Transplanting Keys in CRT TV Repair – Model Supra ST21K33F
One very old CRT TV of Supra make without its remote was brought to me with the complaint that the customer heard a loud hissing noise and the TV became dead. As usual, I opened the TV, cleaned the inside thoroughly and removed the PCB. I saw that the anode wire from the LOT (FBT) was punctured, and the PCBs were in bad shape, with burnt marks on top and bottom, apart from tampering by previous technicians.
Since the boards looked like that of a very old technology, and obtaining replacement for defective parts would be both very difficult and costly, the best option was to replace the PCB with the readily available China Kit of ES Board. The party had not been able to get a replacement for the defective remote. So I proposed a replacement of the Board to customer, which they agreed.
I checked the tube with the new Board and found it to be working fine. Since I have already covered replacement of PCB in my previous articles, I do not want to repeat it here. (An exclusive article for fitting China Kit in detail is in making and I shall be releasing it soon)
What made this case more interesting was the fact that the old Board was having the Control PCB separately mounted on the front, instead of built in the main PCB. So, I, for the first time, studied the possibility of a transplant of the new Control PCB components on to the old Control PCB. Here are the various pictures of the old Control PCB (first two pictures) and that of the new Control PCB of the new board, and in dismantled condition (third, fourth and fifth pictures)
The old panel had ‘Search’ and ‘Sleep’ buttons in addition to identical buttons for Menu, AV/TV, Program (Channel) Up and Down, Volume Up and Down of the new Control PCB, though not in same order. First I studied the connections in the new Control PCB. I connected my multimeter at the two pins that were marked ‘key in and 5V’ in the new Control PCB, and selected ohms range. It showed 198.3 K Ohms. Then I started pressing the keys at random and noticed from the resultant values shown on multimeter, that it followed this order: Menu, AV/TV, V-, V+, P-, P+. See pictures below, which are in the same order:
I looked through the PCB and traced the following resistors connected from the key input (I mean, input to uController IC) to buttons (press to on switches) :- 2.2K from input to Menu, 2.7K from that to TV/AV, 3.9K from there to V-, 4.7K from there to V+, 8.2K from there to P-, 27K from there to P+, which all will total to 48.7K. From the last switch, which is P+, there was a 150K resistor connected to +5V, to provide a state when no switches are depressed. Thus, all these resistors put together will total upto 198.7K, which is what we read in Multimeter as 198.3, because of the varying 5% tolerance of the resistors. The +5V was found connected to the other sides of the switches parallely (In some circuits, these will be grounded. It depends on the configuration of the uController IC used. For those of you who want to learn more about Nor, Or gates etc. please visit this link: http://whatis.techtarget.com/definition/logic-gate-AND-OR-XOR-NOT-NAND-NOR-and-XNOR .)
By this method, the uController IC of the TV gets various pulses (rather high/low stages) with different timings and pulse width, and the inputs match the program built in. Many times I have admired the advancement of technology that has come in due to mixing of software and hardware in digital technology, creating the bus communications, which made the it progress in leaps and bounds. Just imagine that the two teams, one that of software and another that of hardware might be totally different, and with never a need to meet with each other!
Thus, having studied the whole circuit and its ways, I removed all the switches and components from the old Control PCB (Most of the switches were faulty) and transplanted the switches and connecting resistors from the new, one by one. The difficulty was that the sequence followed in the new control PCB and old control PCB was not in the same order. So I had to connect resistors jumping over switches in between. This was a tricky as well as attentive job. Finished the connections one by one. After switches and resistors were transplanted, I studied the circuit for LED light and that of the Remote Sensor. The remote sensor on the old PCB was having its first pin as ground, second pin as IR and third PIN as +5V in, whereas the sensor on the new PCB was having centre pin as ground, first pin as IR and 3rd pin as +5V. Luckily there was one vacant hole in the old PCB. So, I connected the two pins of key in and +5V on the first two holes, and cut the tracks and modified it to connect it properly to the new 2 pin connector removed from the new PCB.
Then soldered the three pins of the remote and once again cut the tracks and modified it to wire the sensor correctly. All these exercises required precise modifications and rewiring, each time pausing and rechecking the connections. Finally the Old PCB was populated with the new components from the new PCB, and connected exactly as it was in the new PCB.Checked the values once again in the Multimeter and found to be matching with what I found first. The only component that I did not change was the LED, as it was fixed with glue (I ignored the possibility of slight current variation in LEDs). Even the resistor of LED was replaced with the one from the new PCB, value of which was slightly different from the old PCB (Let me admit that I am not at all good in any artistic work and mainly concentrate only in accomplishing what I want. So, the connections will look crude and clumsy!) Please see the pictures below:
Forgot to mention that I opened the old sensor from its casing and cut its top shield and placed the new sensor on its base and legs. Since the base-cum-holding-case was grounded, I soldered the centre pin of the sensor to the case to provide ground to the sensor.
Connected the PCB to the Main Board after rechecking all connections once again, connected the main board to the CRT and switched on. TV worked very well and the switches on the panel as well as the commands from the new remote control were properly and accurately responded by the TV. You can see the pictures of the display, which are by operating the front control switches as well as from the remote:
(I am showing only the menu, but checked all other switches on the panel as well as on the remote and found all of them working very well). Finally did the PCB fitting in the cabinet by doing a lot of modifications on the cabinet by cutting, removing, filing etc. Blanked and covered all unused openings on the cabinet rear to avoid lizards getting into the TV set. I am not adding these pictures here, as this article has already more picture in it than desired. The TV set was kept on for many hours and tested OK and is now waiting for customer to pick at the time of writing this note. Another odd job completed functionally satisfactory!
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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