Both Speakers Found Open In ONIDA CRT TV
This CRT TV belonged to the brother of my neighbor, to whom I had repaired a Sony CRT TV a few months back. It was brought to me with the complaint that there was no sound when he switched on the TV next day, though it was working well the previous day night. Before bringing, I had told him to check the input source and try with a different input, which he did with the same result. As usual, the TV was kept in the Veranda of my car porch by the customer, upon which I first applied sanitizer all around. Waited for a few minutes and then opened the back cover. There was not much dust, even though the TV never seemed to have undergone any service before. After doing a thorough cleaning using blower and brushes, I discharged the anode directly shorting it to the ground. Removed the board and cleaned the inside of the front and rear cabinets thoroughly. Used a cotton cloth to wipe clean of the rear of the CRT and made it shine line new. Once again cleaned the board thoroughly using various brushers. After that, the customer brought the front portion, where the CRT was fit, and kept it on my TV stand, which had rollers fit for easy moving. After seeing him off, I brought the board and rest of the parts inside.
On scrutiny of the board, I could see a lot of dry solders in the power supply and FBT (Fly back Transformer) area. I saw one capacitor burst in the power supply. I saw another in the Vertical IC section. This surprised me, as the customer’s report was that the Video was perfectly ok. I checked both the speakers and found these to be open. Here are a few pictures:
This model was using TDA7297 IC for its sound. Whenever speaker failure happens, it is advisable to change the IC. So I replaced the IC. Replaced both the speakers. As the size was not exact, adjusted the rubber washers of the new speakers slightly away by sliding these out and provided a couple of metal washers to hold it in place. That ensured a firm fit. Did all the dry solder patch up. Replaced the burst capacitors as well as other filter capacitors in the Power supply and EHT section. Also replaced the capacitors on the CRT PCB.
After rechecking and ensuring that everything was looking fine and there were no man-made shorts while doing the re-soldering, loaded the board to CRT and applied power. I saw healthy blue raster and the OSD was also sharp and clear. So, applied Video and got a perfect picture and sound. I checked the B+ voltage and found it to be 115V. As a precaution suggested by my techie-friends, provided 470/25V electrolytic capacitors to one side of the speakers. This Sound IC has separate outputs and one end is not grounded like in many other cases.
The output has half of the supply voltage at all the four pins of this IC. So, in case of a power fluctuation, high voltages reach the output and speakers fail. This was a common failure according to my friends. Sometimes, only one speaker would fail and the other would fail after a few days or weeks or even months. Customer would notice only when it fails completely.
This case also looked like that. Anyhow, after running the TV for several hours and ensuring that it worked perfectly well, called the customer to come and pick it up. Here are a few more pictures as a conclusion to this article, without forgetting to add satisfaction to its collection bag. Also seen in the pictures is the snap of components replaced.
This article was prepared for you by Parasuraman Subramanian from India. He is 70 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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