No Picture With Violet Raster And No Response To Controls Solved In BPL CRT TV
Model and Make of the TV: BPL LFR20FA1
Complaint reported: Only violet raster, no OSD and not responding to controls or remote.
History: The TV was not switched on for many months, and was kept covered very well.
Action taken: Opened and cleaned the TV gently as the board looked fresh and clean with not much dust and dirt inside. Looked for any bulgy capacitors or strained components on the TV. Could not find any.
Trouble Shooting done: Downloaded the datasheet of uController IC, Toshiba TMP87CM38N, and studied the voltages present. Noticed that the Reset voltage to the IC pin No.33 was Zero. Tracked the standby and found that one transistor which controls the standby command was defective. It was not switching on to let the voltage pass through to the reset pin. Replaced it with A1015 (PNP) transistor. Switched on the TV with the same Violet raster, though this time the TV was responding to commands.
Checked the voltages to Chroma IC No.TB1238AN. Voltages were present. But the Vertical feedback signals were not alright, when checked in Oscilloscope. Suspected a problem in Vertical IC No.TA8403K. After checking all the components around, replaced it. But problem continued. In order to ensure that the RGB is ok on the Tube, I checked the voltages. It was ok. Then replaced the CRT Socket as it could develop fungi inside the pins because of lack of use. No change. Then removed the RGB connection from CRT Board, after switching off the TV, then used bypass pins to supply the + voltage and ground to the CRT Board from the PCB.
First inserted R in and got Red on the screen, then removed it and inserted the Blue and got Blue on the screen. Did the same for Green and got Green on the screen. This indicated that the CRT was ok and RGB was functioning well. Then the suspicion was the Chroma IC. After checking for any defective components around, I replaced the IC. Then the TV got on with the BPL logo and normal raster. Fed a video signal and ran the TV for hours before closing the back cover, as it passed the ageing test.
(I did not replace all the electrolytic capacitors on the board as the customer said that it was a borrowed TV and they did not want to spend too much on it. There was absolutely no dry solder on the Board! The board had a just finished factory look, which was pleasant to see!)
Conclusion: Thus the result was accomplished giving a different experience, result of which was satisfaction jumping to its collection bag!
Datasheet of uController
Vertical Feedback Signals found on the Chroma IC
The uController, Chroma IC Vertical IC
Very clean inside and fresh look of PCB
Proof for perfect working (Picture quality is not good enough due to not changing the electrolytic capacitors)
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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