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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You may check on his previous repair article below:
https://jestineyong.com/transformer-failed-in-usb-player/
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Mathew Charles karonga
November 21, 2020 at 3:48 pm
Thank you for a detailed report but this seems too much labour for a CRT how much will you bill customer considering the costs of CRT tv nowadays
Parasuraman S
November 27, 2020 at 8:58 pm
I recover the cost of spares and depending on the work, I charge the customer and my customers do pay me without raising an eyebrow!
Many thanks for your comments!
Tyrone Arendse
November 21, 2020 at 3:59 pm
Very good troubleshooting technique, but how is the capacitors connected to the speakers
Parasuraman S
November 27, 2020 at 9:00 pm
One end to IC and another to speaker. The other end of the speaker is connected to the IC. No need to bother about polarity as both are coming separately and one side is not grounded like in other circuits. But I connected the + to IC and minus to Speaker.
Many thanks for your comments!
Waleed Rishmawi
November 21, 2020 at 5:02 pm
you are the king of CRT TV repair especially when are hare old and dirty. good job my friend although too much work for a CRT tV..i hope it was worth the repair. have a blessed day
Parasuraman S
November 27, 2020 at 9:01 pm
Many thanks dear friend! Yes, it was worth.
Albert van Bemmelen
November 21, 2020 at 5:19 pm
Another very nice repair'story' to read! It is obvious that for the extra cleaning service you again provided it couldn't have been in better hands! Only the part when you "Used a cotton cloth to wipe clean of the rear of the CRT and made it shine line new" worried me a bit because as you know the graphite layer on the outside of the glass cathode ray tube must be kept intact. Since it is used as acting flattening capacitor for the HV generator. And if your cleaning is as good as your repairs are, removing that not so shiny layer would of course be a mistake.
Parasuraman S
November 27, 2020 at 9:03 pm
My cleaning the outside of the CRT rear is not that hard to remove the coating! It only just cleans the dust formed. Nothing else is damaged. You concern is genuine! Many thanks for your comments and support!
babums
November 21, 2020 at 10:46 pm
Very good repair work and boosted crt tv performance.I want to know why the capacitor 470uf/25v is connected in series to the speaker. Thank you very much for sharing the article.
Parasuraman S
November 27, 2020 at 9:05 pm
The output is protected in case of a voltage surge. This is a suggestion given by my expert technician friends. Many thanks for your comments!
Yogesh Panchal
November 22, 2020 at 12:48 am
Sir,
Good Repair with modified speaker precaution.
Parasuraman S
November 27, 2020 at 9:05 pm
Many thanks!
Henrique J. G. Ulbrich
November 22, 2020 at 2:10 am
Another successful repair. Greetings! My comment this time is due to the capacitors inserted in the loudspeaker connections, a wrong decision in my opinion. I see three problems on it. First, it´s a bridge amplifier (TDA 7297), which means that this inserted capacitor will work directly with alternating current, not a variable DC current, as found in other types of amplifiers. Obviously, it is not an ideal regime for electrolytic capacitors, that will work out of their normals. The second issue is related to the work philosophy of a bridge amplifier itself. Such type of amplifier has intrinsic resources to keep the same DC voltage at the loudspeaker terminals, which impedes the loudspeakers to receive DC (the only variations observed are those of the signal being amplified). This DC stability is obtained by keeping the four non-inverted inputs at the same potential (internal point Vref of the IC) - see its data sheet at https://www.st.com/resource/en/datasheet/tda7297.pdf. If DC is found imposed to the loudspeakers terminals, by sure it is an IC problem, which shall be replaced. A third comment is related to the natural absence of the capacitor at the output of bridge amplifiers. They are not necessary in good quality (and stable) bridge amplifiers, with the advantage in this case that the response to low frequency (bass response) is optimized at the maximum.
Parasuraman S
November 27, 2020 at 9:11 pm
Many thanks for your detailed technical advice. I have carried out the modification as advised by my techie friends, who do it out of their true experience. Yes, I do agree that there will be some change in the frequencies. But that is better than failure of the speakers often. According to my friends, this solved such problems. So, I just followed it!
Humberto
November 22, 2020 at 11:01 am
Another more CRT TV to your collection and satisfaction bag.
Parasuraman S
November 27, 2020 at 9:11 pm
Many thanks!