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DA100 (50Hz) and DA50W Chassis – Repair Notes 

 

This power supply has been used in many Sharp television receivers during the past six years and is reliable.  It 
works by changing both the pulse width and frequency of operation, this enables correct regulation of the 
secondary supplies.  The major difference between the models is the amount of current that the power supply 
can provide; this is reflected in the wattage consumed from the AC supply.  Note that later versions of the 
chassis have a power factor module fitted between the power supply and the mains input.  The function of this is 
to ensure that current is drawn over the complete input voltage cycle, not just at the peaks. 
It is important to note that when the receiver is in the standby mode, the power supply is turned off, this 
reduces power consumption to less than 3W.  When pin 15 of IC702 is high (5VDC), Q702 is turned on and the 
gate of the FET is effectively taken to ground, stopping the charging operation of C714.  To turn the set on, pin 
15 goes low (open circuit), this allows the charge to build up on C714 and thus the transistor turns on and the 
power supply starts. 

Sharp Electronics (UK) Limited - May 2003 

 

DA100 (50Hz) and DA50W Chassis – Repair Notes 

 

 

145V to 155V 

>10kΩ D720 

15V to 20V 

>10kΩ D718 

-15V to –20V 

>900Ω D719 

7V to 10V 

>10kΩ D722 

5.0V to 5.2V 

>300Ω 

IC707 pin 6 

Sharp Electronics (UK) Limited - May 2003 

 

DA100 (50Hz) and DA50W Chassis – Repair Notes 

 

Depending on the input mains circuit, there is either 320VDC (conventional bridge rectifier/capacitor 
arrangement) or 360VDC (power factor circuit) fed into the top of the chopper transformer.  This voltage is also 
used to charge C714 via R713 and R714, allowing the voltage on the gate of Q701 to increase.  When the voltage 
on the gate of Q701 reaches 4.5V, it turns on and current flows in the primary winding of T701.  This will cause 
pin 15 to increase and when the zener voltage of D716 is exceeded, turning Q703 on and Q701 off. 
 

 

 
When Q703 turns off, after C713 has discharged, the cycle will repeat.  Obviously as the circuit operates in this 
condition, there will be no voltage regulation and the supply will increase to its maximum level, which is about 
220V.  This is undesirable, so there has to be some kind of control based on the HT supply.  Feedback is achieved 
via IC705, its input (LED) is fed with a stabilised 12V supply on its anode by a shunt regulator off the +16V 
supply (R765, R766 and D726).  The cathode of the LED is fed with a proportion of the +150V supply via the 
programmable zener diode IC706.  A potential divider comprising of R743, R744 and R746 set the input voltage 
to IC706. 
As the brightness of the LED increases or decreases, the conduction of the transistor will change in sympathy.  
The collector is taken to a crude supply line generated off pin 15, rectified by D715 and smoothed by C718.  This 
supply will be fed into C713 via the opto transistor; the speed of charge depends upon the luminosity of the LED.  
Q703 will turn on and turn off Q701 
Over current is prevented by Q702.  Its base is connected across the earth return resistor (R716) in the drain 
circuit of Q701.   This resistor has a very low value and therefore a large amount of current has to flow before 
Q702 will turn on and remove the bias on the gate of Q701.  Once the voltage on the gate of the FET has been 

Sharp Electronics (UK) Limited - May 2003 

 

DA100 (50Hz) and DA50W Chassis – Repair Notes 

 

removed, the current flowing from source to drain will decrease, the volt drop across R716 will decrease and 
Q702 will turn off, allowing the gate to rise again.  If the over current condition continues, the power supply will 
trip. 
Q702 is also turned on by pin 15 of IC702 during standby to turn the power supply off.  On Dolby Pro-Logic 
models this signal controls the operation of the Dolby Pro-Logic power supply. 

 
Note that the above waveform has been taken on the primary side of the power supply.  It is therefore essential 
that an isolation transformer be used in conjunction with the set.  This will ensure safe connection to the primary 
side reference. 
This waveform will also change in pulse width depending upon the load demanded from the power supply.  The 
greater the load the longer the on time.  The FET will switch on at about 4.5V, this is indicated as a short 
levelling off period on the upward ramp. 
 

Sharp Electronics (UK) Limited - May 2003