Hitachi 46W500 Training page 63

DP-3X SWEEP LOSS DETECTION CIRCUIT EXPLANATION
(See Sweep Loss Detection Circuit for details)
The key component in the Sweep Loss Detection circuit is QN02. This transistor is normally biased off by the
pull up resistor RN18. When the base becomes 0.6V below the emitter, it will be turned on, causing the SW
+10V to be applied to two different circuits, the Spot circuit and the High Voltage Drive circuit. Either QN03 or
QN04 turning on, will turn QN02 on.
SPOT ACTIVATION CIRCUIT
When QN02 is turned on, the SW +10V will be applied to the anode of DN11, forward biasing it. This voltage
will then pass through DN11. It will then be clamped by DN12, and arrive at pin 4 of PPD3, PPS3. It will then
be directed to the Signal PWB where it will pass through D511 and activate the Video Mute circuitry Q529 -
Q527. This is done to prevent CRT burns. (See Audio Video Mute Circuit for details)
A control (enable) circuit for SPOT is routed from pin 5 of PPS3, PPD3 called "CUT OFF". This will activate
when accessing certain adjustments parameters in the service mode; i.e. turning off vertical drive for making
CRT drive or cut-off adjustments. When Vertical Drive is defeated, the Vertical Sweep loss circuit would acti-
vate. Cut Off is produced from the Microprocessor I001 pin 47 and routed to QN06 to "inhibit" the Spot line
from activating and shutting off the CRTs.
HIGH VOLTAGE DRIVE CIRCUIT
When QN02 is turned on, the SW +10V will also be routed through RN15 and DN09 and applied to the High
Voltage Drive IC IH01 at pin 14. When this occurs, the IC will stop generating the drive signal from pin 1 that is
used to produce High Voltage via QH02, the High Voltage Driver. Again, this is done to prevent CRT burn, es-
pecially during sweep loss.
This high is also routed through RN16, DN10 to pin 3 of IH01 which also kills the internal drive.
CONCERNING QN02
There are several factors that can cause QN02 to activate; loss of vertical or horizontal blanking.
Loss of Vertical Blanking (V Blk)
The Vertical pulse at the base of QN05 switches ON05 on and off at the vertical rate. This discharges CN03 suf-
ficiently enough to prevent the base of QN04 from going high to turn it on and activate QN02.
When the 24 Vp/p positive vertical blanking pulse is missing from CN04 to the base of QN05, it will be turned
off, which will cause the collector to pull up high because CN03 charges up through RN11. This in turn will
cause QN04 to turn on because it's base pulls up high, creating an increase of current flow from emitter to collec-
tor and through RN09. RN08, (which is located across the emitter base junction of QN02), to the SW +10V sup-
ply. This increase of current flow through RN08 will bias on QN02 and the events described in "Spot Activation
Circuit" above will occur.
Loss of Horizontal Blanking (H Blk)
The Horizontal pulse at the base of QN01 switches ON01 on and off at the horizontal rate. This discharges CN02
sufficiently enough to prevent the base of QN03 from going high to turn it on and activate QN02.
When the 11.6 Vp/p positive horizontal blanking pulse is missing from CN01 to the base of QN01, it will be
turned off, which will cause the collector to go high through DN03, RN02 as the SW +10V charges CN02 . This
in turn will cause QN03 to turn on because it's base is pulled up high when DN02 fires. When QN03 turns on, an
increase of current flow from emitter to collector, through RN10, and up through RN08. This increase of current
flow through RN08 will bias on QN02 and the events described in "Spot Activation Circuit" above will occur.
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