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.
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
HEATER = 0 V.
There is no HV.
In which section could this problem be?
Check if the main I.C/microprocessor is getting powered.
Thank you very much sir.
Microprocessor was not getting powered. After supplying the power TV turns on properly.
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