Mitsubishi Electric VS-45605 Technical Training Manual page 64

Blanking Insertion & CRT Protect
Video retrace blanking is inserted in the RGB signals
before they are applied to the CRTs. The Blanking
Insertion circuitry is shown in Figure 7-7. The RGB
outputs from the Gamma IC are applied to the base
of their respective Color Amplifiers, Q2W82, Q2W81
and Q2W80.
The base of each amplifier is connected through a
discrete diode to the collector of Q2W62. When
Q2W62 conducts, all three bases are pulled LOW,
shutting of all three RGB amplifiers, which blanks
the CRTs.
Horizontal pulses, from the Horizontal Output Cir-
cuitry, and Vertical pulses from pin 7 of the V-OUT
IC, are directed to the base of Q2W62. The pulses
drive Q2W62 into conduction during horizontal and
vertical retrace, blanking the CRT screens.
Spot Killer circuitry momentarily turns ON Q2W62
when the TV is switched OFF, eliminating the mo-
mentary spot. When the TV is ON, C2W61, at the
emitter of Q2W61, charges to 9 volts. The base is
also at 9 volts from R2W75.
When the TV is switched OFF, the SW 9V supply is
turned OFF, and the base voltage of Q2W61 drops
to zero. However, the emitter is still positive, due to
the charge on C2W61. Q2W61 conducts until the
capacitor discharges. During this period, Q2W62
conducts, blanking the CRT screens.
The BLK2 output from the Main uPC also connects
to the base of Q2W62, blanking the screens during
channel, and input selection.
The RGB outputs from Q2W82, Q2W81 and
Q2W80, are directed to the RGB Output Amplifiers
on the PCB-CRTs.
Page 7-9
This circuitry is also used for
CRT Protection, against loss
of deflection, and by the Spot
Killer circuit. Horizontal
pulses are applied to the base
of Q522, and a Vertical
Sawtooth from the Vertical
Yokes ground return circuit is
applied to base of Q401. The
conduction of the two tran-
sistors holds their collector
voltages below the zener
point of D456.
If either transistor stops con-
ducting, its collector voltage
exceeds the zener point and
Q2W62 is driven into con-
duction, blanking all three
CRTs.