Repeated Failure Of FBT In CRT TV Solved
This Daewoo CRT TV was brought to me by a new customer, under the recommendation of another customer, whose TV service was in my satisfactory bag! The complaint was ‘phut’ ‘phut’ noise heard from inside and at that time, the display jumps up and down with a lot of lines across.
I opened the set in front of the customer, who had brought his friend with him, as he himself was undergoing treatment for kidney ailment. Cleaned the inside thoroughly, after which, his friend carried the set and kept it on my usual make shift arrangement of a solid wooden chair, with wooden plank kept across the top of the arms (You can see the chair in one of the snaps that follows). They left leaving me to carry on with service.
After noticing a lot of dry solder on the board as well as CRT PCB, I did a thorough patch up of all points on the boards. Disconnected the B+ to the FBT primary, and connected a 100W Bulb in its place and powered the board and found that the 110V was perfectly ok. I also checked the other voltages of the SMPS and found these to be ok. So, switched it off, discharged the Tank Capacitor and reconnected the jumper to the FBT. I also cleaned and lubricated the connectors as well as the anode area of the CRT. This TV was using LA76931 as its uController IC.
After connecting the board to the TV, and rechecking all connections, I switched on and heard the ‘phut’ sound, which was as a result of neck arcing of the CRT, with very bright light on the screen with vertical lines. Switched it off, kept the screen voltage at minimum on the FBT and switched on the TV, with the same result, including the very bright light on the screen and vertical lines. Switched off the TV, and discharged the anode and removed the board and brought it back to my work table. Such problems can be from the ABL (Automatic Brightness Limiter – Automatic Blanking Limiter – Automatic Beam Limiter – These are a few full form of ABL. I am giving this information, as there was a question by one reader in a previous article asking what ABL was. Here is an interesting article in Jestine Yong’s blog: http://www.electronicrepairguide.com/flyback-transformer-abl-pin.html). I checked and ensured that there were no problems in the ABL Circuit. Since I did not want to take any chances of another firework, I also replaced the FBT. After keeping the Screen and Focus controls on the FBT to minimum, connected the board to the TV.
Switched on, and this time no fireworks! Pressed the Menu button, and slowly increased the screen voltage to get a good display. Then adjusted the focus for a sharp picture. Connected a Video input and noticed a good picture, but the vertical height was slightly less. In order not to take chances, switched off the TV, and discharged the Tank Capacitor and replaced the caps and Vertical IC. Then switched it back on. The picture was alright this time. I also bought a new remote control for the TV, as it was not having one. (In one of the snaps below, you can see the wooden chair referred to earlier in the article)
Left the TV on for several hours and watched it on the mirror kept in front of me in the computer, which is my usual practice. On one of my glances, I noticed that the TV was dead. Rushed and checked whether there was anything burning. Nothing was found. Checked the 110V and found it to be fluctuating. Switched off the TV immediately. This is a clear indication of some overload. Discharged the anode cap and removed the board and checked what went wrong.
When I used the ring tester on the FBT, it was found to be short (See first picture below). So, the new FBT has failed! When a new FBT fails, there could be many reasons, as the loads on it are Horizontal, Vertical and CRT. So I removed the FBT, rechecked all components in these areas. Nothing was found to be wrong. As you might have noticed, I used the same number and brand. But still it failed. The earlier one, which looked like a recent replacement by another technician, also had failed. So, it was concluded that the new FBT was of a poor quality or a batch failure.
Bought another FBT of a different brand but with the same number. See how the Ring Tester showed the test result:
Combed the power supply area and replaced the filter capacitors across the four rectifier diodes, though I did not find anything wrong. I also replaced the tank capacitor, as any ripple in the DC can affect the saw tooth generation of the FBT driving it to exceed the limits. As a precaution, I also replaced the Horizontal Output Transistor 3D2102 with MD1803, which is a better sturdy transistor. (I am providing the link for datasheet of both the transistors: https://pdf1.alldatasheet.com/datasheet-pdf/view/410678/JSMC/3DD2102.html http://pdf1.alldatasheet.com/datasheet-pdf/view/151450/STMICROELECTRONICS/MD1803DFX.html
Both transistors have a diode and resistor protection built in.) Removed one side of all the resistors in the vertical section and checked the values.
After ensuring that there was nothing wrong with any of the circuits that would cause another failure, I fixed the FBT and reconnected the boards to the TV. Switched on the TV and adjusted the screen voltage and focus like before. It gave a good picture. Allowed it to run for several hours every day for three days continuously. It was working very well. So, called the customer to come and pick it up, which he did happily. As the cost of components were too high, and the customer was a patient who was in rest due to medical treatment, I waved off my service charges. Here is a picture of the set working and another that of the components replaced.
Another job completed to my satisfaction and got added to the list!
This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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