Troubleshooting a TV For a Dead Condition (Page 1 of 2)
Categories: Televisions and Projectors
When it comes to TV sets, the first thing to always remember is that if you have no electronic experience or training, there really are no user serviceable parts inside of the TV, as they're all solid state. You're going to need to know how to use a simple digital or analog multimeter, and you should also know how to solder and unsolder, as almost all parts in most TV sets are soldered in place on the printed circuit board.
Know the Risks
Before you begin, you must be aware of the safety issues concerning the Hi Voltage in a TV set. All TV sets that have a picture tube, including CRT rear-projection sets which have three picture tubes, will have a Hi Voltage transformer with a heavy red wire that connects to the picture tube with a suction cup. Under there is an average of 32,000 volts when the TV is powered on, and even when powered off can hold a large electrical charge. If it zaps or bites you, it can make a grown man cry, or even stop your heart! So, be warned that you should never stick a screwdriver under the Hi Voltage cap... even if it’s unplugged.
If the cap has to be removed, take a clip lead and attach one end to the metal exposed strap that’s around the picture tube, and the other end to the screwdriver. Slide the screwdriver under the cap slowly, and without the metal blade of the screwdriver coming into contact with you. There should be a little pop if there’s any voltage left. Now, wait for a second or two and remove the screwdriver. Then, pinch the top of the cap perpendicular to the wire while pulling up on it. Never pull on the wire! With the cap off, clip the clip lead to the anode connection on the tube while the other end stays on the grounding strap. Also, never splice the Hi Voltage wire without using Hi Voltage putty on it and heat shrink tubing.
Most sets nowadays have what’s referred to as a "hot chassis". It's called this because the part of the metal chassis around the power supply in some sets are 70 volts DC above ground and have voltage on them. So, never ground anything on a heat sink without first measuring the voltage by using the tuner shield for ground to see if there’s any voltage there.
There are many larger capacitors inside of a set that will also hold a charge. Before unsoldering, jump out the capacitor with a 20 to 50 ohm resistor to discharge any voltage left in them. Also, never jump out any part to see if it works, as they are there for a reason.
If these rules of safety are not followed to the “T”, you could cause serious injury to yourself, or even death.
Troubleshooting a Dead TV
So you have a dead TV set? There could be one, or a combination of many things that could cause a TV not to work or start up. It could be as simple as an AC fuse, a shorted thyristor caused by a power surge, or a defective component. Always remember that a fuse will very rarely blow all by itself for no reason, so NEVER replace a fuse and plug it in without following the instructions below.
Fuses & Thyristors
Manufacturers do not always use thyristors, and like I stated above, it will short out and blow the main fuse in case of a power surge coming through the AC line. If the fuse is blown black, replace the fuse only with the correct amperage rating, which commonly stated on a label around the fuse or etched on one its ends. Now, with the end of the plug out of the wall, measure the ends of the plug with an ohm meter or continuity checker. If it shows a dead short, take out the fuse and look for the thyristor. A thyristor is usually a small black square, right past the main AC fuse. Replace it. If there is no thyristor in your TV, keep the fuse out and continue troubleshooting.
The Standby Power Supply
The best place to start troubleshooting would be the standby power supply. If bad, there won't be any voltage for the remote or microprocessor circuit to work with. There could be what’s called a switch mode power supply, or one that uses a low voltage transformer. If the fuse was black and open, there's most likely a short in the set that has made the fuse blow.
To stop any further damage to the set (just in case there is a short somewhere), would be to use a pigtail light bulb socket with a 100 watt light bulb in it. The light bulb will absorb the current that’s being drawn from the short in the set. Attach some clip leads to the end of the pigtail, and with the plug out of the wall, clip the pigtail wires across the blown fuse. Once connected, plug in the set. If the light bulb glows bright and then goes dim right away, there is likely no short in the set, or at leat it’s not a dead short.
Now, measure a capacitor around the power supply area and see if you have voltage. No? You may have a burnt printed circuit trace or open power transformer. You can replace the fuse and see if you got lucky with only having a blown fuse. If the light bulb glowed bright and stayed bright, there’s a dead short in the set. Sometimes there’s a large, low value rectangle resistor called a current limiter located right after the fuse. This will sometimes open up instead of the fuse. If the resistor is open and the fuse is not, you can then install the light bulb across the resistor IF, and only if, the value of this current limiter is 20 ohms or less.