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Sharp Aquos 42DH77 - Fixes

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Warning: Only attempt this if you are competent to do so! There can be lethal voltages inside the TV even when its unplugged. I'm not liable if you damage yourself or your TV trying these repairs. Read the disclaimer. If your not sure, pay someone else to do it

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Pressing Power button changes power LED colour but TV is dead

99% of the time this is NOT a power supply problem. Its easy to think that because the TV is dead, it must be a bad power supply, so you replace the power supply and find the fault is still present...

In this TV, all the voltages are marked on the power supply connections, so its easy to confirm that the power supply is ok by testing all the voltages. Some of the voltages are high and the board has to be live, so it goes without saying that you should be very careful when taking the voltage readings. If all voltages are reading near to the value listed on the board, then the power supply is ok and the fault lies in the control chip IC9101 on the main board (DUNTKE685WE).

IC9101 is the main control chip, located at the top of the board, above and to the right of the analogue RGB (PC) input socket. It recieves the 'turn on' signal from the power button (or remote command) and sends the signal to the rest of the the TV to start up. In TV's with this fault IC9101 appears not to be sending out the start up signal.

This can be tested by checking the voltage at pin 80 (VON) of IC9101. Plug the TV in, wait 10 seconds, test the voltage, then press the power button, wait 10 seconds and test again. If pin 80 is reading 0 volts or very close to 0 volts on both tests, then its probable that IC9101 is bad. It should read 0 volts and then 3 volts (or 3v then 0v depending on power state) when power switch is pressed. Unless you have the tools required to remove a 100 pin surface mount chip you will need to replace the board. We wait 10 seconds each time because this TV has an energy saving 'slow start' feature and when enabled the TV takes a few seconds to come on and we dont wanna confuse the readings.

Screen Goes Off after a second and TV shuts down

The TV starts ok, but then the screen goes blank, sound stays on and soon after the TV shuts down. The power light turns red and flashes once every 1.5 seconds. This indicates either an inverter or backlight fault. While the power light is flashing red, press the menu button on the remote (pointing it at the TV obviously!). If the OPC light does not come on then it indicates an inverter fault, however if the OPC light starts flashing at the same rate as the power light this indicates that it is a backlight fault and this fix may help you.

The complete LCD screen assembly needs to be taken apart for this fix which is a delicate job and if you are not careful you can severely damage the screen making the TV useless so be careful!! Also the inverter board that powers the CCFL (Cold Cathode Floresent Light) tubes puts out around 1500 volts so NEVER touch the inverter board when the TV is powered up unless you know what you are doing.

The backlight tubes can be replaced if needed, but its usually a fault with the backlight connectors that causes one or a few tubes not to light. If you own a CCFL tester that can test 1000mm tubes you can test the tubes without taking the screen assembly apart. Remove the inverter board, connect one side of your tester to the metal chassis, and then probe each of the 18 backlight connectors. You will probably find that all 18 tubes will light and test ok.

If all 18 tubes test ok, or you are unable to test them, you will need to take the screen apart to be able to take out the connectors to inspect/repair them.

The service manual can be found by searching for it in any search engine this will tell you how to take the screen assembly apart (i hope to add this data to the site sometime soon)

Once the screen assembly is taken apart you will see a plastic lens sheet on top of a diffusion plate. Take care at this point as you remove the sheets to notice on one corner of each sheet there will be a notch or the corner will be cut a little. Also note that one side of each sheet will be slightly rougher than the other side. Make a note of where the notch is and which direction the rough side is facing as if you put them back in wrong, you will have very bad picture quality as I found to my cost when I fitted them incorrectly at first.

When you have removed the lens sheet and diffusion plate you will see the backlight array with a plastic cover clipped on at each end. Gently ease each cover off.

At the opposite end to the inverter connectors you can do a continuity test with a standard multimeter. Test between the metal chassis and each backlight, they will all probably test ok. So you have to move onto the invertor connectors. To remove each of the 18 connectors, very carefully pull the end of the tube straight upwards, just enough to clear, then gently push it to the side of the connector and pop the connector out by pressing from the invertor side towards the backlight side. Always handle the backlight tubes very carefully when moving one end of it around as you dont wanna break it and give yourself more expense.

When you have all the backlight connectors removed you can inspect all of them. You will probably notice that one or more have corrosion or blackness where the inverter board plugs onto it. It is advisible to pull the metal out from the plastic, by gripping one side of the backlight clip with a pair of tweesers or small pliers, and taking some fine sand paper or emery paper and clean off the corrosion, next re-bend the metal slightly so that the two bits that connect to the invertor board are almost touching each other and it looks square (I hope to be able to upload pics of this sometime soon) and then slide the metal back into the plastic.

Look into the clip where the invertor plugs in and make sure that the top of the metal bit is completely up inside the plastic, if it is not, use a small screwdriver to push it up into place. This is important because if the metal is showing at the top, plugging in the inverter will bend it giving you more problems. The only metal you should see from that side is the 2 clips that connect to the invertor. It is time consuming, but advisable to take all 18 connectors apart and re-bend all of them to make sure they all grip the invertor tightly.

Next, look at the inverter board and each of the 18 connections, there may be blackness on some of them. Use switch cleaner spray or very fine sandpaper to clean them all up. Any that are looking in bad shape after cleaning will require you to coat them in a thin layer of solder.

When this is all done, put it all back together and hopefully you will now have a fully working TV! Good luck!

Controls go Mad, Volume turns up, Channel changes on its own

You are watching the TV and all of a sudden the volume turns itself up, or it suddenly changes channel, the menu may pop up or it may change input sources. After you check that the cat is not sitting on the remote, you will probably find that the remote will not do anything.

The controls on the TV itself are in 3 groups. Key 1 = Channel up, Channel down and Input; Key 2 = Volume up, Volume down and Menu; and STBY_POW which is the power button. If the TV has turned it's volume up fully you will find that you can't turn the volume down or get into the menu, Likewise if it is stuck in channel up mode, then channel down and input won't work.

The Key 1 and Key 2 controls work on resistance to ground: 0 ohms for Ch up and Vol up, 8.2K ohms for Ch down and Vol down, 20K ohms for Input and Menu. So first we check resistance between Key 1 or 2 and ground to confirm that none of the switches are stuck. You will probably find that they test ok.

The fault probably lies in the decoupling capacitors: C2034 for Key 1, C2035 for Key 2, C2036 for STBY_POW. All 3 caps are 100pf and can be replaced with standard ones if re-soldering the originals does not work. Its also a good idea to take a close look in that area of the board for any other bad joints and re-solder any you think could be suspect.

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