[Front Page] Article: ITT: Cheap TV Fix

fly

Osharts 11
Oct 1, 2004
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mattressfish
My new $4.10 TV:

oJEev.jpg


== Backstory ==

A couple of weeks ago I took my dogs for a run at a nearby beach, and spotted a TV on the side of the road that someone was throwing out - a 42" RCA 'L42WD22XY6'. Not the greatest TV by today's standards (no 3D, no LED backlighting, no 1080p, "smart TV" features or anything remotely fancy) but definitely not completely useless.

Wondering what was wrong with it, I brought it home and plugged it in.

== Initial diagnosis ==

The TV didn't come with a remote. Using the buttons on the side of the TV to get started, I did a bit of playing.

- Judging by the power LED on the TV, the TV seems to power on and off OK - the light changes from red to green when I push the button, and pushing it again, it changes back to red after a delay. This is a good sign, indicating that the TV's system controller and power supply seem to be working correctly.
- Other than the LED, there are no obvious sign of life.

TVs these days are built like a PC - they have a low power standby supply (equivalent to a computer's +5vsb supply) that runs the 'system controller', which checks the buttons on the side of the TV and listens for codes from the IR remote - this switches on and off the main power supply which powers the LCD panel, main image processor, audio bits and such.

So far this is good. No smoke, no bad sounds, just "it don't work". Time to do a bit more diagnosis.

I programmed my Logitech Harmony remote with the codes for this TV, and hooked up my netbook to the VGA and audio inputs (don't anyone ever tell you netbooks suck, they're awesome). Firing up youtube on the netbook and futzing with the remote, I made a few discoveries:

- Infrared works!
- Sound works!
- And jamming putting my head up against the screen, I could see a very faint picture! Meaning that the LCD panel itself is working OK but the backlight's gone.

This TV has a cold cathode fluorescent backlight - fluorescent tubes run across the back of the screen, and they're driven by an inverter circuit that converts DC voltage to a few thousand volts for the tubes.

Either the tubes are dead (leaked their gas from being improperly made, or cracked/smashed by the TV being knocked over) or the inverters are dead (usually the result of cheap design/component selection). A third possibility is that the TV's power supply has a separate voltage rail that feeds the backlight inverter, and it's a power supply problem.

Time to haul the TV apart.

== The autopsy ==

When you're taking things like this apart, you encounter a lot of screws. I keep a box of 'weed dealer size' ziploc bags around, and put the screws in labelled bags. Makes everything go back together a lot easier.

W0BBg.jpg


Opening door #1 on the back of the TV, we had a power supply. A visual inspection of the power supply didn't show any obviously blown/burnt components, bulging/leaking capacitors or anything. And conveniently they've listed the pinout of the power connector and the voltages that they supply puts out - I measured these and they all check out fine. So it's not the power supply.

(Note: I would NOT recommend that ordinary folk probe things like this power supply when they're plugged in - there's lots of exposed metal here with high voltages on it, which could give you a pretty bad shock or even kill you if you don't know what you're at.)

Onto the rest of the TV. Behind door #2 in a metal box, we have the TV's motherboard:

HQbu2.jpg


Again, nothing's obviously blown/burnt in here. Inputs are on the left, LVDS digital video feeding LCD panel goes out the top, and wires leave this box and spider all over the TV. Lets remove all these wires and get this box out of the way.

iNSY3.jpg


With the power supply and motherboard case taken out, we can now remove the back panel of the TV, exposing the back of the LCD panel itself.

DEoOH.jpg


There's three covers on the rear of the LCD panel, covering up 3 circuit boards - top center is the "timing controller", which converts the LVDS digital signal from the TV's image processor to raw drive signals for the LCD panel itself. The two covers on the left and right hide two inverter cards that drive the LCD's backlight - since we're chasing a backlight problem, let's have a peek at them.

aJJSQ.jpg


Well that's not good... each inverter has two bulging caps. Seems this TV has fallen victim to the "capacitor plague", which is an interesting story of industrial espionage.

Electrolytic capacitors use a complex chemical called an 'electrolyte' to store electric charge, and manufacturers of capacitors guard their electrolyte compositions as tightly as Coca-Cola and KFC guard their own secret recipes. A scientist working at a japanese capacitor company planned to steal the company's electrolyte recipe and sell it to companies in China and Taiwan, but his co-workers figured out what he was doing and set him up with a recipe that was missing critical ingredients. Capacitors made with the botched recipe would work OK at first, but eventually would blow up. WHICH WOULD BE A COOL STORY BRO, BUT MY FUCKING ELECTRONICS SHIT BLEW UP, SO FUCK YOU.

I pulled the busted capacitors off the inverters. Just for fun, I measured them:

sg1xK.jpg


Original value was 470uF, this one measured 5.8nF, or about 0.001% of its correct value. Two more of them measured similar values, and the fourth capacitor was short-circuited. I went over the rest of the inverters - removed a few parts and tested them, etc. I can't really get into details without being too confusing, but everything tested fine except for a single 10A fuse that was blown on the 'master' inverter - probably caused by the shorted cap.

So I ordered new fuses/capacitors, crossing my fingers and hoping that's the only thing the TV needs. They came in, and we're all fixed up:

mOOYx.jpg


I reinstalled the inverters in the TV, screwed everything back together and surprisingly didn't end up with any extra or missing screws.

Using the girlfriend's Blackberry Playbook as a HDMI source, I fired it up, and...

sFREG.jpg


Success! I suddenly found myself with a 42" TV and nowhere to put it - we already have a 40" in the living room and a 37" in the den. Ultimately we ended up taking the mirror off our bedroom dresser and putting the TV in its place. The girlfriend was awfully pleased to not have a broken TV taking up space in the spare bedroom anymore...

Anyway, I'm constantly fixing electronics for myself and others. If anyone else has broken shit around the house and wants any advice, just ask.
 

Domon

Robotic Dexter
May 19, 2011
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pffft, spare bedroom is just another word for workroom. Girlfriend is incorrect.

Also, awesome work sir.
 

fly

Osharts 11
Oct 1, 2004
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Hey gee, I've got a buddy who has an older Samsung DLP TV. It randomly shuts off. If he waits a min or two, it will turn back on. He's recently replaced the bulb which didn't help. Any ideas what could be broken from where you're sitting?
 

Domon

Robotic Dexter
May 19, 2011
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Hey gee, I've got a buddy who has an older Samsung DLP TV. It randomly shuts off. If he waits a min or two, it will turn back on. He's recently replaced the bulb which didn't help. Any ideas what could be broken from where you're sitting?

dlps run really hot, and if it runs then turns off, id bet on a heat related issue (aka, a component that fails spec when it warms up). If it flat out shuts off, power board. If he hadnt already replaced the lamp, i would have said it was the lamp drawing more current than it should, thats pretty common with dlps too. It could still be something in that lamp circuit too though
 
Last edited:

gee

Blame It On The Gassa Nova
Sep 29, 2012
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Hey gee, I've got a buddy who has an older Samsung DLP TV. It randomly shuts off. If he waits a min or two, it will turn back on. He's recently replaced the bulb which didn't help. Any ideas what could be broken from where you're sitting?
Sounds thermal. To get a good color gamut, DLPs run their light bulb "could burn down your house" hot - and have thermal protection in case stuff gets too hot inside. I'd say it's tripping.

Haul the thing apart and clean everything - it may have accumulated dust, fur, etc. Also make sure it's not jammed against a wall with no ventilation, or sitting in front of a baseboard heater.

(edit: beaten by domon)
 

Domon

Robotic Dexter
May 19, 2011
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last time i found a tv on the side of the road and plugged it in, nasty smoke just poured out, such a bummer.

thats still fixable probably. If nothing else, it makes it really obvious which part is bad.

Unfixable things on a tv (for the most part)

A) Shattered screen
B) Broken backlight tubes
C) Bad tab bonds

anything else can be fixed for far less than the tv cost.
 

gee

Blame It On The Gassa Nova
Sep 29, 2012
15,553
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Halifax
thats still fixable probably. If nothing else, it makes it really obvious which part is bad.

Unfixable things on a tv (for the most part)

A) Shattered screen
B) Broken backlight tubes
C) Bad tab bonds

anything else can be fixed for far less than the tv cost.
Friend has a really nice LG TV, barely over a year old but out of warranty, with a bad "tab bond" (I think that's the right term) - the flat cables feeding the LCD have ICs bonded to them, and one of them has a bad connection causing horizontal banding.

New TV is the same price as a new panel. Frustrating.
 

Domon

Robotic Dexter
May 19, 2011
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Friend has a really nice LG TV, barely over a year old but out of warranty, with a bad "tab bond" (I think that's the right term) - the flat cables feeding the LCD have ICs bonded to them, and one of them has a bad connection causing horizontal banding.

New TV is the same price as a new panel. Frustrating.

I just RMAed my 55 for tab bond issues, thankfully within warranty.

If it wasnt, there are homemade fixes, but theyre really really risky. You have to disassemble the entire tv, including the panel, and then you put little rubber bumpers on the inside of the chassis. If you do it right, they exert minimal pressure against the tab bonds when you reassemble, reconnecting the bad joints by simply pushing on them.
 

fly

Osharts 11
Oct 1, 2004
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mattressfish
I just RMAed my 55 for tab bond issues, thankfully within warranty.

If it wasnt, there are homemade fixes, but theyre really really risky. You have to disassemble the entire tv, including the panel, and then you put little rubber bumpers on the inside of the chassis. If you do it right, they exert minimal pressure against the tab bonds when you reassemble, reconnecting the bad joints by simply pushing on them.

Why is that risky?
 

Domon

Robotic Dexter
May 19, 2011
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Why is that risky?

because too much pressure shatters the tab bonds. Theyre really fucking fragile. Theres also a huge risk of damaging the panel when you have to essentially break it out of its casing. You see, its glued.
 

Applesauce

The Gypsy-The Acid Queen
Dec 9, 2008
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Holy cow...People who know their way around stuff like this and other "boards" are wayyy complex.
 

gee

Blame It On The Gassa Nova
Sep 29, 2012
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Halifax
Holy cow...People who know their way around stuff like this and other "boards" are wayyy complex.
Nah, most people who know electronics as well as I do tend to be complete retards at other things. Like being social.