76DW-18H (serv.man3). Revision 1 - Sharp TV Service Manual (repair manual). Page 5

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.

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.).

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 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.

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.

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).

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.

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.

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.