFL OFF.
2) The input pin 6 remains "high" as transistor Q12 is blocked and D18 conducts
via R28 to keep pin 6 of IC1 high. This requires that the set be powered off to
reset this circuit.
3) The red LED D10 ( HOLD DOWN DEFLECTION) is illuminated in order to show
that "scan hold down" has occurred.
4) As the deflection is stopped, there is also a horizontal scan failure and the
associated circuit will drop the EHT voltage and blank the three CRT's to prevent
damage to the CRT phosphorus.
c) Too low drive protection (+17V monitoring).
It is imperative that the Mosfets are fully switched on, so that the internal resistance
will be as low as possible. Due to the large deflection current, even a small amount
of excess resistance, will cause the Mosfets to generate too much heat.
This Mosfet drive pulse amplitude depends in part on the +17 volt supply and the