Sharp 76DW-18H (serv.man3) Service Manual ▷ View online
Wide Screen, Colour Television CW100 Chassis
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 5
At this point, the magnetic field within T701 will collapse causing the e.m.f at pin 17 to go negative
turning off Q703.
turning off Q703.
Q701 will not turn on until sufficient charge has built up on C707. However, currently C707 negative
plate is approximately -12V; due to the collapsing field within T701. This result in the time period
required to build sufficient charge on C707 to biased Q701 'On' increasing i.e. Q701 off time = the
time for the back 'e.m.f.' to decade to zero, plus C707 charge time form zero to 4.5V approx.).
plate is approximately -12V; due to the collapsing field within T701. This result in the time period
required to build sufficient charge on C707 to biased Q701 'On' increasing i.e. Q701 off time = the
time for the back 'e.m.f.' to decade to zero, plus C707 charge time form zero to 4.5V approx.).
V
OLTAGE REGULATION
The feed back through the opto-coupler (IC701), provides voltage regulation. The 140V line is used
as the controlling source and a reference is provided from the +12.5V supply. Protection is also
incorporated via the +8V supply for faults that may occur on either the +8V rail or the 5V rail, which is
derived from the +8V rail.
as the controlling source and a reference is provided from the +12.5V supply. Protection is also
incorporated via the +8V supply for faults that may occur on either the +8V rail or the 5V rail, which is
derived from the +8V rail.
Increase in load
As the 140V rail falls due to increase in load, Q705 will turn off causing Q712 to turn off thus
decreasing the brightness of the LED section within IC701. This has the effect of decreasing the
conduction of the light sensitive transistor which will increase the voltage drop across the
collector/emitter junction; thereby increasing the time taken to charge C708, delaying the point when
Q703 turns on, causing Q701 to turn off. Allowing more energy to be transferred across T701
increasing the HT rail; hence, the frequency of the power supply will fall.
decreasing the brightness of the LED section within IC701. This has the effect of decreasing the
conduction of the light sensitive transistor which will increase the voltage drop across the
collector/emitter junction; thereby increasing the time taken to charge C708, delaying the point when
Q703 turns on, causing Q701 to turn off. Allowing more energy to be transferred across T701
increasing the HT rail; hence, the frequency of the power supply will fall.
Decrease in load
As the 140V rail increase due to a decrease in load, Q705 will turn on, turning on Q712, and the
brightness of the LED section within IC701. This will increase the conduction of the light sensitive
transistor; thereby decreasing the voltage drop across the collector/emitter junction; which will
decrease the time taken to charge C708, thus advancing the point when Q703 turns off, causing
Q701 to turn on earlier. Allowing less energy to be transferred across T701 decreasing the HT rail;
hence, the frequency of the power supply will increase.
brightness of the LED section within IC701. This will increase the conduction of the light sensitive
transistor; thereby decreasing the voltage drop across the collector/emitter junction; which will
decrease the time taken to charge C708, thus advancing the point when Q703 turns off, causing
Q701 to turn on earlier. Allowing less energy to be transferred across T701 decreasing the HT rail;
hence, the frequency of the power supply will increase.
It can now be seen that this is a variable frequency power supply, typically the frequency will vary
between 144kHz (no load) to 70kHz (high beam current).
between 144kHz (no load) to 70kHz (high beam current).
O
VER
V
OLTAGE
P
ROTECTION
D707 is used to prevent the secondary rails increase beyond safe limits. If the e.m.f. @ T701 pin 17
increases beyond safe limits D707 will conduct, increase the voltage across C708, thereby reducing
its charge time, turning on Q703 earlier, turning off Q701, allowing the HT rails to fall. Once the HT
starts to fall Q703 will turn off allowing the Power Supply to restart.
increases beyond safe limits D707 will conduct, increase the voltage across C708, thereby reducing
its charge time, turning on Q703 earlier, turning off Q701, allowing the HT rails to fall. Once the HT
starts to fall Q703 will turn off allowing the Power Supply to restart.
O
VER CURRENT CONTROL
Over current control is performed by monitoring the amount of current passing though the R706
(Q701 drain, earth return resistor). This resistor is an extremely low value, 0.18R, and Ohms Law
dictates that the current flowing through it must be proportional to the voltage across it. The base
voltage required for turning on Q702, is +0.6V then the voltage across R706 to ‘trip’ the circuit would
have to be approximately 1.2V (this includes 0.6V drop across D720). Therefore, it can be seen
(using Ohms Law) a current of 6.7amp is required to operate the trip circuit.
(Q701 drain, earth return resistor). This resistor is an extremely low value, 0.18R, and Ohms Law
dictates that the current flowing through it must be proportional to the voltage across it. The base
voltage required for turning on Q702, is +0.6V then the voltage across R706 to ‘trip’ the circuit would
have to be approximately 1.2V (this includes 0.6V drop across D720). Therefore, it can be seen
(using Ohms Law) a current of 6.7amp is required to operate the trip circuit.
The voltage developed across R706 will turn Q702 on via D720 & R707. Once Q702 turns on, C707
will discharge rapidly, removing Q701 gate bias. Q701 will remain turn off until the current through
R706 decreases At this point Q702 turns off allowing C707 to charge turning on Q701. If the overload
is still present then Q702 will turn on again, thus repeating the cycle. In the case of a short circuit line
output transistor the power supply will keep repeating this cycle. The HT rail & base of Q702 (if
measured with a standard DVM) would read normally read zero volts and the power supply appears
not to be working. On some occasions when the short is not as heavy then a whistle may be heard.
will discharge rapidly, removing Q701 gate bias. Q701 will remain turn off until the current through
R706 decreases At this point Q702 turns off allowing C707 to charge turning on Q701. If the overload
is still present then Q702 will turn on again, thus repeating the cycle. In the case of a short circuit line
output transistor the power supply will keep repeating this cycle. The HT rail & base of Q702 (if
measured with a standard DVM) would read normally read zero volts and the power supply appears
not to be working. On some occasions when the short is not as heavy then a whistle may be heard.
Wide Screen, Colour Television CW100 Chassis
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 6
Note:
To check that the power supply operating, disconnect R648 & measure the voltage at the cathode of
D708:
D708:
140V.
Power Supply working, short in line stage
<40V.
Short on a LT rail
0V.
Power Supply not operating (check that the CTV is not in standby mode)
Self Assessment Question
Name two conditions that will turn on Q702?
What is the supply voltage to IC1001 during standby?
What will be measured with a DVM at the base of Q702 during over-current?
If Q703 is open circuit, what would happen to the power supply frequency?
If load increase on the power supply, what will happen to the frequency?
Wide Screen, Colour Television CW100 Chassis
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 7
Slave Processor
30 Min Study Period
Wide Screen, Colour Television CW100 Chassis
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 8
SLAVE PROCESSOR
As described previously, to control the operation of the set in the standby mode (including timer on
function) a Slave Processor (IC6702) is utilised.
function) a Slave Processor (IC6702) is utilised.
This Slave Processor has a volatile memory therefore a copy of its operating system (software) is
stored in the EPROM and any customer/engineering settings are stored in the NVM's (EEPROM)
which forms part of the Main Microprocessor circuit. For this reason, the default condition Slave
Processor standby function (<> standby), pin15; is low; enabling IC6702 software to be downloaded
from the Main Microprocessor circuit each time power is applied to the CTV.
stored in the EPROM and any customer/engineering settings are stored in the NVM's (EEPROM)
which forms part of the Main Microprocessor circuit. For this reason, the default condition Slave
Processor standby function (<> standby), pin15; is low; enabling IC6702 software to be downloaded
from the Main Microprocessor circuit each time power is applied to the CTV.
Figure 1 - Slave Processor Circuit (CA10)
S
TANDBY OPERATION
Due to legislation, which limits the standby power consumption, We ensure that this CTV will
consume as little power as possible during standby. This includes not fitting a standby indicator.
However, legislation states that there must be an indication that power is flowing through the On/off
switch, therefore, fitted across the On/off switch is a 'neon'. The amount of power which a neon
consumes is negligible when compared with an LED or incandescent lamp.
consume as little power as possible during standby. This includes not fitting a standby indicator.
However, legislation states that there must be an indication that power is flowing through the On/off
switch, therefore, fitted across the On/off switch is a 'neon'. The amount of power which a neon
consumes is negligible when compared with an LED or incandescent lamp.
If the CTV is in standby mode when power removed, it will remember to switch to standby the next
time power is applied.
time power is applied.
The data that determine whether the CTV comes on in standby is stored in one of the NVM’s ( IC6702
has a volatile memory). At switch on IC6702 will not know what to do, but the default condition for the
standby signal IC6702 pin15 is low. Therefore, the power supplies will start, enabling the Main
Processor circuit to function Once IC1001 has reset, and downloaded its data from the EPROM,
IC1001 will communicate with IC6702 via the two opto-couplers (IC6703 & IC6704); downloading
IC6702 software. Once this has finish IC1001 will check the data contained within the NVM's relating
to standby. If it is then decided that the CTV should be in standby mode; IC1001 will instruct IC6702
to take its pin15 high, removing the FET gate voltage from both power supplies.
has a volatile memory). At switch on IC6702 will not know what to do, but the default condition for the
standby signal IC6702 pin15 is low. Therefore, the power supplies will start, enabling the Main
Processor circuit to function Once IC1001 has reset, and downloaded its data from the EPROM,
IC1001 will communicate with IC6702 via the two opto-couplers (IC6703 & IC6704); downloading
IC6702 software. Once this has finish IC1001 will check the data contained within the NVM's relating
to standby. If it is then decided that the CTV should be in standby mode; IC1001 will instruct IC6702
to take its pin15 high, removing the FET gate voltage from both power supplies.
5V
HT
Reset
Reset
L
N
D6704
D6703
R6704
R6703
R6713
R6712
R6705
D6705
R6707
IC6701
R6706
D6701
C5V1
C5V1
C6703
R6715
R6714
Q6701
R6718
R6716
R6717
R6719
Q6702
R6708
C6704
R/C
IC6702
Slave Processor
(Volitile)
Slave Processor
(Volitile)
M1000
R6710
R6709
Key pad
IC6704
IC6703
Standby
15
8
1
4
3
2
6
13
7
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