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Dead CRT TV brought back to life and how it snubbed my nose!
I got one Nikai NTV2102 with the complaint that it went dead after seeing only a vertical line on the screen, subsequent to which even sound went off.
So, no light and sound! As usual, I opened the set and did a thorough cleaning of inside, keeping it out on the Car Porch, with a blower and set of brushes. I noticed that one 220uF/160V was bulgy on the top in the B+ line that goes to FBT (LOT) and HOT. Took it out and see, what it measured:
Replaced the capacitor. Since the fuse was intact, and there was no further visible damages, except burnt dark marks in CRT PCB as well as around FBT and SMPS areas, where dropping resistors are used, I connected the TV to mains and switched it on. It came on but with a 1″ gap at top and bottom, indicating problems elsewhere.
So, disconnected the PCB after discharging the anode cap, and started probing for the ‘culprits’! (I did not suspect anything wrong with the vertical IC, though the CTV was reported to have died after a vertical line, as the SMPS was starting, LED light was glowing, no fuse was blown, and there indeed was a raster though slightly narrow and further I had checked for any shorts using my analogue multimeter. The ring test on the yoke also showed no fault)
Checked ESRs of capacitors in the SMPS, around FBT and CRT PCB. Changed all capacitors which were found with its ESR value crossing the limits. Did a thorough dry solder patch up and board cleaning. Removed the jumper wire that goes to the Horizontal Output Transistor (HOT), and connected a 100W bulb as a load, connected to mains and switched on. Nothing happened! The set was now completely dead! By trying to be a very ‘smart’ technician with a lot of confidence and handling things so easily, with varieties of testing gadgets and methods with me, I got a snub on my nose, perhaps the CTV would have smirked, if it could!
So, after taking a break to play some games in the computer, I turned back my attention to the TV. Checked fuse and then ensured that B+ voltage at the Tank Capacitor was there. Yes, it was reading almost 280VDC. This set was using a 330uF/400V capacitor in its SMPS. Discharged the capacitor. Used my Ring Tester on the SMPS Transformer and noticed that it was lighting upto the yellow LEDs and not full. Well, that could happen as all the Transformers would not show all LEDs lighting up due to its load on the secondary, and the feed back circuit on the primary. This SMPS was using IC TDA16846P as its PWM controller, with IRF840 mosfet as driver. So, checked all components in the SMPS primary one by one. Noticed that one resistor was showing a funny reading! The colour band indicated Brown, Green and Red, for a 1.5K resistor. But see the reading, it was showing 7.46K.
Checked all the diodes and other resistors and capacitors by pulling out one end of it! (I had already replaced all electrolytic capacitors except the Tank Capacitor in the SMPS primary and secondary section) All else were OK! So, replaced the resistor, and with the feeling of a Police Inspector, who caught the thief, I switched the CTV on, but, again, nothing happened! It remained dead, or shall I say, deader? Another chance for the CTV to smirk! Another vexed snub, and another round of games in computer!
I continued my probe for the fault on the next day, starting fresh. Removed the SMPS transformer this time. Checked it with ESR meter as well as Ring Tester. It was ok. Checked all the secondary diodes and components by pulling out one end. OK. So, removed the mosfet, and connected an Universal Module and switched it on:
This proved that the Secondary side and the SMPS Transformer primary winding were all ok. Removed the PWM controller IC and placed a 14 pin socket in its place. Replaced the Run DC diodes, as suggested by Jestine Yong in his SMPS repair book. Replaced a few resistors and capacitors in the primary side, doubting probable failure in load. Connected the SMPS transformer and put back the IC, checked again. Same result, CTV’s smirk darkened further! While checking live again, wrongly shorted the IRF pins and the fuse blew and mosfet got dead short! Got new IC and mosfet and put it in. Switched on, nothing happened, it remained dead. Looked like it got habituated in it! But see my plight, every time I have to discharge the capacitor, disconnect the mains and continue to probe, by tilting the board to both sides, causing strain on the LOT and CRT wires and board. After some time, I got accustomed to this process, and it looked like some sort of exercise for me and I was rather amused in it and enjoying!
Forgot to mention that when things were going out of control, I had downloaded the datasheet of the IC, studied the function of each pin and the minimum voltages that should be present for proper functioning. I even drew a circuit of the SMPS by looking at the board on both sides, and noted the minimum and maximum of voltages at various pins of the IC:
Continued my work on the CTV on the third day! Well, was I getting fatigued by the tough fight by the CTV? Well, perhaps! This time, I rechecked all the capacitors replaced with the one on the board. I do this by putting a mark on top of the capacitor on the board, when a matching removed one is found and finally count both replaced and removed to tally. Traced that I had put in 4.7uF/25V in the SMPS primary, in place of a 47uF/25V!!! This was to pin 14 of the IC, where the + power supply is fed from the start up resistor and then from the second primary winding of the SMPS. (Incidentally, this circuit does not use an optocoupler and no feed back goes from the secondary to the primary. So, during my failed attempts in the earlier rounds, I had tried keeping only the bulb load to the 110V B+ and disconnecting the other two diodes from the secondary side.) Replaced the capacitor. With no hope, switched the CTV on, with the same result, but no disappointment for me this time, as I was seasoned now! Bought two more ICs from different shops with different batch numbers, thinking that the one bought by me could be a fake. Replaced the IC. This time there was a change! The output pulsated slowly on, off, flip-flop, and though this is an improvement on the situation, the efforts were a flop! So, resorted to replacing the tank capacitor with a new one, replacing the run DC diode once again, as suggested by Jestine Yong in his book. Same result! Replaced the IC again from a different batch! Same result! Rechecked all the components in the primary side once again for a possible failure again! Nothing at all! What to do now?
Continued my work on the CTV for the fourth day! By this time, I was like a warrior whose energy is all used up and continuing his fight for the sake of it! Look at the defective as well as suspected components replaced by me! All these look like soldiers who lost their life or given compulsory retirement, in a battle! The cover you see is that of the Universal SMPS Module that I used for checking.
Finally, on the 5th day, I decided, this is it, no more nonsense! So, I concluded either it is due to fake ICs or the problem in the second feed back winding on the primary of the Transformer. So, decided to take off the IC and Mosfet and connect the Universal Module in its place. Temporarily connected it and after restoring all open jumper wires and other connections, connected the board to the CRT and switched on. It worked just fine! So, discharged the anode, disconnected the board and fixed the universal module firmly on the heat sink of the mosfet, and reconnected the board to CRT.
Switched it on, it worked well. Adjusted the brightness, contrast, colour and sharpness to a pleasing level. See the pictures below:
I tested the CTV for 5 hours continuously on that day and again for several hours today. It worked very well! So, did I win the battle, but lost my pride in a truce? Well, it is for you all to decide the climax of this CTV story!
This article was prepared for you by Parasuraman Subramanian from India. He 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 and presently working as Consultant Manager, Purchase & IT, in Irinjalakuda Cooperative Hospital.
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