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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.

Colour Television – DA100/DW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

September 99

Page 3

This part of the circuit has changed compared to previous chassis, where this supply was derived by
means of a bridge rectifier with two additional 68k resistors and a 5v1 zener diode to limit the voltage.
This has been replaced by a series regulator circuit, which is supplied from one side of the mains, with
an earth return via the bridge rectifier.

D708 and D707 forms part of a bridge rectifier, two diodes of the main bridge rectifier are used for the
other part. Therefore, at the junction of D707 and D708 there is a 100Hz signal comprising of positive
halve cycles at mains potential.  Each time this signal exceeds 47v D729 will conduct, turning on
Q717.

When Q717 turns on the gate of Q715 is taken to ground. Since this is a N Channel MOSFET Q715 is
turned off and no current will flow through this device.

During the period when Q717 is turned off Q715 will turn on allowing C739 to charge to approximately
7v which is limited to 5v by R777 and D710.

It can now be seen that the period of Q715 conduction is very short (it is only turned on when the
100Hz signal at the junction of D707 and D708 is less than 47v). Therefore, a top-up supply from the
bridge rectifier circuit comprising of D734 and D731 is added to the cathode of D710 via R721 and
R703.

The 100Hz signal at the junction Q717 and D733 is also used as a timing signal for the real time clock
within the slave processor. However, before it can be used the signal is passed through a Schmitt
Trigger circuit (Q708 and Q709) to ensure that the signal free from noise.

HT

+5v

Clock

R721
150k

R703
150k

D734

D731

D708

D707

R771
470k

D729
47v

R774
470k

R775
470k

R776
82k

Q717

Q715

D733

D732
15v

R777
47

C739
220

C724
220

D710
5v1

R739
47k

R738
47k

R737
180k

R735
470k

Q708

Q709

R736
470k

R770
220

R787
47k

C725
4n7

Colour Television – DA100/DW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

September 99

Page 4

N

L

T702

IC708

R763
100

POR701

Q714

From IC702
pin 14

From T701
pin15

D727

R762
220

R768
220

C702
10nF

Degause
Coils

D728
4v7

This circuit is configured to ensure that the degauss circuit will not operated unless the switch mode
power supply is operating and IC702 pin 14 = low.

A positive bias is applied to the emitter of Q714 from T701 pin 15 (rectified by D727 and limited by
D728). Via the opto-coupler; IC708. Since Q714 is a pnp device once the base is taken low compared
to the emitter, Q714 will turn on allowing current to flow through IC708 (LED Section), turning on the
triac T702, allowing current to flow through the degauss coils.

Current will continue to flow through the degauss coils until either the PTC (POR701) has increased in
value sufficiently to cut off the current flow, or IC702 pin 14 goes high, turning of Q714, preventing
current flow through IC708, turning the triac T702 off.

Since the default condition for IC702 = low, it can be seen that the degauss circuit should now operate
each time the CTV is turned on from the main on/off switch or switched from standby to normal
operation. Whether the main processor has set up communication with the slave processor or not.

Colour Television – DA100/DW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

September 99

Page 5

This is another new circuit, but depending on whether you are dealing with the DA100 (4:3) chassis or
DW100 (16:9) chassis will determine the exact circuit.

The FET Q704 is a series regulator producing 5v, which is controlled by Q713. A voltage is fed-back
from the output of Q704 (junction of L705 and C723) which will control the conduction of a variable
zener diode (IC707). D723, Q705 and Q706 provide protection against excessive supply voltage.

D722

D714

IC707

D723
2v7

Q705

Q706

Q704

+5v

+16v

From T701
pin 1

Q713

L705

C723
2200

C734
470

C721
470

R760
4k7

R729
4k7

R757
2k2

R726
22

R706
8k2

R710
2k2

R707
1k5

R725
10k

C722
10nF

C735
150pF

R731
10k

R756
10k