Monday, November 23, 2015

Videocon 21' TV is not turning on

Usually I get a lot of inquiries from my customers and non-customers with regards to CRT television repair, for me to give the best possible solution to the problem I usually need crucial information from the tech.to make any serious conclusion.

The most important information to analyze a technical problem is actual voltage testing figures, the beauty with figures is that they never lie.
This is the data/information I process in making any suggesting as to the circuit causing the problem in question.

This means the accuracy of my advice will actually depend on you (tech), in fact 90 percent is on your side because you are the person on the ground.
Let us look at this question from one of my customer from India Mr.Mahadev Pawase.


Hello Sir,
I have Videocon 21' CRT TV for repair. The symptom is there is nothing on screen also front led is off.
I have checked voltages
RGB Supply= 128V
VR= 67V
VG= 67V
VB= 66.8V
HEATER = 0 V.
B+= 129V

There is no HV.
In which section could this problem be?
Please guide.Regards

Check if the main I.C/microprocessor is getting powered.
Regards Humphrey

Thank you very much sir.
Microprocessor was not getting powered. After supplying the power TV turns on properly.
Regards

Note that this technician has done his homework very well and to the point, Now let us try to analyze these figures (voltage) provided a bit.
First of all to be able to solve this kind of problem it is important you get the circuit diagram of the TV in question and in my case I was not lucky so I decided to use common sense.
The most important voltage on the secondary side of a CRT Television is B+ and this is present at 129 Volts Dc which to me is also normal for 21 inch Television.
He has also provided the RGB supply at 128 Vdc, this voltage is suspicious because normal voltage for this circuit is 180-200Volts.
Looking at voltage B+ 129v and 128 RGB supply, you can see this voltage is almost the same, this made me to conclude that the fly-back is not working.
Why? This is because a fly back works like a step up/Down transformer and this B+ of 129V  goes in the primary of the fly back transformer coil and out after being stepped up to over 180 Volts used for the RGB circuit and what we have here is In-129v and out-128 volts.
We can conclude that the drop in 1 volts (129-128) is due to a small resistance of the primary coil.

See the figure below for more info on this.




Another Voltage which made me to conclude the fly back is not working is the heater voltage which was registered as Zero. If the fly back is working one should expect 3 VAC heater voltage, please note this voltage is not DC but VAC and therefore when measuring it your meter should be set to VAC mode.
Another nice tip he also provided which made me to point him to check the supply to the micro/main ic is this (The symptom is there is nothing on screen also front led is off).

Front LED is off! Actually if he omitted this I could have pointed him to the drive circuit which also causes the same symptom…NO HV

So I figured that if the Power On led which use the same voltage with the microprocessor (5Vdc) is off then most probably this voltage is also not getting to the microprocessor.

Please note that in some TV models the microprocessor is part of the main I.C (Jungle) that is why I directed him there.

All the best guys

Stay safe

13 comments:

  1. Thanks for this contribution, we are frout of you sir.

    ReplyDelete
    Replies
    1. Hi Bala, Thanks and thanks for passing by my friend.
      Regards Humphrey

      Delete
  2. Hi Mr Kimathi
    Thanks for this very analizingly article .
    Beh

    ReplyDelete
    Replies

    1. Thanks for your feedback Beh.
      Regards Humphrey

      Delete
  3. Replies
    1. Thanks and welcome Gideon
      Regards Humphrey

      Delete
    2. Hi Humphrey,
      That experience has also taught me something. I am currently working on the 14 Inch TV with standby light on but no HV. The B+ voltage detected is 110v and RGB voltage 110v too.
      But would you
      please advise me on how to measure the voltage at the base of either HOT or SOT by using DMM. When I try to measure that with DMM range at 2vdc I get 0v. Then when I change to ac lower range, I get fluctuating figures.
      How best do you measure these voltages. I am trying to match them with the correct voltages that are found in your book but fail. Please assist!!!
      Peter Bwalya Mutale

      Delete
    3. Hi Peter, long time, hope you are doing well that end, regarding your question on testing the base voltage of HOT or SOT, for the HOT the Dc voltage is very low but current is high courtesy of the drive transformer which act as impedance matching..usually this transistor fail by shorting collector-emitter junction and therefore if you measure that with a meter in diode mode and find low and high reading you can assume the hot is okay. for the SOT transistor here i expect some reading like 0.6 volts but if it is switching will get some funny reading but definitely not zero.in AC mode you will always get funny reading when both are switching but this is not recommended.
      Regards Humphrey

      Delete
  4. Great artice. i really appreciate it.LG-Rp20ca10-chassis-mc83c-sm which has the res standby light but when i press the switch it goes off but nothing else happens. all voltages are registered but really don,t know what else to do. can somebody help. the switching is not staying on.even when the switch is pressed the other voltages are on up to the tuner. please help. the schematic can be found on the net. i could not email it. thanks

    ReplyDelete
    Replies
    1. Hi Cj, all voltages are registered is not good enough for me to make conclusion, is the B+ present? if so is it normal? start from known to unknown.
      Regards Humphrey

      Delete
  5. if power LED is flash or blinking what that mean ?

    ReplyDelete