crt tube replacement and total restoration of sharp ctv 51en7
This CRT TV belonged to a person running a grocery-cum-vegetable shop just in front of my house, whom we know from my childhood. The TV was dead and lying in the attic for many years. Though he wanted me to make it ready and sought my consent to bring it, it took a few ‘years’, before that was finally done! That’s how this CTV reached my table on the 30th of June, 2019! Need not say that the TV was in a very bad shape. A lot of dirt, dust and cobweb mixed with rat urine and excretions were found on opening the back cover. After two hours of thorough cleaning work, his assistants moved it inside the sit-out of my house (normal ‘delivery’ room for all finished devices). The photograph you see above is after that.
I continued cleaning the PCB thoroughly using various brushes and blower. Saw a burst capacitor near the CRT Socket. After this entire marathon cleaning work, I took the PCB to my worktable to check whether it has some life left in it to revive. I disconnected the B+ to the FBT and loaded a 100W Bulb in its place and applied power through a 200W Series Bulb. The 200W Bulb remained lit indicating that all was not well in the power supply. This TV was using the very old PWM IC – TEA2161, uController IC – PCA84C844P, Chroma IC – TDA8361, Memory IC – PCF8582E, Vertical IC – XO640C etc.
I checked the SMPS transformer with ring tester and found it to be OK. As a part of reviving the TV, I replaced all the suspected components and capacitors in the PS. Switched on to see the same result. So, checked the startup voltageat pin 16, which is strangely through a direct tapping from AC input, with dropping high watt resistors. But no external rectifying diode was used, as it is built in the IC. The Feedback positive voltage is fed to the same pin, but taken to pin 15 through a limiting resistor. Looks like the function of pin 16 is disabled once DC is present in PIN 15. After studying the technical specifications of this IC, let me tell you all, I really liked it! Please see an extract from the datasheet:
Though this was found normal (around 16V without load), switching was not taking place. So, I replaced the IC, upon which, it started working. After getting relieved that the PS was working, and the secondary voltages were ok and the uController was also working (standby LED Lit and responded to control switches), disconnected and discharged the Caps. Replaced all the control switches, electrolytic capacitors on the board, and the fixed capacitors in CRT PCB as well as in the Horizontal high voltage section. Also replaced strained and suspected resistors, including the POTs on the board. Did a thorough dry solder patch up by retouching all the points on the board. Cleaned the PCB thoroughly and looked for any man made shorts, found and rectified in a couple of places and then reconnected the jumper wire to FBT.
Found the focus wire to CRT Socket attacked by fungus and it had come off. Cleaned and replaced the CRT Socket also, as it might have fungus formed inside. The first picture is that of the corroded capacitor found in the CRT PCB, which was mentioned earlier.
Connected the board to CRT and checked. The Raster came on for a brief time, but suddenly the vertical size shrunk to the middle, indicating that all was not well in the Vertical circuit or driver circuit. I checked the voltages for the Vertical IC and found this to be normal (24V DC). Replaced the IC with LA7830, which is equivalent to the one used. As I had similar CRT TV circuits, compared the voltages at various pins and found these to be ok. Replaced all the resistors and fixed capacitors in the Vertical , drive and feedback sections, as failure while in use is something that cannot be ruled out in such cases. Checked the voltages at pin 41, 42 and 43 of TDA8361 and found these to be abnormal. Even when the wave forms were checked, these were not correct. Disconnected the Vertical drive point that goes to Vertical IC and touched the input with a screw driver, holding it on the metal side (removing sandal and touching my foot on the ground) and got full raster, indicating that the Vertical section was working well, but the output from the IC was not good enough (Caution : Please be very careful in doing this, to avoid accidental touching of other high voltage circuits around). This type of touching feeds some signal (50Hz or so) and that is enough to drive the vertical IC. Checked the feedback circuit and Sync circuit that goes to uController for OSD, and did not find anything wrong. As the Chroma IC TDA8361 was suspected, I replaced it. Then the Vertical problem got solved. Here are some date, wave forms and other snaps for reference purposes.
I had reinstated the original Vertical IC, and found that also was ok. But while removing the LA7830, one of the pins developed a minute crack. I therefore decided to discard it.
Once the vertical problem was resolved, I reconnected the board to the CRT and allowed it to run for soaking test. After an hour or so, heard a cracking sound and the TV went off with blinking LED. I disconnected the power immediately and looked for the source and was shocked to see that the CRT had developed a crack near the anode cap! What a sad thing to happen! These types of cracks are very rare, but likely to happen, when a CRT is lying in humid conditions for a very long time, and the minute molecules of moisture gets settled between the metal cover and glass. The anode portion gets very hot quickly and there is a chance of this type of cracks, though very, very rare. In my experience, this is the second CRT out of several hundreds of TVs I have handled. First the crack developed only near the anode socket, but slowly it got extended further. As the CRT Tube is designed not to explode (it will only implode), there is no danger as such. But if the anode connector comes off, and touches any ground path, it can cause fire. But anyhow, the protection circuit will definitely turn the PS off.
I informed the customer about this and he agreed to my proposal to hunt for a replacement tube with old TV dealers. After combing several possible places including my techie friends, I decided to get it from an Old TV dealer located around 30 KMs away from my home. I loaded the front Cabinet which was fit with the CRT (I had removed the yoke and other connections from it) into my car and drove to his place. I had carried the board with me for checking the replacement tube from any suitable old TV. This was a 20” Flat CRT Tube, black stripe made by JCT. He showed me a working Sharp TV. When I checked the contrast, it was in maximum and even then the picture was not sharp enough. So, the owner went to his home and brought an LG CTV, which had a better tube (Samtel, Blackstrike 20”) in it. But the board in it was defective. So, he connected the board carried by me and checked. The tube was alright. Paid the money, after bargaining. He was kind enough to remove the old CRT from the cabinet and fit the replacement tube in its place. He had two full time technicians, who repaired old TVs for resale. Since Kerala has many people coming from other states for casual labor requirements, these people buy such reconditioned TVs, as they require it only for a short period, say for a few months. There is a good market for such old TVs.
Brought it home and fit the Board and adjusted the focus, screen voltage etc. and also the brightness/contrast to get a good picture. The result was more than what I expected it would be. Have a look:
Here is the picture of parts replaced (sans the TDA8361, which I forgot to put in the tray)
Thus the curtain fell on this long story; as in any classical comedy, the king and queen lived together happily thereafter (In this case the CRT and its board)! (LOL) But the ‘satisfaction intensified’ rocketed itself to its 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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Albert van Bemmelen
June 29, 2020 at 1:11 pm
You still are a very good crt TV repair engineer Parasuraman! If not the best! You didn't even stop after also the cracked crt tube needed to be replaced. (I believe I didn't receive this repair or its newsletter it was in? And maybe that is why no one commented on your superb repair with so many failing components?)