Restoration of a Samsung CRT TV by Rejuvenating CRT
I got a Samsung CRT TV Model CB20E4V2X with the complaint that the picture diminished day by day and only sound remained, after which it became dead completely. So, I went through the routine of confirming the complaint by switching it on at my desktop, opening the cover, cleaning it up and removing the board for a visual inspection, after discharging the anode. I noticed that a few capacitors were bulgy in the secondary, 125V line, CRT PCB and around the LOT. The LOT (Line output transformer or FBT – Flyback Transformer) itself was not in a good shape, as the wire going to the anode was wrapped with tape and had collected a lot of black dust. You can see it from the first picture I took before removing the board:
Nevertheless, I cleaned the CRT as well as this LOT to the best possible limit. Replaced the capacitors. Noticed dry solder on the board as well as on the CRT PCB.
Patched up all the dry solder on the main board as well as the CRT PCB. Cleaned the board and CRT PCB thoroughly. After doing another visual examination, reconnected the board. The set came on, with completely dark screen. When I increased the screen voltage, there was mild green raster with retrace lines. Neither the menu nor any other uController based displays could be seen! When I switched off and did a discharge of the anode, there was no EHT discharge, which showed that the LOT must be weak. So, I decided to replace the LOT.
Got a replacement for as low a price of Rs.220 (about 3.25 US$). Replaced the LOT. Simultaneously followed the testing methods described in CRT Television Troubleshooting Guide by Humphrey Kimathi (Chapter 11: HV is present but the screen is dark). Heater voltage of 9V AC was ok. There were no resistors on its path. After checking the other points mentioned by him like screen voltage, RGB voltages etc, replaced the TDA6107Q IC on the CRT PCB. Then tracked the ABL circuit. Noticed that one resistor had increased its value. Replaced it. Connected the board, with the same result. So, once again discharged the Anode and found there was a crackling noise indicating that enough EHT was present now! Removed the board and connected it to another CRT that I have for testing purposes. The raster was present!
So, there was nothing wrong with the board now! I had connected a China ES board to the Samsung TV CRT and found green raster with retrace lines, confirming that the tube indeed was defective. In the Samsung circuit, there is a feedback sensor that shuts down the system, if there is an imbalance between the guns. So, it will not open out RGB. Whereas, in the China Kit, there is nothing like that! I turned my attention to the CRT and guessed that the RGB Cathodes were weak. I had come across a guidance given on the inner cover of every Modern Colour Television Circuits released by BPB Publications, New Delhi, about rejuvenating old CRT. Here are the circuits:
I bought one eliminator box, and other components mentioned there in and assembled the CRT rejuvenator. This took many hours, as drilling on the cabinet and fixing the knobs, transformer, bulb holder, wiring, connecting a CRT socket securing the loose wires on an insulated wire to the ground pin of the CRT socket etc. are all a laborious job! The whole thing cost me around Rs.600/- (around 8.60 US$)
The knobs have 6V, 9V and 12V AC selector for the heaters and Blue, Green and Red selection for the RGB Cathodes. The methods to be followed are as given below:
Discharge the anode and remove the board. The CRT should be bear without any connections on its neck. Insert the CRT socket. Select the desired heater voltages (in this case 9V), and select any one of the colours, and switch on. As soon as we switch on, after the filament glows, the 15W bulb will glow momentarily and might blink a few times and go off. This is the time the carbon deposits inside the CRT gets burnt or displaced. After a few minutes, the filament of the 15W will start glowing slowly and become very bright. We have to wait for about 3 to 5 minutes after its full glow. Then switch off and change the Cathode selector switch to the next colour.
Repeat the process. After completion of the charging of 3 Cathodes, once again check whether these are properly charged, i.e., whether the bulb glows to its full within a few seconds of switching on and stays steady. If not, we have to repeat the process several times, until this stage is reached. It might take several minutes and even hours. If the cathodes are not getting charged, we may have to increase the voltage of the filament to 12 and retry. If there is no glow of the bulb, it means that the CRT cannot be rejuvenated.
In my case, the Cathodes got rejuvenated very well! See the bulb glowing (I used a 15W, 230V AC thread type bulb connecting it to an adapter to fit in the bulb holder)
After finishing the recharging, and rechecking each cathode once again, disconnected the rejuvenator and connected the board back.
Caution: When the board is connected and switched on, there were rare cases of heavy internal arcing and neck cracking. So, as a precaution, it is suggested by one of the experts in the field to disconnect the supply to the yoke, keep the focus and screen control to the minimum and then switch on. After switching on, the focus and screen control should be increased slowly until we get a spot on the screen. This spot should not be allowed to be there for too long, as it will burn and leave a permanent mark on the front screen. The moment we see the spot, just switch off, remove the power plug, discharge the anode and then connect the yoke. It is learnt from experience, this process will eliminate the chances of internal arcing due to carbon deposits falling on wrong places, when the recharging of the tube takes place. In yet another place in the website, I remember to have read that we should keep the tube on its face for sometime, and tap the neck with a rubber mallet or with a screwdriver handle wrapped with a rubber ring, gently so that the carbon falls far away from the neck. I did both before switching on.
Hurray! The TV started working like a new one! The CRT guns started shooting (I have deliberately selected an appropriate picture to imply it, rather to give a James Bond Victorious effect!)
Here is the picture of the components replaced:
Another job done satisfactorily, learning many things new in the process!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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