AMAZON multi-meters discounts AMAZON oscilloscope discounts
<< cont. from part 3
Video Projectors
LCD and DLP video projectors are quite popular, and each type has its characteristic failures. Let's look at projectors and how to work on them.
How They Work
Front projectors and modern, non-CRT rear projection TVs use the same technologies, with a bright lamp, a "microdisplay" device that forms the image, and a series of lenses to magnify the results. In LCD projectors, there are three small LCD panels, one for each primary color of red, green and blue. Light from a very high-intensity arc lamp is filtered to remove ultraviolet energy and then split into three beams, with color filters for each color. Each beam illuminates its own panel. The resulting three images are recombined with a prism and focused on the screen with the projection lens.
DLP projectors have no LCD panels. Instead, they use a special chip called a Digital Light Processor, invented and manufactured by the Texas Instruments Corporation. On the surface of the DLP chip is a matrix array of microscopic mirrors, each separately addressable. Depending on the resolution, there may be hundreds of thousands to a few million mirrors. Feeding power to a mirror makes it flex, deflecting the light at an angle and reducing the amount reflected straight toward the lens. The result is a projected video image of high contrast.
That'd be all there is to it, except for one small detail: color. DLP chips are expensive, and they require a fair amount of circuitry to drive them. Some very pricey, professional-level projectors have three DLPs, combining their outputs like LCD units do. The home units you're likely to service, though, have only one chip and accomplish color projection by rapidly flashing the three color images in sequence with a high-speed rotating color wheel of red, green and blue segments between the lamp and the DLP chip. Your visual system, which can't keep up with anything coming at it that quickly, combines them into one full-color image. DLPs can move their mirrors much faster than is required for normal video rates, so it's possible for them to flash two or even three complete sets of tri-color images in the time span of one frame. When the specs say the unit has a 2X or a 3X color wheel, each frame of video is being flashed at that rate, compared to a normal video frame, with three flashes of color each time. So, a 3X-rate projector flashes nine images in the time it would take a CRT to scan one frame.
The increased rate helps diminish the "rainbow effect," an annoying consequence of the single-chip, frame-sequential color projection method that occurs when the viewer's eyes move. Especially in darker images with bright points of light, like night scenes with streetlamps, the visual trail left by the bright spot can break up into its component colors as the eyes change position, because each color frame strikes the retina in a slightly different spot, so they don't blend together. Some people find the effect very distracting, so manufacturers keep speeding up the color wheels and frame rates to minimize the time between projection of the different colors, keeping them closer together in the moving eyes of the viewer.
What Can Go Wrong
There's lots to malfunction here. The most troublesome elements in a projector are the very expensive lamp and the circuitry powering it. The brightness required is so extreme that only a high-pressure mercury vapor arc lamp will do the job. Operating an arc lamp is not as simple as just applying power. First, it has to be "struck," or started, by applying a fairly high voltage until conduction across the arc is achieved.
Then, once current starts flowing, the mercury inside vaporizes and makes the lamp conduct much more readily, with lower resistance. The voltage must then be reduced to typically less than 100 volts. The circuitry driving the lamp is called the ballast, though it's much more complicated than the simple ballast that starts an old-fashioned fluorescent lamp.
Wait, there's more. Arc lamps exhibit some pretty odd behaviors. As they age, they tend to develop bad spots on their electrodes, increasing the resistance of the most direct path across the arc. Because the vapor conducts, other paths arise, and the arc can jump around, causing flickering of the light. To combat this annoying malady, the ballast may adjust the operating voltage or add pulses to keep the lamp at its best.
Even with all this effort, lamps go bad, they go dim, they fail to strike, and now and then they explode violently.
The second most failure-prone part differs between LCDs and DLPs. In LCDs, the polarizers, sheets of plastic film in front of each panel, get burned by the residual ultraviolet output of the lamp, even after its light has passed through the ultraviolet filter. The blue polarizer, in particular, tends to burn, resulting in a yellowed image or splotches of yellow.
In DLPs, the color wheel, whizzing around so fast, often experiences motor failure or catastrophic mechanical failure. Because of lamp heat, color wheels are made of glass, not plastic, and some are assembled with nothing more than glue! In time, the glue degrades, also from lamp heat and ultraviolet, and the delicate red, blue and green segments fly off, smashing against the inside of the projector's case and shattering into a million pieces.
DLPs also use a light guidance arrangement quite different from that in LCDs.
LCD panels are considerably larger than DLP chips, and the lamp's output is spread over enough area to illuminate them fully. That's easier than the DLP scheme, in which the light has to be formed into a small beam. To do that, DLPs use a mirror tunnel, sometimes called a light tunnel, made of four mirrors arranged to form a rectangular channel. Like the color wheels, the mirrors may be glued together, and the glue can fail, collapsing the tunnel.
Cooling the lamp is a critical function in all projectors, so they all have fans blowing air through the lamp housings. Most projectors have multiple fans. LCD units usually have one just to cool the panels, because, as blocking elements, they absorb a lot of heat from the lamp. DLPs, which reflect light instead of absorbing it, don't overheat their imaging chips, but they sometimes use fans to cool the rest of the optical chain, along with the lamp fan. Some projectors have power supply fans as well.
A failed fan, especially if it's the lamp fan, will cause the projector to overheat and shut down. It takes a few minutes for the thermal sensor to heat up enough to cause shutdown, so the projector will run for a short time before it dies.
Most projectors have dust filters on the lamp housings, and they get clogged with room dust to the point that airflow is severely restricted, triggering an overheat shutdown.
And, of course, projectors suffer from the usual power supply issues, especially bad capacitors. The units are remote-controlled, so at least part of the power supply runs all the time, even in standby. After a few years, a cap or two is shot, and the projector stops turning on.
Is It Worth It?
An expired lamp might seem like an obviously worthwhile repair, but the lamps cost so much-from around $100 to more than $400-that you must consider whether the rest of the projector will survive long enough to use up a new lamp. Those hot lamps put tremendous stress on the other optical components, and many projectors are designed to last about as long as one lamp. It's no fun to spend two-thirds the cost of a new projector for a replacement lamp, only to have a polarizer or a color wheel go bad 100 hours later. Some expensive, pro-level projectors are built to last through several lamp replacements, but the relatively inexpensive home units are not.
Burned LCD polarizers are pretty much a dead end unless you can scare up a parts unit. It's almost always the blue polarizer that goes, so a unit old enough to be cut up for parts probably has the same bad polarizer and will be of no use. Manufacturers don't sell the polarizers separately; they want you to buy the entire light engine (optical system) or replace the projector. Believe me, you don’t want to spend what a new light engine would cost.
Color wheel costs vary greatly among manufacturers. The wheels for some rear-projection DLP TVs can be had for $50, while those for some front projectors cost an eye-popping $500. There's no basic difference between the parts; it's all a matter of marketing and volume. Parts for TVs, including lamps, are generally lower than those for front projectors.
Light tunnels can often be repaired for nothing with some epoxy and a steady hand.
Dangers?
The lamp is not your friend! If you look directly into it while it's running, even momentarily, I hope you like dogs, because you're probably going to need one. The brightness is higher than anything the human eye can withstand. There's a fair amount of ultraviolet, too, which is very damaging, even after most of it’s absorbed by the lamp housing's UV filter. Seriously, don't ever look directly into a running projector! The lamp gets hot enough to burn you badly, too. After it's been operating, let it cool down quite awhile before going near it. When it's hot, the glass is more fragile as well. The actual lamp envelope is only about the size of two pencil erasers, but it's under tremendous pressure. I've seen one explode, and it's not pretty. They go off like a shot, and a fine mist of glass particles tinged with mercury gets ejected from the projector's fan vent; you wouldn't want your eyes in the vicinity.
The voltages used to strike and drive the lamp are hazardous. Figure around 1 KV for striking and 80-100 volts during normal operation. Don't go poking around with your scope in the lamp supply (ballast), especially while the lamp is striking.
A DLP's rotating color wheel could cut you if you contact its outer edge while it's spinning, but it's more likely you'd destroy it.
How to Fix One
See FIG. 10 for a view of the optical path, or light engine, in a typical DLP projector.
Most problems are found there or very nearby. The three most common failures are no operation at all, no lamp strike, and overheating with subsequent shutdown. If the unit won't turn on at all, suspect the usual power supply issues. Projectors and TVs spend virtually their entire existences plugged in, waiting for a remote-control signal, so bulging power supply capacitors are pretty much a foregone conclusion eventually.
One difficulty in servicing projectors is that restarting a hot lamp damages it badly. So, once you turn the unit on, you don't want to turn it back off, take a few measurements or check a few parts, and then fire it right back up again. Always let the lamp cool before restriking it. That can take a half-hour or so.
===
FIG. 10 DLP optical path Light tunnel DLP chip Light condenser Lamp
compartment Fan Projection lens Mirror Color wheel
===
Lamp Problems
If the projector powers up but blinks a warning light on the control panel, the lamp or its ballast may be bad. A sensing circuit checks for current draw through the lamp; that's how the thing knows the lamp has struck and it's time to reduce the striking voltage to its normal running level. If the lamp won't strike, the warning light is as far as it'll go.
You can't check the lamp for continuity with a meter; at room temperature, the bulb is an open circuit until it has 1 KV or so applied across it. Assess the lamp's condition visually. Being certain it’s cool, remove the lamp. Unless the bulb itself is completely enclosed by its housing, it's a good idea to wear goggles, just in case you bump it or drop a tool on it, because it could explode in your face.
Some TVs use bare lamps, with nothing enclosing them. That little stalk protruding from the front is the end of the actual bulb, and it's fragile! At the bottom of it, nearest the back of the reflector, is the high-pressure envelope. The lamps in most projectors are enclosed in a housing with a UV filter in front, so they're a little safer, but you could still get showered with glass blown out the sides of the housing, through the dust filters.
It's important never to touch the bare lamp or the UV filter, because skin oils will make them crack when they get up to operating temperature. A cracked UV filter may leak UV radiation, and a cracked bulb…well, you know what will happen. Kaboom! Look at the envelope head-on, slightly from the side. You're looking down the focal point of a parabolic reflector, so it's hard to see the arc gap, but if you look a bit off-axis, notice it. It'll look greatly magnified by the reflector, and that's helpful.
If the envelope's glass looks clear and clean, the lamp doesn't have lots of hours on it and is probably good. If it looks charred, it's an old lamp and may be shot. Examine it carefully, and you can also see the actual electrodes and their condition.
Almost all projectors have a time counter in their menus telling you how many hours are on the lamp. Without a working lamp, of course, you can't see it! Ballast Problems
The ballast is really a pretty fancy power supply of its own. It has to supply the high striking voltage and then the lower operating voltage. Operation can require a few amps, for around 150-250 watts of lamp power. Output transistors supply it, and they can pop. Also, a fair amount of heat is generated in the output stages of some ballasts, so check the circuit board for burn marks and degraded solder joints.
Don't try scoping the ballast's output stages. It's far safer and easier to disconnect power and pull and check the output transistors. There may be an onboard fuse, too.
Be sure to discharge any large electrolytics before desoldering anything.
Look for optoisolators in the path between the lamp and the ballast. The output of one of them will change state when the lamp successfully strikes, relaying the information that it's time to lower the voltage. If you can't find that signal, the lamp is not being struck, or it's bad.
Overheating Problems
Overheat shutdowns don't happen instantly; it takes a little while for the heat to build up. If the projector runs for five or ten minutes and then quits, it's probably overheating. A lamp very near the end of its life can run excessively hot and cause this condition, but most of the time it's due to lack of airflow over the bulb. Check first for blocked dust filters. Some projectors, especially LCDs, have them at the air intakes for both the lamp and the LCD assembly. Blockage of either can trip the shutdown.
DLPs usually have filters right on the lamp housing. Some units are designed to be dust-resistant and have no filters. They rarely clog up, but check the air channels at the lamp housing just to be sure nothing has gotten in and blocked those.
There's a thermal sensor over the lamp. If the fan doesn't do its job, the lamp overheats and the unit shuts down. Never defeat this sensor, even just for a repair test.
Even if you don't cause a fire, you'll probably destroy the lamp, and it could get hot enough to burst.
The fans themselves can fail, but sometimes the problem is the circuitry powering them, especially in units with variable-speed fans. Many projectors have normal and economy settings for lamp brightness, with greatly increased lamp life at the lower setting. To reduce noise, they slow the fans down in the economy mode, using power transistors to lower the voltage. Over time, a transistor may fail, and the fan will go dead. The fans should always be turning when the projector is on. If one of them isn't spinning, disconnect power and try turning it by hand. If it's not gummed up with dirt or dried-out lubrication, it should move freely.
The fans are the same types used in desktop computers, and they usually run on either 5 or 12 volts when at full speed. Check to see if any voltage is getting to them. It may be less than the fan's specified voltage, but it won't be a fraction of a volt, and it'll never be zero if things are working. Expect at least a couple of volts. Don't use circuit ground for this test; check directly across the fan's positive and negative leads. If a fan shows voltage across its leads but is not moving, the fan is bad. You can probably find a compatible replacement fan from a computer supply house. Just be sure the new one moves at least as much air.
If there's no voltage, the driving circuit is out. Trace the fan's wires back to the board and look for a small power transistor. Scope to see if it's getting power supply voltage and if that is getting fed to the fan. The transistor may be between the fan's negative terminal and ground, with supply voltage going to the fan's positive terminal.
If you see the same voltage on the fan's negative terminal, the transistor isn't pulling current to ground and is probably open. That's why you can't use circuit ground when testing the fan; you need to be sure there's actually a voltage difference across it.
Light Tunnel Problems
If the light tunnel collapses, the projector will run but you'll see a darkened area along an edge or even large portions of the image blacked out. Gluing it back together is a chore, but the price is right! Carefully disassemble the optical path and remove the tunnel's mirrors. The shape of the tunnel corresponds to the aspect ratio of the imaging chip. You should be able to deduce how it went together from the position in which you find the pieces, and from the glue remnants. Clean off the old adhesive and glue the tunnel back together with epoxy; instant glue won’t survive the lamp's heat. Use the good stuff that takes awhile to dry, not the 5-minute quick-set type. If you can find high-temperature epoxy, that's your best bet. Let it dry for a night or two and then reassemble.
Color Wheel Problems
The DLP's color wheel is driven by a small motor, with a position sensor to tell the video circuitry when to flash the correct image for whatever color is in front of the lamp.
Most of them use a Hall-effect sensor that picks up a magnetic field from a magnet embedded into the assembly's rotor, generating a pulse for each revolution of the wheel.
Without that signal, the unit's microprocessor assumes the wheel isn't turning properly, or at all, and stops operation, shutting down the lamp.
Treat the color wheel gently. It's delicate, and breaking it means the end of the projector unless a petal has come off intact, in which case you can glue it back on.
When the unit starts up, see if the wheel is turning. It should spin very fast. If it seems sluggish or isn't moving at all, unplug the projector and gently turn the wheel by hand. If it doesn't turn freely, the motor's lube may have dried out. Often, dried lube will cause a horrible screeching sound from the wheel, too. The normal remedy is replacement, but the cost can be ridiculous.
I've saved a few screeching color wheels for nothing, though, and the fix has lasted a long time. Get some silicone lubricant spray. Don't use anything else, like oil or other kinds of sprays. Remove the wheel and spray into the motor bearings, saturating the motor as best you can. You'll probably be spraying under the color disc itself into the top of the motor. Let it soak in for a few minutes, and then dump out what's left into a tissue. Very gently clean excess lubricant off both sides of the color disc with an alcohol soaked swab. Don't put pressure on that disc or you may snap off the glass petals. Give the motor a few careful spins and then reassemble the unit. Sometimes it works.
Video Processing Problems
The circuitry driving the LCD panels or the DLP chip is dense and complex. It's all digital and not very serviceable. If you're losing a color on an LCD, or the image is flickering or freezing on a DLP, check for bad cable connections to the boards driving those imaging devices. Also look for leaking surface-mount electrolytic capacitors. The heat of a projector can really shorten their life. If there's no video at all, look for on-board voltage regulators, and scope to see if they're putting out voltage. Beyond that, you'd need a service manual and some serious, high-speed digital test gear to make much sense out of the signal processing. These kinds of problems aren't common, though. The vast majority of projector failures can be traced to the power supply, the ballast, the lamp and the optical components.
Blobs in the Image
These are caused by dust on the LCD panels or the DLP chip. A little carefully applied compressed air will take care of them. You may have to open a housing or remove the lens to gain access. Be careful not to spray the air too close to the imager, in case some of the cold propellant winds up on the optical surface, where it could mar the transparency. The actual mirrored surface of a DLP chip is behind a sealed window, so there's no chance of harming the mirrors. LCD panels and their polarizers are more easily damaged.
Have at It!
I hope you've enjoyed our extended romp through consumer electronics and its repair. Armed with the techniques presented in this guide, you should be ready to dig in and have some fun! Just always remember to put safety first, and keep in mind that, as with any skill, expertise develops over time. If you wreck a product while trying to fix it, toss its remains on the parts pile and chalk up the loss to a valuable lesson learned. We all lose a few along the way. The more you practice, the better you'll get.
Ready to go? Scope on? Soldering iron hot? T minus 3, 2, 1…blastoff !! Have fun!