Jerking And Vanishing OSD In Sharp 21RK21N CRT TV
This CRT TV was brought to me in dead condition. At the time of bringing, and even opening and cleaning in front of the customer, who helped me move the heavy CRT TV to a place inside the house, he did not say anything about getting any burning smell.
In fact, I did not notice it until I brought the Board to Table for troubleshooting. I saw the Fuse was intact. But noticed that the FBT has a puncture on it and the PCB at the bottom portion had burnt, which I saw when I removed it. I saw one diode also broken near the same spot.
As you can see from the last picture, the number was not clear enough, but managed to get a replacement, because the electronic shop owner has a list of FBT numbers used in almost every model of CRT. After getting this in hand, when I tried to tally with old one, it perfectly matched. So replaced the FBT.
As I found strains, and burnt marks in several other places, I replaced all the electrolytic capacitors and push to on switches on the control panel. I also replaced the High Voltage Fixed capacitor in the Horizontal section, as it was found punctured.
After retouching all the solder points, which was very difficult as the board had SMD uController and many SMD components, I disconnected the supply to primary of the FBT and checked for stable power supply.
And also checked whether the Horizontal Oscillation was taking place, by placing an LED between the Base and Emitter of the Horizontal Output Transistor (I had removed it). You can see the result yourselves:
Having ensured that the B+ was ok, and the Horizontal oscillation was taking place, I disconnected the power, discharged the high voltage electrolytic capacitors, and reconnected the jumper wire to the FBT. Then connected the Board to the CRT and switched on. That is when I noticed the jittery (Horizontally and vertically shaking) OSD and vanishing of it after about one to one and half minutes. Even the adjusting control was blurry and shaking.
This can happen due to the defects in pump out of Vertical IC, the Horizontal and Vertical sync pulses to the IC or the connected components in these sections. Rarely it can also be due to the memory or IC itself. After checking all the relevant components in those areas, and even trying by replacing the Vertical IC, I decided to put a blank memory IC in place of the old one. Since this was having an SMD IC, I removed it first. Then placed an 8 PIN IC socket from the other side, as it had provisions for fitting both SMD and through hole. First loaded a blank 24C16 IC in it and switched on.
Whenever a blank memory IC is loaded, we should take care of the following: 1) Before putting it, switch off the TV directly from the mains and not using remote. This is to ensure that the TV does not start in standby mode. 2) Discharge residual power in capacitors both in Primary and Secondary of Power Supply. 3) Insert the new memory IC. 4) Switch on the mains. 5) Wait at least for a few minutes during the time of which, never ever touch any controls or remote. 6) Switch off the TV after a few minutes directly from the mains and not from Remote. 7) Then Switch on the TV after a few minutes and then you can operate it through remote as usual. If we do not follow this procedure, self-copying from the uController might be interrupted and it is likely not only to spoil the memory but the program in the uController itself. This is the expert advice that I got from my Techie Friends.
After doing this, I noticed that the original settings of the IC had got affected, as a result of which, the quality of picture was not good enough and the vertical/horizontal signals also got affected. But OSD was stable. This was a clear indication of defects in the original memory IC. So wired the SMD IC to another 8 pin socket, in which I had inserted 8 paper pins and fixed the buds with glue. This was done so that I can insert the pins of that socket into another IC socket for copying purpose. I also tested that every alteration done was ok, by inserting the IC into the socket on the PCB and switching on TV.
Then went to one of my Techie Friend’s shop and collected a memory copier, pictures of which are given below. As the instructions were in Chinese, I used Google translator for reading the instructions in English. I took the snap of the instruction sheet, and fed it to the translator in my mobile and I got the translation instantly. Then followed it to copy the memory to another 24C16 IC (an equivalent one).
(The model you see is no longer available for purchase!) After copying, I put the new memory IC in the CRT TV Board watched whether OSD did the dancing and vanishing trick. It did not!
Fed Video signal and found the TV to be working very fine indeed! Thus, another tedious but fruitful work finished and the resultant satisfaction gem jumped into its collection bag as usual!
Here is a picture of the components replaced in in the TV.
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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