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DP-4X 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 RN08, RN04 and RN09. When the base becomes 0.6V below the emitter, it will be turned on,
causing the SW +10.5V to be applied to two different circuits, the SPOT circuit and the High Voltage Drive cir-
cuit. Either Horizontal Sweep Loss detection QN03 or Vertical Sweep Loss detection QN04 turning on, will turn
QN02 on.
1. SPOT ACTIVATION CIRCUIT
When QN02 is turned on, the SW +10.5V will be applied to the anode of D716, forward biasing it. This voltage
will then pass through D716. It will then be clamped by D717, and arrive at pin 4 of the PDS2 connector. It will
then be directed to the Signal PWB where it will pass through DY12 and activate the Video Mute circuitry QY57
and QY48 which then Mutes the output from the Rainforest IC IY04. 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 PDS2 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 activate. Cut Off
is produced from the Microprocessor I004 pin 47 and routed to Q718 to "inhibit" the Spot line high from activat-
ing and shutting off the CRTs and shutting down Horizontal Drive..
2. HIGH VOLTAGE DRIVE CIRCUIT
When QN02 is turned on, the SW +10.5V will also be routed through RH06 and DH02 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 R758, D719 to pin 3 of IH01 which also kills the internal drive by pulling up the
H. Blanking input to a High and stopping the internal Oscillator.
CONCERNING QN02
There are two factors that can cause QN02 to activate; loss of Vertical and/or Horizontal deflection which is
sensed by monitoring H and V Blk pulses.
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 CN04 suf-
ficiently enough to prevent the base of QN04 from going high to turn it on and activating QN02.
When the 24 Vp/p positive vertical blanking pulse is missing from CN05 to the base of QN05, the transistor will
turn off, which will cause the collector to pull up high through RN13. CN04 charges up through RN12. This in
turn will cause QN04 to turn on because its base pulls up high, creating an increase of current flow from emitter
to collector. This will increase the voltage drop across RN11. The junction of RN11 and RN09, which is sup-
plied pull up voltage through DN01, to the SW +10.5V supply. This increase of current flow through RN11 will
pull the base low and 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 CN03
sufficiently enough to prevent the base of QN03 from going high which would 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 DN04, RN05 as the SW +10.5V charges CN03 .
This in turn will cause QN03 to turn on because its base is pulled up high when DN03 fires. When QN03 turns
on, an increase of current flow from emitter to collector through RN10. This increase of current flow through
RN10 will bias on QN02 and the events described in "Spot Activation Circuit" above will occur.
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