Mitsubishi V19-V21, Техническое руководство

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8-6
As beam current increases, the
voltage at pin 8 of the Flyback
decreases. When the voltage
at pin 5 of IC5A02 drops be-
low the reference at pin 6, the
output at pin 7 goes Low, shut-
ting the set Off.
Other circuitry in IC5A00 pro-
vides additional protection.
By monitoring the voltage
across R5A51 at pin 7 of
IC5A00, the HV Output tran-
sistor current is sensed. If the
voltage becomes excessive,
IC5A00 removes drive to
Q5A51.
Pin 10 of IC5A00 is used to
remove HV drive if a sudden
arc occurs. Q5A06 normally
does not conduct. If an arc
occurs, the voltage at pin 8 of
the Flyback drops suddenly:
• Exceeding the zener
point of D5A60.
• Turning Q5A06 On
• The resulting High at
pin 10 of IC5A00
removes HV drive.
Deflection Loss Detection
In the chapter on Video/Color, the CRT Protection
Circuit was described. This circuit cuts off all three
CRTs to prevent phosphor damage due to loss of
deflection.
Figure 8-6 shows the Deflection Loss
Detection circuitry.
Q5A38 monitors horizontal deflection. Pulses from
the collector circuit of Q5A31 continually drives
Q5A38 into conduction. The average collector volt-
age is below the turn on point of D5A12. If hori-
zontal deflection is lost:
• Q5A38 shuts Off
• Q5A38 collector voltage rises.
• D5A12 is forward biased
• VBLNK goes High, activating CRT Protect.
Q4B01 monitors vertical deflection. The vertical
sawtooth from the Vertical Yokes ground return
drives Q4B01 into conduction. If vertical deflec-
tion is lost:
• Q4B01 stops conducting.
• Q4B01 collector voltage rises
• D5A13 is forward biased.
• VBLNK goes High.
Since the 24V supply is the source of the HV 12V
supply, the 24V supply is monitored for further
protection. Q5A07 does not normally conduct. If
the 24V supply is lost:
• The emitter of Q5A07 momentarily remains
High, due to the charge on C5A12.