Edition 47
Technical Trends - Caps and Blown Tops
by Bryant Underwood, Cassidian Communications

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I read a recent report from Warranty Week regarding the cost of repairing the new technology of Flat Screen TVs and the value of service plans to reduce risk to the end users for the high cost of repair. Warranty Week placed the repair cost for the TVs at ~$300-$1000 each! I had a recent opportunity in performing some repair on these TVs myself and I thought I would let you in on some of the details of the repairs.

Several months ago some friends provided me with the ‘gift’ of two failed flat screen TVs. The TVs were from differing manufacturers and both failed at about 14 months after purchase. They both got quotes for the repair and became very frustrated at the vague estimates for repair and overall high cost that would be “at least $300”. A cost that closely matches the data reported from Warranty Week. I was busy and just did not have time to take a look at these and they sat in an unused bedroom collecting dust. During the holidays I had some spare time and received some prompting from by wife to ‘get rid of these things’. With that motivation I jumped in for the repair.

The symptoms from both were the same. They were dead and would not power on. I suspected power supplies to be the problem. I downloaded a service manual for one of the units. The actual troubleshooting flow chart from the factory is presented below. Notice that the repair is only at the board level. Then notice that the first task required if the unit will not power on is the swap out the main board, it is not to replace the power supply board. That is a very odd process and if followed, the outcome will only be to increase the repair cost unnecessarily.

For clarity I am going to jump to the end in explaining why. The failure for both of these TVs was the same component and the same defect-a bad output filter capacitor. My total cost for the repairs was $16 and I fixed them both in about 45 minutes, on the kitchen counter while watching TV (Top Gear from the BBC). By all accounts bad output capacitors are the number one failure mode on most current flat screen TVs and a very lucrative repair. If you have friends that run TV shops, they will tell you how much they love this repair. The diagnosis is pretty quick and the turn time is fantastic. In addition the failures are assured. The reason that the failures are guaranteed to happen is because of the parts themselves. If you look up the specifications for the typical parts used you will see endurance values for the capacitors rated at very limited hours of life. Notice the specification below for one of the capacitors that failed was just 2000 hours or just about 250 days of use. The part actually ended up failing at ~3500 hours. The reason it was able to last so much longer is that the operating temperature on the power supply board was reduced enough to allow the life of the part to almost double. Not too bad but still a failure that WILL occur before most consumers would expect it to happen.


While the defective parts were the same the failure modes were not. I snapped photos of the two parts below.


Notice the capacitor on the right is bulged at the top. This one was on a power supply board that used a discontinuous flyback power supply. These types of power supplies cost much less and allow the capacitors to be exposed to very large ripple currents. This is a very bad thing. The result is that they will fail quicker and more catastrophically, often they will vent out or even explode. The great thing about capacitors that fail in this manner is that there is a clear failure and quick diagnosis. All you need to do is look for stuff that looks like it has, or is about to explode and replace it. The capacitor on the left is different. From outward appearance there is no issue. Also at room temperature the ESR (series resistance) of the part would not indicate it was bad. This is a tougher failure to diagnose. What would happen is that has the part heated during operation, the ESR would increase and cause a thermal cascade further reducing the capacitors performance. This power supply was of a better design, and had protection circuits that would shut down the power supply before a major failure. All good to be sure, but it makes the troubleshooting much tougher. Because once the TV would get hot enough to fail, the time that would pass after I removed the covers and then pulled the power supply board out and then un-soldered the capacitor, allowed the part to cool enough that it would not test as bad. In this case I just had to trust my experience and spend the $8 for a replacement part to validate the old capacitor was intermittent. The net of all this was two TVs for my daughters and a little insight for you regarding the complexity and opportunities from a little spare time spent with TV repair.
RLM
Bryant Underwood manages Public Safety Sourcing for Cassidian Communications, an EADS North America Company in Frisco Texas.

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