Lightning Strikes On Onida UltraSlim 300 TV
Those of you, who had not read my previous article on this CRT TV, may please read it to know the previous service work done on this set; https://jestineyong.com/servicing-onida-slim-300-crt-tv/
The TV was visiting my workshop after a gap of more than two years, when it got struck by lightning. After opening and cleaning the set, which I did in front of the customer, I took his assistance to move the heavy CRT TV to my service work table.
I observed that the fuse was intact, one 4.7uF/450V Cap bulgy in the B+ primary line, and there were no other visible damages. I replaced this capacitor, disconnected the B+ jumper wire to the FBT, connected a bulb in its place and found no output from the power supply. Disconnected and discharged the Tank Capacitor and checked the primary of the SMPS Transformer using Ring Tester. Only one LED was lit, indicating some short in the primary or secondary of the SMPS. The STR was also warm, indicating a possible short in it. I replaced the STR and looked for any other shorted components in the primary or secondary. Found one transistor A1273 short. Replaced it. Checked further. As I did not find any other, rechecked the primary of SMPS transformer with Ring Tester and found all the LEDs lit! So, applied power and this time the bulb lit and the Multimeter showed around 125VDC, indicating that all was well with the power supply.
But there was no standby light! Checked the 5V and 3.3V supplies that go to the uController IC and found these to be ok. Checked for any short in Remote Sensor or standby LED and did not find any. Looked for voltages at the pins of crystal oscillator and did not find it. This was a clear indication of failure of the uController. Anyhow, in order to be sure, I checked all the active components on the board that are likely to be a cause and also disconnected the tuner feedback tracks and turned the set on again. But the standby LED was not getting on. After informing the customer about the results, decided to replace the uController. This is an eighty pin SMD IC and on close observation, I could find that the IC had burn marks under it, on the other side of the PCB. Now, as all know, it is very difficult to remove an IC, which had got bonded with the PCB this way.
But, I had no alternative but to remove it. As my attempts were not successful, I took assistance of a younger technician friend, who had more experience in removing such ICs. We tried our best to remove it using blower as well as solder-heat method, but in vain. We then cut the pins and tried to remove the IC. But it never budged! So, as a last resort, I used a wooden peg, and tried to hammer it out, which brought the IC out, but not without damaging the tracks! Afterwards, when I observed the board, it gave me a shock as it looked like a battle field and had got spoiled beyond restoration!
Very sad thing to happen! On enquiry with my technician friends I learnt that even though a China Kit for a higher price was available for use in Ultrathin CRT TVs, the result was not successful, as it failed too often and the picture clarity was not good enough. Moreover the Pin-Amp circuit used in this TV was so critical that any variation will affect the picture. Then I posted panic messages to my technician friends and enquired whether anyone had a spare board for this TV. None had. I even checked with the company technician for a replacement, but they also had no stock, as this set was discontinued long back and support was available no more. But I never gave up and finally located one with an old TV recycling guy, whom I had contacted for getting boards and CRT Tubes on previous two occasions. As I had to check the PCB using the tube, I loaded the CRT TV to my car (without bothering about the likelihood of low back strain again) and picked up my technician friend on the way and went to the shop, which is located around 30Kms away. Unfortunately, there was no power when we went there, and on enquiry with the Electricity Department, it was not likely to be restored until late in the evening. Well, you can imagine our plight! Both of us had skipped our lunch to reach here and though I could afford to wait until the evening, I could not hold my technician friend, who would lose his income for the rest of the day! So, glancing through the board and getting reassured by the dealer that it was a working board, paid the money he asked for and returned.
Back on the service bench, as a precaution, I disconnected the jumper wire to the FBT and connected the 100W bulb as a load and switched on the Board. The DC output was ok and standby LED was also lit. Disconnected and discharged the Tank Capacitor and connected the jumper wire back to IFT and fit the board to CRT. But did not find any display and observed that the switching was not taking place and filament of the CRT was not lit. On an expert advice, I removed the Horizontal Output Transistor, connected an LED at the base and emitter, and found the LED was lit.
So, the problems were in the pin-cushion circuit of the TV which is a special circuit to manipulate the sweep to suit the ultra slim tube. I replaced a couple of transistors in the section, high watt resistors from the old board, as malfunction of these can also prevent from Horizontal Oscillation. Along with this trouble shooting, I also replaced all the electrolytic capacitors in this old board and touched up all soldering joints on the board. Replaced the Pin Cushion coils taken out from in its original board to this board.
Then connected the board back after cleaning it thoroughly. I was happy to see Horizontal oscillation taking place and the set coming on with blue Raster! But there was a problem; all the four corners of the TV were drawn in. This needed service adjustment. But customer did not have the original remote. Even though I tried getting different models of Remote available in the market, I could not enter into the service mode using it. Hence there was no other go, but to leave it as it is, which the customer also agreed. The gap is not very prominent where there is video.
Thus, this work was finished with not full satisfaction, but worth its collection to the bag.
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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