76FW-53H (serv.man13). DA100 and DW100 training course notes - Sharp TV Service Manual (repair manual). Page 5

Colour Television – DA100/DW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

September 99

Page 2

At this point, the magnetic field within T701 will collapse causing the e.m.f at pin 15 to go negative
turning Q703 off.

Q701 will not turn on until sufficient charge has built up on C714. However, at this time the negative
plate of C714 is approximately -12V; due to the collapsing field within T701. This results in the time
period required to build sufficient charge on C714 to bias Q701 on increasing i.e. Q701 off time = the
time for the back 'e.m.f.' to decade to zero, plus C714 charge time form zero to 4.5V approx.).

The feed back through the opto-coupler (IC705), provides voltage regulation. The +150V line is used
as the controlling source and a reference is provided from the +16v supply.

As the +150V rail falls due to an increase in load (high beam current), the trigger voltage of IC706 will
fall, increasing IC706 cathode voltage, reducing the current flowing through the LED section of IC705.
This has the effect of decreasing the conduction of the photo transistor part of IC705. This increases
the voltage drop across the collector/emitter junction; thereby increasing the time taken to charge
C713, delaying the point when Q703 turns on, turning Q701 off. Energy is transferred across T701 for
a longer period, increasing the HT rail; hence, the frequency of the power supply will fall.

As the +150V rail increase due to a decreasing load (low beam current), the trigger voltage of IC706
will rise decreasing IC706 cathode voltage which increases the current flow through the LED section
of IC705. This has the effect of increase the conduction of the phototransistor within IC705. Thereby
decreasing the voltage drop across the collector/emitter junction; which will decrease C713 charge
time, advancing the point when Q703 turns off, turning off Q701 to turn on earlier. Energy is
transferred across T701 for a shorter period, 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).

D716 is used to prevent the secondary rails increasing beyond safe limits. If the e.m.f. at T701 pin 15
increases beyond safe limits D716 will conduct, increase the voltage across C713, 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 R716
(Q701 drain, earth return resistor). This resistor is an extremely low value, 0.47R, 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 a current of 2.5amps
is required to operate the trip circuit.

The voltage developed across R716 will turn Q702 on via D713 and R719. Once Q702 turns on,
C714 will discharge rapidly, removing Q701 gate bias. Q701 will remain turn off until the current
through R716 decreases. At this point Q702 turns off allowing C714 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 and base of
Q702 (if measured with a standard DVM) would 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.