Sharp CRT TV Model # 21V1-L the power LED turns on but nothing else happens-email support

Hello Humphrey!

This is Juan Rash from Belize again asking you for help on TV repair.

I am trying to repair a Sharp CRT TV Model # 21V1-L. The power LED turns on but nothing else happens.

Is the power LED ON green? If yes then it means the power is doing well and the micro processor is being powered.

There is a voltage regulator IC with 4 pins on the secondary side of the board. This is the first time I am dealing with this kind of regulator. The part number is KIA78R05PI. 

Pin 1 is getting good voltage from the SMPS transformer but there is no output voltage on pin 2. 

Pin 4 is marked SW but there is no voltage on this pin. 

Pin 4 is the control pin or switch as you have rightly put it, when things are normal it should be around 5 volts. Now what can cause this voltage to disappear? This could mean there is a problem elsewhere like over current or overheating, so check for any shorted component on the secondary side of the supply…by the way is the H.O.T ok?

I want to know if Pin 4 is output or input terminal. I want to know if pin 4 need to get voltage to turn on the regulator.  The data sheet does not explain this part. 

Pin 2 and pin 4 go to the large IC. 

Pin 4 is the control pin, 5 volts normal ON and Zero volts OFF; as I have said this voltage can go low if the I.C is in protection mode

Thank you very much for your help!

Most welcome Juan.

Last but the least Juan always when you do a repair on TV and everything seems ok  and still the set appear dead, then try to power  ON the set using remote control power on button because sometimes the TV can be ok but be in standby mode.

Let me know how your are progressing on this

Regards Humphrey

Conclusion

From what Mr. Juan has said that it is his first time to deal with this I.C KIA78R05PI I feel I should highlight a little about this I.C for other tech out there, the best place to start is checking for the datasheet of the same I.C for assistance.

I checked this I.C on the internet and this is what I got, in our case the internal structure of this I.C will give us some insight about this I.C.

From the diagram about we see pin 1 (Voltage In) is connected to Pin 2 (Voltage out) via PNP transistor (you can actually call it a switch).

If Voltage is applied to pin 4 it will move to the Base of the NPN transistor inside this IC. Once the voltage get at the base of this transistor it will cause this NPN transistor to Close, This will then take the base of the top transistor(PNP) to the ground and then voltage will appear at  Pin2(Vo).

Humphrey Kimathi

Author CRT Television repair Course