Read Sharp 28JW-74H (serv.man2) Service Manual online
50Hz Widescreen Colour Television – GA20 Chassis
Sharp Electronics (UK) Ltd
Technical Support
Issued June 02
Page 6
Q751
Q752
R743
4K7
R746
4K7
R747
2K2
R758
10K
D759
D758
R749
2K2
R756
12K
Q753
R755
5K6
D761
D760
R754
18K
R766
1K
D774
Q759
D770
R765
1K8
R764
8K2
D766
R774
1K
D775
R781
1K
D773
+5v Stby
-16v
+16v
IC703
R771
15K
+5v Stby
Stby
+10V
R794
1K
Q762
D783
C787
D784
D787
C12
D785
C6v2
C6v2
D786
R795
39K
+14v
CRT
Heaters
C774
1µF
Excessive EHT
If the EHT increases then the horizontal flyback pulses will also increase. The +14v is horizontal
flyback pulses rectified by D601, Therefore if the EHT increases then the +14v would also increase.
The +14v is monitored by D785, C774 and R795 and applied to the base of Q762 which is normally
turned off. When Q762 turns on it will forward bias D770, D759 and D758 causing Q752 to turn on.
When Q752 turns on Q751 turns on, turning on the thyristor D706 via IC703, removing IC701 Vcc,
resulting in the power supply entering burst mode.
If the EHT increases then the horizontal flyback pulses will also increase. The +14v is horizontal
flyback pulses rectified by D601, Therefore if the EHT increases then the +14v would also increase.
The +14v is monitored by D785, C774 and R795 and applied to the base of Q762 which is normally
turned off. When Q762 turns on it will forward bias D770, D759 and D758 causing Q752 to turn on.
When Q752 turns on Q751 turns on, turning on the thyristor D706 via IC703, removing IC701 Vcc,
resulting in the power supply entering burst mode.
CRT Heater Voltage
An increase in CRT heater voltage has a similar effect to excessive EHT, but this time the monitoring
component is D787.
An increase in CRT heater voltage has a similar effect to excessive EHT, but this time the monitoring
component is D787.
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