LCD monitor repair
(Last modified 16 January 2014)


Most of the monitors I use (including this one) are repaired units that people have discarded or recycled.  If you have just a little bit of tech know-how and a decent parts bin, you can usually get these discarded monitors running again.  This saves space at the landfill and cuts down on the hazardous waste generation.  Plus, you can donate any extra monitors you have to friends, churches, clubs, or charities that can use a hand.

And consider taking some fixed monitors down to the city government offices, to discourage them from running reclaiming drives and instead running reuse/repurpose drives!

I recently donated four of my repaired monitors to the local Seniors Center.  Lots of people on fixed incomes may have been given a system or had one for several years, but can't afford to replace a monitor that goes out.  My afternoon's entertainment fixing a monitor may help someone out of a tight spot


The external power supply
In a surprising number of cases, nothing is wrong with the monitor itself.  Those monitors that use a lump-in-the-line or brick power supply usually had a bad supply.  If you get an LCD that has a small power jack in the back (like the coaxial power connectors common on most devices), try finding a suitable replacement power supply.  You will need to check the ratings for the monitor on the back panel to get the voltage and current required.  This is usually something like 12 VDC at 2.0 Amps or so.  Go to a surplus shop, root through the junk power supply bin, and find a supply that matches these ratings.  Note that going to a larger number on the current (Amps) is OK, but you have to match the voltage exactly!

Note that you must also find a matching power connector!  Take the monitor with you when you look for a power supply.  When you find a suitable supply, check that the connector on the supply mates properly with the jack on the back of the monitor WITHOUT APPLYING POWER!  If the two connectors don't mate, find a second supply with a connector that does mate, even if the voltage or current are wrong.  You will later cut the connector off this second supply and splice it onto the cord of the first supply.

When you get home, you have one last item to check before powering up your new monitor.  The center pin of the monitor's power jack could be either negative or positive.  Whichever it is, your replacement power supply must be wired the same way.  If you're lucky, the back of  the monitor will give you that information.  If it doesn't, use a voltmeter to check the resistance between the center pin and some metal point on the monitor's frame.  If you get less than 5 ohms, the jack is wired center-negative (the less common).  If you get more than 5 ohms, you should wire your power supply as center-positive.

Compare the polarity your monitor expects to the polarity of the power supply you chose (the one that provides the proper voltage and current).  If the power supply plug is the right polarity, it fits, and the voltage/current are correct, you are ready to go; move to the next step where you try out the supply.  If the polarity is not correct, you need to cut the power supply output cord (the one with the plug on it) about six inches from the power plug, then rewire the plug with the two wires swapped; be sure to solder your connections and use heatshrink or tape over the splices.  If you also need to change the plug by using the second power supply you chose, cut the output wire on the second supply about six inches from the end of its plug and splice the plug onto the output cable of the first supply; be sure to get the polarity right and to solder your connections and use heatshrink or tape over the splices.

Now that your power supply is ready to go, plug it into the monitor, hook the monitor to a PC, plug the power supply into the wall, and boot the PC.  If the power supply was the problem, you should see the monitor light up.  You're done!

New plug on an old power supply

Here I've replaced the plug on an old lump-in-the-line power supply to match a discarded ViewSonic VX500 (a really nice monitor, BTW).  The photo shows the voltage and current rating of the supply.  I had to solder the two wires from the power supply to match the polarity required by the VX500; it wants center-positive.



120 VAC monitors (and some that use external supplies!)
Often, defective monitors have issues on the main power board.  Such monitors can show a wide variety of failures, such as not powering up, flashing the display on and off, or working for a while, then going dark.  The first step is to unplug the AC cord, open the back of the monitor, and inspect the power supply board (the board with the largest, heavy-duty components on it).  Look for any electrolytic capacitors that are damaged or disfigured in any way.  Sometimes you'll see one that has burst its top or that has a bulging top.  The top of each electrolytic cap should be flat and shiny.  Also look for damage on the bottom of each electrolytic; sometimes these caps will rupture where the leads come out.

If you find a damaged cap, you have to replace it with a similar unit.  Note the capacitance and voltage of the bad cap; this will usually be something like 10 uF at 25 VDC.  You must use a replacement that has at least the same working voltage and at least the same capacitance.  You can go as much above the original voltage as you want, but don't go any more than twice the original capacitance.

IMPORTANT!  The replacement capacitor MUST be rated at 105 decC!  Do not try to use a cheaper 85 degC capacitor!  You can find 105 degC caps at Fry's, Digikey, Mouser, or most TV repair shops or suppliers.

Remove the defective cap, taking care to note the polarity.  Electrolytic caps have positive and negative leads and have to be soldered into the circuit with these leads in the proper location; put the replacement cap in the board in the same polarity as the original.  Solder the leads in place, then clip them off on the underside of the board so you don't short-circuit something later when you put the monitor back together.

Double-check that there are no more bad caps (you can have two or more bad caps in one monitor).  After you have replaced all the bad caps, put the back of the monitor back on, hook up the monitor, and apply power.  In most cases, this will fix the monitor.

A bad cap next to a good cap

Here you see a bad capacitor on the left and a new capacitor on the right.  You can clearly see how the bad cap has burst at the bottom near the leads.  All caps in the monitor's power supply should have smooth, flat tops like you see on the new cap.  If the top is bulging or burst, replace the cap!

Sometimes, a cap will go bad without bursting or showing any physical damage.  One monitor I worked on would run fine for a few seconds, then go dark.  It would also smell hot long before it had run long enough to get hot under normal circumstances.  After it shut down like this once, I hurriedly reopend the back of the monitor and found a pair of electrolytic caps that were far hotter than anything around them, after only a few seconds of operation.  I am waiting for replacement parts, but I strongly suspect these caps were causing the problem.

Note that most power supply boards usually have a very large electrolytic cap somewhere near the 120 VAC connector.  I have never seen one of these go bad and comments from repair sites, notably badcaps.net, indicate that this cap almost never fails.  Doesn't mean it won't happen, but it seems very unlikely.


Bad solder joints
I am seeing more and more monitors with bad solder joints.  I suspect that this is an issue with the move to lead-free solder that was started a few years ago.  In years past, the solder joints on five-year old equipment were still bright and shiny.  Today, equipment only five years old has grainy, dull, or open solder joints.  Typically, ALL of the solder joints on a power supply board are in this condition.

I fixed one monitor where the only problem was a defective solder joint on the main power connector.  An easy fix, but it should never have happened.

Part of my routine now is to replace whatever components are obviously defective, then visually inspect (under magnification) all solder joints.  I have found the most problems around large-lead parts, such as inductors or through-hole connectors; for some reason, these connections don't age well.  To repair such connections, heat the solder joint and suck all of the old lead-free solder from the pad.  Then resolder with old-fashioned rosin-core lead solder.


Removing adhesive residue

Sometimes, a store or company will plaster an adhesive label across the screen of a monitor they intend to dump.  This always irritates me; just because they can't/won't fix it, why do they have to make it that much harder for someone else to work on the unit?

Regardless, you can clean up the monitor screen easily.  First, carefully peel away all of the paper label that you can.  DO NOT use a knife or other sharp instrument!  Just carefully scrape the paper away with your fingernail.  Don't try to scrape off the adhesive, you'll just mar or damage the screen.

When you have as much of the paper removed as you can, set the monitor flat on a table with the screen facing up.  Carefully pour just enough olive oil or other light vegetable oil on the screen to cover the adhesive.  You want a small pool of oil completely covering the adhesive.  Make sure none of the oil drips into the seams of the frame or you'll have to disassemble the unit to clean out the oil.  Leave the oil sitting on the surface of the monitor for at least an hour. 

Mop up the oil with a soft paper towel.  Then wipe the screen with a soft paper towel that has been dampened with water and a small amount of liquid hand soap.  Follow this by wiping the screen with a dampened paper towel (water only).  Finally, clean the screen with Windex and a soft paper towel.

If there is still adhesive left on the screen, just repeat the oiling and cleaning treatment.


Nothing wrong!

One recent monitor, a CTL 170Lx 17-inch unit, actually didn't have anything wrong with it at all!  When I applied power, the screen stayed dark but the little LED on the frame would blink green, then yellow, then green, over and over.  I did some web searches and discovered that this monitor sometimes gets confused after a power-surge or power failure.  To recover, I had to reset the monitor by holding down front-panel buttons two and three (counting from the left) while powering up the unit, then wait until the LED stopped blinking.  Once I did that, the monitor worked perfectly!  You don't often get LCDs that are this simple to repair, but it never hurts to check the web; it could save you the hassle of opening up a perfectly working monitor.

(Props to Chris McLellen of Boise LCD for this reset tip.  Chris has a nice website and looks like he offers a good service.  I haven't done business with him, but I like the attitude he shows and the fact he was willing to blog this very helpful tip for all to use.  Thanks, Chris!)


Worst case
The above paragraphs describe the problems with maybe 75% of the monitors I've fixed.  But sometimes you get a monitor that doesn't work after trying these suggestions.  At this point, you may need to find someone with more troubleshooting skills and tools than you have.  Check the local vocational college, repair shops, or high-tech clubs and see what you can do.  At the worst, you will end up discarding this monitor, which is how you got it in the first place.

If you do choose to discard, PLEASE try to find a good home for the monitor!  See if there is a local Freecycle or high-tech collection point that will hopefully get your monitor into the hands of someone who CAN fix it and CAN reuse/repurpose it.




Monitors I've rescued...
Here are some examples of monitors I've repaired, complete with the stories behind them.


Planar PE1700 (added 1 Nov 2010)
I found this monitor at RE-PC in Tukwilla several months ago.  This 17" monitor has a coax power connector on the back panel, so I started out by making a new wall-wart power supply with matching cable.  However, when I plugged in the cable, the monitor still didn't work, so I had to open the back and take a look.  I did not see any damaged or burned components, so I hooked up the power supply and started tracing voltages.  Oddly, there was no voltage present on the PCB, even though the wall-wart power supply was working.  Turned out the coax power jack was defective.  I replaced it with one from my junk box and the monitor lit right up.  Until I got the Dell 1905FP (below), this has been my best and favorite monitor.  I think I paid $7 for it...


Dell 1905FP (added 1 Nov 2010)
I picked up this 19" monitor at RE-PC in Tukwilla for $10.  This monitor has a 120 VAC connector on the back, so I figured I'd find bad caps in the power supply.  When I opened up the unit, I noticed three capacitors on the power supply board were distorted.  All three caps were 1000 uf at 10VDC.  I didn't have any of these in my parts bin, so I motored down to Vetco in Redmond and picked up those caps plus a good assortment of other values to keep my junk box stocked.  Vetco's prices are a bit high, but the quality is the best.  You gotta love being able to walk into a parts store and load up on Nichicon 105 degC electrolytics on a Sunday afternoon!

I replaced the caps, put the unit back together, and fired it up; works like a champ.  This model is no longer stocked by Dell, but a search on the Internet shows it goes for about $250 new.  Highest normal setting is 1280x1024 resolution at 75 Hz refresh, which is certainly useable.  The colors are rich and the stand it came with has smooth, sure motion.  Not bad for $15 (monitor plus caps).


LG L1933TR (added 11 Mar 2011)
I bought this non-working 19" monitor at PC Recycle in Bellevue for $10.  It uses a 120 VAC power cord, but plugging it in yielded nothing, not even a lit power LED.  I opened the case and immediately noticed that all of the solder joints had a grey, grainy look to them.  In the days of leaded solder, I would have called all of them bad solder joints.  Now that we're using lead-free solder, I guess this is as good as it gets.

I replaced three caps in the power supply board; one was obviously bad, and I replaced the other two just because.  When I applied power, the unit still didn't turn on.

Next up was resoldering the AC power connector on the main board.  I didn't see anything obviously wrong, but I have had such connections go bad before.  When I resoldered the pins, I made sure to use leaded solder.  When I applied power, still no joy.

I checked voltages at the computer board and everything looked good.  It seemed logical then that the signals from the computer board weren't making it to the graphics connection on the LCD panel.  I reseated the small multi-pin connector at the LCD panel.  I also reheated all of the pins for that connector on the underside of the computer board.  When I applied power, I had a beautiful LCD display.

Unforutantely, I can't say definitely that the fault lay with the solder joints on the underside of the computer board.  I suspect that was the culprit, but I can't prove it.  Oh well.  $16 and a couple of entertaining hours, and I have a new monitor.  Great way to spend an afternoon.


AOC LM729 (added 20 Mar 2011)
Another $10 purchase from PC Recycle.  This monitor uses a 120 VAC power cord; when first powered up, it showed the AOC logo and a Windows display for about ten seconds, then the screen went dark.  I opened up the power supply and did not find any suspicious-looking caps, but I replaced the three highest-value electrolytics on the PCB anyway, in the "it can't hurt" philosophy.  Unfortunately, it also didn't fix the problem.  It wasn't that the screen stopped receiving graphics signals from the processor board; the drive voltage to the backlights was cutting out.

Research on the web turned up a reference to inductors on the power supply board of this model occasionally showing bad solder joints.  A quick look showed everything seemed fine.  However, a closer inspection under magnification showed that the solder joints for both L201 and L202 appeared to have aged badly; one of them was clearly open.  I sucked the solder from each connection and resoldered it; problem fixed.

This monitor has an excellent picture with good color, supports 1280x1024.  It also rotates 90 degrees on its stand, but I don't have drivers to support that feature yet.


AOC LM729 (added 20 Mar 2011)
This is a second LM729 monitor I picked up at the same time as above, also for $10.  It showed the same problems as the first one, so I was filled with confidence as I opened it up.  Again, all of the caps looked perfect.  This time, I went straight for the two inductors on the power supply board.  As before, the solder joints for both had a grainy appearance and while I've seen worse, I resoldered them anyway.  I also touched up several other connections, notably those on a pair of power transistors nearby.  Plugged the monitor into a test PC I keep handy; monitor worked like a charm.

I find it odd that so many monitor makers can't get the caps right, but AOC did.  However, AOC had problems with solder joints aging on the inductors where other makers don't.  Regardless, to the consumer the issue looks the same; the monitor stops working.  Oh well...


Viewsonic VA902b (added 27 Mar 2011)
I was in the Arlington PC Recycle and picked up this unit for $19.  This is a no-frills 19" monitor; no speakers, no DVI connector, no USB.  When I plugged it in and fed it a graphics signal, the monitor would work fine for a second or two, go dark for a couple of seconds, then repeat the cycle continuously.  When I opened it up, I found and replaced two bulging caps (1000uf/25V and 220uf/25V).  I also replaced a second 220uf/25V cap in the same area, just because.  I double-checked the solder joints on the underside of the power supply board and resoldered several that looked grainy.  Reassembled the monitor, plugged it in, looks great!  Total cost: about $23.


Dell 2005FPW (added 10 Apr 2011)
This was a $20 purchase from the Arlington PC REcycle.  This is a nice, 20" wide-screen monitor, no speakers, with DVI and USB connectors.  When I powered this one up, it showed an excellent graphics picture for about three minutes, then the screen went black.  After a few seconds, the graphics image came back on, but again when out after a few seconds.  I replaced ALL of the electrolytics on the power boards EXCEPT the largest cap near the AC connector.  The symptoms improved slightly, in that it took longer before the screen first went black, but the problem persisted.

Research on the web shows that two caps on the logic board (C637 and C635) are often at fault, so I replaced them; problem remained.  I replaced a couple other caps in the same area on the logic board, but the problem was still there.

I soldered a wire to the GND connection and a wire to the 12 VDC connection (pin 1) on the power supply board, partially reassembled the display, then powered up the display with a meter on the wires so I could monitor the value of the 12 VDC supply to the logic board.  The 12 VDC supply started out at about 6.35 VDC.  This voltage slowly dropped until it hit about 6.01 VDC, at which point the monitor went dark.  The supply voltage immediately started climbing until it hit about 6.14 VDC.  At this point, the display came back on, the supply voltage dropped back to 6.01 VDC, and the display went dark again.  This cycle repeated until I shut off power.

I unsoldered the 12 VDC regulator (a KIA278R12) from the power board and removed it from its heatsink.  On my bench, applying 12.5 VDC to the input gave me 7.5 VDC output with no load, so the regulator is shot.  I was able to find a suitable replacement from Mouser (the Fairchild KA378R12C) and have some on order.  The Fairchild part is a 3A regulator and should prove more robust than the 2A part it is replacing.

More on this fix later...

OK, I received the KA378R12C regulators.  The first thing I did was hook one up on my bench and check the output.  As expected, it outputs 12 VDC.  I spread a little heatsink compound on the device's back, bolted the regulator to the original heatsink, and put the monitor back together.  Works like a champ (I'm using it to write this update).

This monitor was a bear to take apart and put back together.  I ended up loosening the adhesive tape that holds the flat ribbon cable connecting the front-panel switch board, to give myself a little bit of extra slack for putting the front panel back together.  Even so, this is an awkward monitor to work on.  Hopefully, I've dealt with all the issues and won't have to reopen it for a long time.


Dell E198FPb (added 11 Apr 2011)
I picked up this 19" plain-Jane (Dsub-15 VGA only) monitor from the IT department of a local business.  It was out of warranty and had been stuck on a shelf, waiting for the recycler.  I didn't even bother to power it up so I don't know why it was labeled BAD with a big sticker on the screen.  The power supply board had three bad caps, a pair of 680uf/16VDC and a 330uf/35VDC.  I replaced them with suitable units from my parts bin, put the monitor back together, and it powered up just fine.  This unit has a very nice image with good color; it will make a great utility display.  Another one saved from the recycler!


Dell 1908FPt (added 1 May 2011)
Another orphan rescued from a local business, this 19" monitor is loaded with VGA, DVI and USB connectors.  As above, I didn't even power this one up, so I don't know what the symptoms were.  This fix required replacing four 330uf/35VDC caps.  I didn't have that many such caps in my parts box, but a quick trip to Vetco (on Sunday!) took care of that.  While I had the unit open, I wiped the two major heatsinks free of thermal compound, then replaced the compound with new from a tube I keep in my parts bin.  I also reheated a couple of suspicious looking solder joints on the power board.  The repaired monitor has excellent color and brightness, and will make a nice addition to my computer room.

Dell 1908FPt (added 16 Jan 2014)
I picked this one up from a local company that had it destined for the recycle bin.  When I powered it up, I heard a sizzling sound from the lower corner of the case and smelled a lot of ozone.  I removed power, opened up the case and noticed a large, discolored area on the metal case, near the lower set of high-voltage wires that connect between the PCB and the CFLs that light the display.  I carefully connected the 120 VAC and switched on the monitor, being VERY CAREFUL to keep both hands away from the high-voltage leads.  I could see an arc between the wires and the frame, and the insulation had been melted on both the blue and pink wires on this harness.

I powered down and removed the AC cord, then cleaned up the brown crud that had piled up around the wires.  I laid down a piece of high-voltage insulation on the frame beneath where the wires run.  I also wrapped at least ten layers of electricians tape around the blue wire and around the pink wire, to provide additional insulation.  I then reconnected the 120 VAC and switched the monitor back on; no sizzling sound and no ozone.  However, the brighness on the lower half of the screen is noticeably dimmer, though the monitor is certainly still usable.  So far, so good; looks like one of my easier and cheaper fixes.

BTW, after I reassembled the monitor, I tried to adjust the brightness, but hitting the Menu button brought up a big padlock image on the display and no access to OSD features.  A quick check of the web shows that for this monitor and several other Dell models, you just keep the Menu button pressed for about 15 seconds and the locked icon switches to an unlocked icon.  Press the Menu button again, then one more time, and you should have full access to the OSD features.





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