In 2007, my brother got an LG 26LC7R HD-ready TV, and we shelved the still-working(!) 20 year-old CRT set. The new TV had analog and two HDMI inputs. Now, ten years on…
The LCD TV started exhibiting delayed video functionality after power on, which progressed from two minutes to 45 minutes over a few months; audio remained normal. At power on, a white screen (without OSD) was gradually replaced by vertical bands of finer lines (Figure 1 ), followed by normal video after the delay. This was common for all video inputs. It looked like some faulty component warmed up during the delay and recovered.
Figure 2 Layout of the four main boards
The company technician diagnosed it as main board problem, with no spare boards available. Well, no EE with DIY leanings can stand a dysfunctional TV with bright CCFLs and otherwise good build for long. Armed with a multimeter and some forum suggestions, I opened it.
The SMPS board had three bulging 680µF/35V capacitors near the heatsink (Figures 3 & 4 ) on the 24V 3A CCFL inverter supply. Heat from the heatsink doubtless helped vaporize the electrolyte. All large electrolytic caps were replaced, and output voltages measured on the SMPS connector were normal, but the first problem continued.
Figure 4 Closer view of the caps
After working for 45 minutes in HDMI mode, the video became a still picture, fading into white, and it seemed to run hot. Again, audio was okay. Switching off for a few minutes to cool restored operation immediately. With power unplugged, I felt hotspots on the secondary SMPS heatsink, main board ICs, main processor (hottest), and HDMI switch IC.
To confirm thermal shutdown as a possible cause, I set up a fan to cool the open TV’s main board. It operated continuously. The main board had a number of LDOs and (aging) SMD caps, though all voltages were okay. Could unstable regulator outputs be the cause of the heating? I recalled EDN articles cautioning about LDO instability, and added electrolytics at the outputs as shown in Figure 5 to the existing SMD ones. However, they did not change anything, and were later removed.
Figure 5 Caps added and removed
I measured temperatures (Figure 6 ) on the heatsink, body plate, main processor case, and near the top vents. The blue LED visible in Figure 7 indicated that the 12V supply to TCON (the LCD controller board) was on during shutdown. The readings are plotted in Figure 9 .
Video shutdown happened in HDMI mode after 39 minutes. Shutdown (and recovery) of main processor (video output) is correlated with the slope change at this time.
Figure 8 Brownish vias with traces between them instead of ground plane
At shutdown the main processor (VCT 7993P-A1-H000) may have been at 100°C (Rth(J-C) = 35K/W, 2W assumed consumption, 208-pin TQFP). Operation recovered at 67°. The processor has a thermal copper slug underneath, but the PCB designer did not provide a sink plane (Figure 8 ). The TV may work in a cold ambient, but had problems in tropical countries where room temperature can reach 44°!
Figure 11 Adding heat spreaders to vias
Extra heatsinks were added along with fans. Note the copper plates on heatsinks in Figure 10 and the new sinks on the video processor, HDMI chip, and Class-D audio amp. The new, magnetically-fixed 402 mm fans aid convection and were positioned to clear the VESA mounting flanges inside the back cover. The blue clips were later removed and the copper plate folded tightly onto the heatsink fins.
In Figure 11 , copper strands conduct heat from the vias to copper strips underneath the main processor board, and small aluminum heatsinks on the ground plane spread the heat. Readings were taken, first, only with heatsinks, and later with fans turned on (Figure 12 ).
Temperature reductions after mods ran from 10-24°. The fans are powered by a 12V adapter.
Figure 12 Modified TV thermal profile
While checking the TCON board, I accidentally shorted the pins of the LCD bias voltage IC (TPS65161). After replacing it, the board worked again. A spare TCON board bought on eBay for $13 had a blown fuse and a damaged TPS65161 and was also restored, but the first problem remained.
The LVDS cable and its connections were okay. The PCB strip between TCON and the LCD panel was too. This lead to the FPC (Flexible Printed Connection, or TAB bond): ACF bonded to the PCB strip. When the flex connection was pressed as shown in Figure 14 , (by a fiber clamp), the display jumped to life, showing TV tuner noise. Thermal cycling (daily on/off over 10 years) may have caused it to contract (into disconnection) when cold and expand (into connection) when hot. The induced mechanical stress may have caused bond delamination. This fits the delayed video or warm-up effect, confirmed it by twisting the TV frame, jolting the screen into life!
The few LVDS lines are expanded and decoded into thousands of lines, driving millions of LCD pixels. All these lines and decoding ICs are on flex connections to the LCD, challenging Murphy’s Law. As a chain is no stronger than the weakest link, FPC connections and thermal design seem to be the weak links here, limiting TV life. A lower internal ambient temperature would enhance the TV’s life.
A few weeks later, there was an attractive offer on Amazon for a 43” 4k TV, as well as an $80 trade-in discount. Yes…after one month of renewed use, the old TV was sent packing.
The new TV came with a two-pin power cord, and the ungrounded metal border and rear metal plate provided a shocking experience during installation. Voiding the warranty, I decided to convert it to three-pin power input. Given this experience, the next story may not have to wait for 10 years, and I’ll be making preemptive measurements soon enough.
Buddha, eons ago, cautioned about (4k) desires being the root cause of all (TAB Bond failure and other) miseries. The learning will continue.
This experience helped me understand the general design (and under -design) issues in modern TVs. In their urge to lead, manufacturers are pushing the technology to its limits, disregarding some values to the customer. Despite the growing number of service personnel in some locales who repair and salvage spares, LCD graveyards are on the rise. Complexity seems to be increasing at the expense of reliability.
—Cherukupalli Rama Murthy is an EE with experience in embedded systems applications and EMC.
“Yes, all the mods were left in TV except the (little noisy) fans and 12V adapter as two power cords for one TV may not go well with all -Figure 12 being the rescuer.”
“….Muntzing in the 21st century no doubt.”
“You may be lucky to get 3 years out of your new TV. Over the last 10 years we have burned through 3 large screen TVs in my mother in laws bedroom. Meanwhile in the living room we are still watching a large screen CRT Toshiba that is over 19 years old an
“I agree. My 40+ year old Heathkit TV was still working when I replaced it with an LCD. My current LCD had LED backlight failures after only three years which required complete disassembly of the panel to repair. Most people would have had to trash the TV.
“Thanks for leading me to Bob Pease's Muntzing stuff.”
“Was the CRT TV reliable because its screen did not need even one electrical connection (just the electron gun) whereas the LCD needs thousands of connections to screen?The speakers were bigger unlike the small inaudible flat ones now in the slim LCD TVs.”
“I believe that even though the guns are very hot and a usual point of failure besides the phosphor burnout, the vast number and complexity of connections to flat panels seems to be the cause of early failures. The flat screen failures I had were all sect
“Did you leave in your mods when you traded in the TV?!?”
“It's wonderful to be able to fix your appliances when they start to get wonky, but to be honest, when something like this happens in my house, I find it a lot easier to just get out there and buy a newer model! Especially for televisions and that sort of
“Thank you for sharing the article on usual suspects in TV Repairs. This will be used for many users reading your blog like me.nhttps://www.doorstephub.com/tv-service-repair/bangalore”
“That is why it is essential for us to have our own residential electrician. Thanks for this!nnhttp://www.crelectrician.com/residential-electrician”
“Hopefully the new TV designers will do away with the LCD glass screens which need the unreliable TAB bonded Flex connections (flex PCBs with ICs mounted) and LED backlights. The current mis-labelled LED TV, which is actually an LCD TV with LED backlight)
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