Philips FIMI Field Support Manual page 27

3.2.3 HORIZONTAL DEFLECTION AMPLIFIER
At the initial switch-on of the monitor current flow in the horizontal
deflection coil is zero. Transistor V54 is forward biased deriving base volts
from transformer T51, which has 9.4V on its primary derived from the +12V supply
vi a R 78. During the scan period, therefore, V54 is switched on and this, in
conjunction with +12V charging C62 and C65, causes the generation of a current
ramp via an LC network comprising the primary of T52, inductors L51 and L52, and
capacitors C62, C65. The current ramp increases with linear amplitude (amplitude
and linearity are adjustable by means of L51 and L52 respectively) until retrace
time.
At the commencement of retrace time a sync pulse from pin 5 of U51 switches V51
on, with V51 on transistor V53 conducts, and in so doing a voltage pulse is
developed across V53 collector load, the primary of T51. The voltage bias on V54
base goes sharp
1
y negative and V54 switches off. The LC network co 11 aspes
rapidly so causing a high level voltage pulse to appear across the deflection
coil. The scanning drive current drops and retrace is effected. At the falling
edge of the sync pulse V51 and V53 switch off and V54 switches on, this
initiates the current ramp generation process and horizontal scan is repeated
until the next sync pulse from U51.
The primary switching function of T52 is utilized in the generation of CRT
supplies, which will be fully described in section 3.4.
3.3 VIDEO AMPLIFIER
Transistor VlOl is switched on being forward biased by a base potential of 6.2V
established by zener diode VlOO, transistor Vl02 is off. When the incoming video
signal is applied to the base of Vl02 it is switched on, causing VlOl to conduct
hard-on , collector current is developed t hrough R104 and LlOO and this is
coupled to the CRT cathode pin 7 via Rl05, thereby modulating electron beam
intensity. On the falling edge of a video pulse, Vl02 switches off and VlOl
collector current drops. Diode Vl03 serves to limit the VlOl collector current
to 250mA during video pulse amplification to increase the operational life of
the CRT cathode.
3. 4 CRT SUPP LI ES
Various supplies for the CRT and circuits employed in the monitor electronics
are developed across the secondary of transformer T52. This transformer has, as
its primary input, a 250V AC 19KHz signal developed by the horizontal retrace
circuit (see section 3.2).
3.4.1 14.5KV EHT SUPPLY
The 14,5KV anode voltage is developed by means of a screened secondary winding
w hich has a high turns ratio. This voltage is fed directly to the CRT via an EHT
lead terminating in a diode (mounted withi n the CRT) for rectification before
appear ing at the anode.
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FIMI MON 12
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