Strange Symptom Found In Onida CRT TV Model CO21BLL250BG
This CRT TV had visited me for a speaker replacement in October, 2020 and those interested can see the article: https://jestineyong.com/both-speakers-found-open-in-onida-crt-tv/
I am skipping images of the TV, as the above article already contains these. This time, in November, 2023, it was brought to me with the complaint that they heard a ‘tick’ noise and the TV became dead. As the customer was my neighbor’s brother and known to me for many years, I took up the work on condition that all physical movement of the heavy CRT TV has to be handled by him, to avoid contracting another low back pain.
After the TV was brought in and kept on the veranda of the car porch, I opened the back cover and subjected it to a thorough cleaning using blower and brushes. Then I removed the main board, after which he moved the TV to a make shift arrangement made on a sturdy wooden arm chair and left. I took the board to my service table and did a thorough visual inspection. I found that the fuse was blown, which was the cause for the ‘tick’ noise the customer heard. There were dark marks on the power supply section, which was using STR G8656. I removed the STR in order to check up the area thoroughly:
Since the fuse was blown, the first step was to locate any short immediately after the AC/DC inputs. I checked the EMI filter capacitors and rectifier diodes and did not find anything wrong. There were no cracked or burst components in the area. (The small capacitor that looks like cracked in the first picture above was stuck to the heat sink and I detached it to take out the STR) The value of resistors were checked and found to be ok, for which purpose, I pulled out one end wherever it was found necessary. Surprisingly, the resistance reading across the tank capacitor (of-course, I had discharged it to avoid a shock from any residual voltage) showed 80 Ohms. Well, that was a major short and I started investigating the reasons for such a short. The ring test of the SMPS transformer showed short too. My doubt was on the secondary. So, I removed one end of the rectifier diodes in the secondary one by one and continued checking with Ring Tester.
Then I did the same with the rectifier diodes in the primary. There were only two fixed capacitors in the SMPS circuit. I checked both with my analogue multimeter and it showed very high resistance, which is common. So, I did not suspect it. I had even removed the tank capacitor to check. My suspicion therefore was on the SMPS transformer itself. I removed it and published it in my group explaining the problem and seeking a replacement. I even mentally planned to open the transformer and do a hand rewind, like how I did on a mobile adapter before, if I could not fetch a replacement.
On a feedback from my techie friends, I removed the blue capacitor 103/12KV and then the short had gone! Vow! I should have done this in the first stage itself, as this was known to be the culprit for most of the power failures in Onida CRT TVs. My over-confidence and absence of exposure to CRT TV servicing for a prolonged time wasted a lot of my time and efforts! I put the SMPS transformer and tank capacitor and reconnected the components that I had lifted and did the ring test to find a very healthy result. Then checked the blue capacitor (C911) and noticed it was showing very high resistance. Then, how and why it caused an 80 ohm reading across the tank capacitor? That remains as a strange mystery! Let us have a look of the board:
As you might notice from the above pin out details, the capacitor was connected between pin 1 (drain) and ground. When the cap itself was showing very high resistance, how it influenced to cause an 80 ohm short across the capacitor? Probably through the TX coil? Well, perhaps the experts in this forum or readers might throw some light.
Anyhow, after this mystery got dropped to the back of the mind, I put back the STR and replaced the C911 capacitor. Then did a retouch and cleaned the area thoroughly. Then disconnected the B+ coil to the LOT (FBT) primary and connected a 100W bulb as load and applied power. I was glad to see that the lamp lit up with a reading of 111.96V which was very healthy.
Then disconnected the bulb and reinstated the coil and loaded the board to the CRT TV. Then applied power and allowed it to run for several hours before calling the customer to come and pick it up. Mission accomplished with great satisfaction added with lessons learnt and an additional bonus of an unresolved mystery!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 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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Albert van Bemmelen
March 30, 2024 at 5:11 pm
Glad that you again were able to find and fix another defect tv, and help your good neighbour at the same time.
Absence of exposure to CRT TV servicing for a very long time is what certainly continues to limit my knowledge in tv repair I'm sure!
Parasuraman S
March 30, 2024 at 10:09 pm
Many thanks for your expert analitical comments, dear!
Tayo
March 30, 2024 at 6:24 pm
Another excellent article Maestro!
I just acquired a blue ring tester but Im sorry to say I havent taken the time to really understand how to use it.
This article is a big encouragement to do just that.
And sorry about your back pains. Wishing you a speedy recovery.
Parasuraman S
March 30, 2024 at 10:10 pm
Many thanks for your encouraging comments, dear! Thanks for your health wish too!
Yogesh Panchal
April 4, 2024 at 4:39 pm
Good JOB! Sir
Parasuraman S
April 6, 2024 at 9:06 am
Thanks, dear!
DougIa
November 23, 2024 at 4:54 pm
Hi Mr jesting is repairing a crt tv,it using str ic on power supply it's not power on its led light blinking
What should I do
Jestine Yong
December 3, 2024 at 8:18 pm
Hi Dougla,
I suggest that you visit the link below for more repair tips and ideas on CRT TV repair:
https://jestineyong.com/category/television-repair/
Jestine