LED backlighting a 22″ lcd monitor

This one’s kinda messy, but I was visiting my parents so I only brought my soldering iron and basic tools with me.  I had a friend mention that their lcd monitor was broken, of course the first thing out of my mouth was “I can fix it no problem”.  It was a little more complicated than I thought.  Let’s review the most common failure mode of electronics this decade: bad caps.  What I mean by that is capacitors that have failed, exploded, had their ESR go up, heated up due to that and then exploded, or bulged, or just plain leaked.  The reasons are numerous: there could be an unexpected cooling failure causing the caps to heat beyond their tolerances; there could be too high an ESR as I mentioned; they could even just be, gasp, old! But by far the most common reason for failure is espionage.  It’s a little more mundane than it sounds, there are news articles, wiki entries, and even a website and forum dedicated to it.  I’m just hereto say it has been a godsend for those of us that can open consumer electronics and solder reasonably well.  When you have people throwing out devices worth several hundred dollars each because of a few cents in parts that are fantastically easy to identify and fix… well, let’s just say I know some people who could major in monitor repair and minor in general debugging.

the whole setup, mountain dew and all

This problem monitor happened to be a Sceptre 22″ lcd monitor.  Now, due to the rapidity that people are throwing out monitors I tend to have a large backlog from which I can pull parts (usually entire power or logic boards, sometimes button panels, etc…) but I don’t have any spare dead monitors this big.  Once open (although I should have done this first) it was obvious that the LCD worked, the logic board worked, and the main part of the power supply worked.  I could tell because the only thing that didn’t work was the back-light   This doesn’t preclude it from being bad caps, but it lowers the likelihood   In back-lights I tend to see one of four things go wrong.  First there is the simple bad caps on the inverter input, this wasn’t the case here.  Next there’s the blown PWM chip or blown FET driver, usually SOT-8 or DIP-8, can be blown by heat, time, or voltage fluctuations (sometimes traced to caps). Another potential failure mode I know is shorted transformers; I haven’t ever gotten a monitor I cared enough to diagnose this far, but the PWM chips create a square wave that gets stepped up in the transformers to drive the high voltage tubes.  The problem this time was bad back-light tubes.  These tubes were seriously blown, though I have come across ones that glow red briefly and then the over-current protection kills the back-light inverter.  Well, I don’t have any spare tubes this size, what about LED strip lighting? that should work, right?

assembled with strips installed

I ordered a 12v LED strip off of amazon and it is pretty nice stuff.  The stuff I bought had a rubberized coating on it meant to be used outside, which didn’t really help, but oh well.  The rest of the mod went as expected, I pulled the tubes, carefully disassembled the panel and fitted the new lights, ran power wires through a switch because I didn’t feel like working out the back-light auto-on feature and powered it off of the 12v tap on the power board.  That simple, I could have done with some better diffuser  but I didn’t have much time to work on this.

monitor off, back-light on, diffusion problem

working. Next patient!

entire disassembly album


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