Rejuvenated A Weak CRT Tube Of Samsung Model KS9A21M16 And Fixed Broken Back Cover
This CRT TV was brought to me by a new customer (another recommendation done by a satisfied one, related to him) with the complaint that there was no picture, but sound was ok. Sometimes bizarre colors were seen with picture but disappeared within a few minutes. Sometimes, the picture did appear after a prolonged time, and stayed for the day. But same problem was repeated. Well, to me even though the reason for this was obscure, I started from my first routine of thoroughly cleaning the inside. Just see how bad the interior was in this TV:
After thorough cleaning of inside using blower and wiping the hard-to-go- away-dust with brush and cloth, I brought the PCB to my work table for investigation. The PCB was removed from the CRT TV after discharging the anode to ground. This is a precaution to be taken while handling CRT Tubes, as the large capacitance of the CRT anode side would sometimes retain some charge, though there are bleeder circuits.
On visual examination, I found two 1000uF/25V Caps slightly bulgy. One 681/2KV Capacitor that comes as a feedback from the collector of the Horizontal Output Transistor to the uController, was found cracked. I replaced these caps first.
The PTC was found cracked and I replaced it too. Then did my routine of retouching the board from one end to another; CRT PCB, Vertically fit Hi-Fi Sound Processor Board (I removed it) and the Video Input board on the side. I did this after replacing all the electrolytic capacitors on the board, because of the peculiarity of the complaint reported. This will rule out possible filter capacitor problems. In the process I had a close and thorough look at the tracks for any tear or wear.
I replaced the bleeding capacitor of the screen value of which was 472/3KV, which could cause a dark screen. I also replaced the 681/630V Cap in the ABL section, as I noticed it was strained. Cleaned all the boards thoroughly using IPL. Looked for any man made shorts, using a lens.
After completing all the precautionary checks and refitting the Hi-Fi Sound Processor Board, loaded the Board to the CRT and switched on. There was no raster. I was groping in the dark as to the real reasons for this situation, because the B+ of 125V was ok, the RGB voltages on the CRT PCB were ok; the 180V on the secondary of FBT was ok. So, there could never be a problem in the secondary or primary side or in the power supply. The secondary voltages of the SMPS were also ok. The reason was RGB was not opening up from the uController as it goes to some protection. After intercommunicating with my techie friends, who always come to my rescue, I carried out the suggestions given by them one by one. (1) Charge the CRT guns first which I did:
As you can see all the three guns, Blue, Red and Green were showing peak results evenly. Those of you who are interested in knowing details of the CRT Rejuvenation, please read my article: Restoration of a Samsung CRT TV by Rejuvenating CRT | Electronics Repair And Technology News (jestineyong.com)
After taking precautions before switching on the TV, as described in my previous article on rejuvenation, I loaded the board once again to the CRT and switched on, with the same result of dark screen. As you might have noticed from the other article under reference, I had success in getting display on the screen in the same Samsung CRT TV, but a different model. But in this case, I was not lucky enough! So followed the next suggestion given to me : (2) Keep the screen control at low on the FBT (3) Keep pressing the channel button up or down repeatedly, while adjusting the screen voltage very, very slowly. According to them, the Samsung CRT TV gets locked and RGB drive opens up only if it gets proper feedback of screen voltage setting. Since manually pressing the front panel button was not possible, as I had to insert my fingers taking due care not to go near the EHT wires, I fetched a new remote (customer had lost the original remote and did not bring one with the TV) for easy control from a distance. Another method suggested by them was to go to service mode (switch on TV, wait for filament to glow, press standby, followed by info, then menu, then mute and then standby) adjust G2 to ok level, while turning the screen voltage. Anyhow I never tried this. After making futile attempts to get RGB opened by following step (3), I went to yet another suggested option of (4) shorting the 3.3 Ohms current limiting resistor that goes to the heater from the FBT. Then I changed over to Video mode and played a Video, upon which sound came, indicating that all was well with the TV except the screen.
Then when I kept the screen control to the minimum and slowly turned it, the RGB opened up and display came on, bringing smile of relief on my face. So, I did not have to look into feedback from ABL/Vertical/Horizontal etc. which are other areas that play an important role in this kind of problem. Adjusted the focus to its perfection. Then allowed the set to run for a few hours. I reported my progress to the group and thanked them profusely for guiding me to success! (Shorting the resistor thereby slightly increasing the voltage to the heater was the only way to bring life back to the tube made in 2003!)
Now the next job was to patch up the cracks on the back cover, caused by a fall or rough handling by the customer or technicians who handled the TV before (The board had enough evidence of previous repairs!). The cracks were multiple in nature and ran crisscross! In some places, the access from inside was difficult, and in some other from the outer. I applied Fevibond (a type of rubberized compound similar to quickfix) then kept the pieces together in its place. Then applied Feviquick (similar to superglue). Then used my soldering iron and inserted cut pieces of component terminals which I store, crisscross at several places along the lines of cracks. In some places, I could add strength by doing it from inside also. Well, this took almost about two hours for me! Though the finish looked very clumsy, the result was good, because the back cover got held in place strongly, when fixed it! Here are the ‘nasty’ pictures of this crude operation: (LOL)
As my trouble shooting and rectification took days, the customer’s old aged parents were finding it to manage without an entertainment in the house, especially due to social mingling restrictions. The father was my old classmate (something I learnt late!) and I hurried with the last job without bothering about doing some filing and polishing the stitched up ‘wounds!’
Now, finally the much awaited picture of the toll: (Oh, I forgot to mention that one speaker was found defective and I replaced it!) Well, well, now time to declare that profound satisfaction got added to its collection bag with a bang! Goodbye from your beaming and whistling SPR, until we meet in the next article! (I was being addressed to as ‘SPR’ by my friends as well as foreigners when I was working in the US based company!)
This article was prepared for you by Parasuraman Subramanian from India. He is 71 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/funny-tuner-ic-complaint-found-in-sony-car-cd-player/