10
HORIZONTAL and VERTICAL DEFLECTION
HORIZONTAL and VERTICAL DEFLECTION
In addition to decoding and switching, IC504 provides
deflection processing for the horizontal and vertical time
base circuits. Using video from the IF or external source as
appropriate, the time-base section of IC504 produces
horizontal drive pulses at pin 40 to switch horizontal drive
transistors TR401 and TR402, and a differential vertical
ramp voltage at pins 46 and 47 to drive the vertical output
circuit (IC401).
All geometry adjustments are performed via the I²C bus with
the service parameters (
see "Adjustments" in
Servicing
) with
the exceptions of horizontal width being adjusted by L404
and horizontal linearity being fixed by L405.
Horizontal Time base
The horizontal time-base is controlled in IC504 by a dual
PLL referenced to the 4.433MHz crystal oscillator. Feedback
is obtained from the flyback transformer via clamp diodes
D501 to D503 and fed onto pin 41 of IC504 as a reference
pulse.
Vertical Time base
A differential ramp is generated at pins 46 and 47 of IC504
by the charging of C534 with a simulated current source
from high value resistor R534. This drives a DC coupled
vertical deflection stage contained in IC401.
Vertical Protection
When operating normally, the vertical output stage
generates a +5.5 volt pulse during vertical flyback blanking.
This pulse is fed via TR404 and TR406 to IC504 beam
current input. Should the pulse fail the picture tube outputs
are ‘blanked off’. Zener diode ZD911, on the Aquadag, is
critical in ensuring that this circuit does not blank the picture
tube during normal operation. For diagnostic purposes,
vertical protection may be disabled by pressing the ‘2’ key on
the remote control hand unit when in service parameter ‘OA’.
This should always be re-enabled after repair (
see
"Adjustments - Vertical Protection Disable" in
Servicing
).
Deflection and CRT supplies
43 volt and 16 volt supplies for the vertical deflection stage
are taken from secondary windings on the diode split
transformer (T402). A further secondary winding provides
power for the picture tube heaters.
A reference flyback pulse for IC504 is derived from a tap on
the primary winding providing high rise-time, and therefore,
low jitter on programme and text. The output from this tap is
also rectified and smoothed by D406/CE416 to provide a
210 volt supply for the video output amplifiers on the tube
base panel. The EHT. supply to the picture tube (23.5kV
nominal) is produced within the diode split transformer
(T402) by a 3 stage diode split overwind. The leakage
inductance of T402 and the distributed capacitance of its
overwind are tuned to harmonics of the horizontal flyback
frequency to ensure good EHT regulation. An integral thick
film resistor network is connected across the first section of
the overwind to provide adjustable focus and A1 voltages for
the picture tube.
Vertical Protection
Disable Select service parameter OA (S correction) and
press the number 2 key on the hand unit to disable the
vertical protection of IC504. This will isolate the vertical
protection circuit. After repair, vertical protection should
always be re-enabled by pressing the number 2 key again
and storing it with the TV key. (The value of parameter OA
toggles between two values when the 2 key is pressed, the
higher of which indicates vertical guard is enabled).
Auto Black level Disable
Select service parameter 02 (hue) and press the number 2
key on the hand unit to disable automatic black level
sampling in IC504. This isolates the black current signal from
the tube base. After repair, the auto black level sampling
should always be re-enabled by pressing the number 2 key
again and storing it with the TV key. (The value of parameter
02 toggles between two values when the 2 key is pressed,
the lower of which indicates black level sampling is enabled).
FBT Protection/Disable
The protection circuit works by monitoring the 16V Field HT
supply (which is rectified from active scan voltage) and
putting the TV into STANDBY mode when Field HT falls to
below 14.4V in normal operation. The under voltage
detection / Shut-down is provided by TR891, R892, R893,
and ZD893, If the Field HT drops below 14.4V, TR891 is
switched OFF allowing its collector to rise and activate the
shutdown transistor TR892 via R895,R896. This places the
TV into its STANDBY mode.
