Archive for the ‘Electronics’ Category

Dell E172FPb LCD Monitor (Benq OEM) CCFL Backlight Repair

July 13, 2007

UPDATE – READ FIRST (3 June 2008)

K, so its been a while since I’ve added anything on this page (and in general for my blog) but theres been quite a few concerns around the use of the replacement transistor by people who I would assume know more then me (i know very little).

I’m glad to say that the 2 fixed LCD’s I use still works so that in itself is good but in saying that, based on some of the comments, I dont want to help someone burn their house down, I love my home and I think you do too :-> . Please only do this at your own risk.

One other thing to note here is that I live in New Zealand, which in general is a bit colder then the rest of the world. The LCD doesnt seem to get too hot here but I would think that cooling is would be a bigger issue elsewhere in the world.

Please read all the comments before deciding to use the replacement parts and if you can get hold of the original parts, Please do!!! I’m thinking that if my transisters go sometime soon, I will end up ordering new ones (the propper parts) from some of the places indicated on some comments.

 To everyone who has made comments, thanks, Its always great to see feedback 

E172FPb beforeBeing the second faulty Dell lcd monitor passing my desk in the last 6 months, I thought I’ll make a posting to describe the successful fix to this guy (and the first one, which is still running good).

The symptoms: When pluged in, This guy fires up just long enough to get past the initial “logo” screen and onto the test pattern (its not plugged into the computer at this stage). After about 2 seconds of being on the test pattern, it turns off, or more specificually, the protection circuits kick in, turning off the backlight. So in this case, the monitor was still working and the LED was indicating that the monitor was on (it was green). A good way to test if it is just the back light is to grab a strong light and shine it onto the lcd, If you can see a dark picture underneath then the lcd is def still working and it should be a pretty minor fix.

So what is this protection circuit? The protection circuit on Lcd’s is generally a way of protecting the actual fluorescent tube from getting overdriven (too much current going through them). So when this protection circuit kicks off, it generally means there is either a short in the tube or a short in one of the components near / on the inverter. unless the LCD is quite old, its usually a short somewhere in the inverter circuits.

So after seeing these symptoms and having seen the excact same on the other dell lcd that I had fixed, I was feeling pretty confident of a good outcome….

Opening up the monitor:

  • Remove the screws holding the Stand in place.
  • Remove the stand
  • Flip the monitor so that the bottom is facing up.
  • Grab a flathead screw driver and use it to ply apart the bottom of the monitor.
  • Now that the bottom is released, Start working yourself u8p the sides.
  • Once the sides are sorted, you should be able to seperate the top without a screwdriver, infact, I found that using a screw driver to ply the top made no difference. I think the top is a unique case….
  • Remove the plastic backing
  • Remove the taped shield
  • Disconnect and unhook the 2 ribbon cables which connect to the lcd pannel from the processor board.
  • Disconnect the cable from the processor board to the front control panel.
  • Remove the front plastic cover (they are cliped around the side to the main panel)
  • Remove the 2 hex nuts by the VGA plug
  • Remove the Metel shield along the side of the monitor (that long glossy metel part, it slides right off).
  • Disconnect the 2 cables connecting the back light from the power/inverter board.
  • unscrew the 4 screws from the side of the panel and remove the back metel plate.
  • You should now have all the plastic parts, the main panel (keep this away from anything / anyone who migth step or damage it!!!) and a metel casing part which holds all the pcb’s, one for the Power/inverter and the other for the graphics processor.
  • Unscrew the earth cable and unclicp the plastic holding the power socket in place.
  • Remove the remaining screws which hold the pcbs in place.

You should now be able to pop out the circuit boards, be careful though, although the power baord would be quite amune to static, the processor board (with the dsp chip) is not! be careful with this board, you want to put it on an antistatic bag or newspaper and not on plastic or carpet (the whole static thing again).

Some photos:

dell_e172fpb_opening_1.jpg   dell_e172fpb_opening_2.jpg   dell_e172fpb_opening_3.jpg   dell_e172fpb_opening_4.jpg   dell_e172fpb_opening_5.jpg   dell_e172fpb_opening_6.jpg  

The Internals:

Dell E172FPb Internal 1   internal_2.jpg   Dell E172FPb Internal 4

The Fix

Faulty transistors to checkLooking only at the power/inverter board, there are a small group of 4 transistors which are used to drive the inverter, these fail easiy and when they do fail, they usually fail in pairs. The main problem with testing these is that you can not test these transistors in circuit, you have to remove them to test them. So start by unsoldering one pair and checking them using a multimeter (more info on testing NPN transistors, click here). if these are good, start unsoldering the other pair and do the same.

dell_e172fpb_fixed.jpgIn my case, I found that there was a short in one of the pairs. The origianl transistors were ‘c5707‘ NPN transistors, which are impossible to get hold of here in New Zealand (they can be found on ebay though if you live in the states), so hunting through the jaycar catalogue, I found something quite similar, the “MJE3055” Transistor. The main problem with this guy is that it has a much bigger casing (TO-220 package) and isnt going to fit nicely into the lcd monitor case….

The first dell I had, I moved the new big transistors to the left of the board and moved the original good ones down to the right, then using some insulation tape  and cardboard to isolate the heat sink part of the transistor from the metel (earthed) chassi case. This worked quite well but I’m always worried about how hot these transistors get and and that the insulation tape wont hold (should have used some better quality tape really) so I thought this time around I’ll try something abit different…

MitresawSo what I ended up doing was taking the case down to the garage and starting the Mitre saw up, Made a nice slit where the inverter circuit lives and ripped out that chunk of metel. Although It wasnt as clean as the first Dell I dealt with, It did mean i had some piece of mind around knowing it will never short circuit again the casing.

I think If I were going to do this again, I would go down the insulation tape route based on the clean finish.

The Result

 Putting the whole thing together and having a working product, Priceless 🙂


Dell E172FPb


Bad caps on a compaq Machine (motherboard)

June 2, 2007

Afew days ago I had a call from some friends of mine, it appears that their fathers old compaq (amd athlon 1100mhz) had given up on them and they had alot of good stuff on it, including alot of the photos from their recent wedding. To say the least, it was vital to recover what i could from the machine. I was told that the sympton was that it would nolonger turn on and that for payment, my partner and I would be shouted to their delecious home cooked mussles, who would be the fool to give up an offer like that?!

Anyway, after having dinner at their place and a good catchup, I had a chance to test out the machine for myself. There was definitely power and it would turn on but only showed a black screen, it wouldnt post, no beep codes or anything 😦

Bad CapsAfter opening up the machine, I quickly spotted a good handful of bad Capactors lingering around the cpu (tell tale signs are when theres brown oze appearing from the top and when the top looks like its under alot of presure / not flat). Theses were common faults awhile back, affecting alot of motherboards of verious brands and there is a whole conspericy theory around why this happened (checkout for more info), but these are realitively easy and dirt cheap to fix if you have the time, the tools (tools being a 25W to 40W soldering iron) and a steady hand.

When looking for replacement caps, theres heaps of places online these days that will sell you a cap kit for specific motherboards but as living in new zealand, postage would cost more then its worth, however, sells low esr caps thet may fit into some of the older motehrboards, else you can always find a cheap doner/faulty motherboard (this being the case with me) from your junk pile, a friend, ebay, trademe etc. The one thing to be aware of is that when replacing caps, usually only the higher value caps need replacing (look for anything 1000uf or higher as potentially being faulty), this doesnt mean lower value caps go bad but its alot less likely. The otherthing is although value isnt entirely all that important (I have replaced afew 1500uf and 22oouf  caps with 1200uf before and will in this case), Voltage is everything, When a cap says its 6.3V, do not use anything less, you could do more damage and potentually damage other parts such as the precious CPU and less precious Power supply!.

It is also very important that you use low ESR capactors as when passing power through the caps, they generate a resistance which is dependent on the frequency being passed through it (and dont say you are passing DC, cos you wont be, well not clean DC as its a switching power supply and also, the motherboard is using switching to get the different voltages it needs). The greater the resistance, the hotter the cap gets and the shorter they last (very short lifespan). The Low ESR caps cope better with the high frequency going through them.

bad caps on motherboardIn my case, the original caps were Nkcon 2200UF (6.3v). Luckly I had a old motherboard around which although wasnt useful, it had some good caps onbaord, so off they went. The best way I have found for removing caps of the board is by heating up one of the legs from the bottom of the board while applying presure from the top side of the cap (same side as the led you are ehating up). As it lifes out abit, repeat on the other leg and keep repeating the process untill out. This method also works well when removing the bad caps from the board you are trying to fix (however you really do need to be careful on this as the pcbs have heaps of layers and can be damaged very easily).

When applying the good caps to the board, fistly, make sure the legs are clean, straight and spaced such taht they should slide into teh board easely. Apply some solder to the upper side and lower side of the board wher the cap is going to sit. Position the cap into its final position (ensuring that the positive and negitive pins are in their correct orentation).

Apply heat on the bottom of the board (board should be rested verticually if possible) while applying presure on one side of the cap. repeat process for the 2nd leg and keep repeating to wiggle the cap onto the board. The cap should soon be sitting on the baord (doesnt need to be a tight fit as that may over time and the heat cycles, damage the cap).

Repeat process for all the caps (11 in this case).

As a final step, make sure you clean up after yourself, remove any splatters of solder that may have jumped as you heated it, resolder any of your work that looks unclean etc.

Finally, test your board and pray for success (I would recommend you test your board using old parts if you have them just incase, else use only 1 memory stick, no hard drives, no unnessery addon cards etc). once you are happy with a job well done, connect everything else up.

In this case, the hard drive was still good and the motherboard was the only fatal fault. What was better was that when they came to pick the machine back up, they droped off some dim sims that Serahs father had made!, yum!!!!

If you plan on trying this fix then afew words of advice:

  •  When replacing the caps, Make sure the voltage of the new caps is atleast the same
  •  The new caps should be the Low ESR type and rated for 105 degrees
  •  Be extremely careful when removing the damaged caps, if you are not, you can easily damage the board.
  •  Tint the leads of the new caps first before putting them onto the motherboard
  •  Make sure you have the caps positioned correctly (the ‘-‘ should be marked on the cap and the ‘+’ should be marked on most boards)
  •  Make sure you do a good job on cleaning up before putting the baord back into teh case, old solder bits could get traped and short circuit things (really bad, best to tidy up well)
  •  Before turning the board on for the first time, make sure you install only the minimum parts to get the machine started (pick the smallest ram chip, dont install hard drive, if you have onboard graphics, use that instead of the standalone one, dont install any unneeded addon cards such as modems, unplug usb ports and even the monitor if its an lcd etc), that way, you will cause the lest damage if somethign went wrong (you should know if somethings not right within seconds of turning it on).
  •  Finally, have some faith in yourself, it isnt a hard job to do.

 Good luck!

AOC LM722 LCD repairs

January 27, 2007

100_1343.jpgOne of my real bad habbits is picking up faulty junk off trademe in hopes of me repairing them and making use of them, I just cant stand how people end up throwing away good stuff which only costs $1 to fix (no really, only $1 or even less!!!). Luckly for me, someone elses loss is my gain and also luckly for me, Ive had some good luck in this area, in this case, picking up this faulty AOC LCD Monitor for the cheap cheap price of only $45, Ofcourse, it did have this minor problem of actually being faulty.

After having it arrived at work in its original packaging (really must stress the packaging, once the screens cracked, or have dead spots, nothing left to do but dump the poor thing), I pluged it in and sure enough, the monitor would turn on for about 2 seconds and then nothing.

Suspecting the backlight as bing the issue, I grabbed a tourch and tried to see if there was a dark faint image on the screen, no luck, the monitor had actually gone on standby mode (indicated by the orange led). Further diognostics required taking the unit apart…. heres some quick notes on how to do this (without putting too much stress on the plastic and the actual lcd).
You will need only 2 screw drivers (one philips for the screws and one flat head, for best results and to avoid damaging the plastic, go for the widest flathead you can find) and a small spanner.

Ok, some pictures on removing the plastic casing.

100_1358.jpg   100_1356.jpg   100_1355.jpg  100_1354.jpg   100_1353.jpg   100_1348.jpg

The insides (click for high res)

 100_1099.jpg   100_1350.jpg   100_1349.jpg  

The problem!

 100_1100.jpg   100_1101.jpg
A visual inspection quickly showed the croperit, the pluge of the bad capactors strikes again.
After a quick replacement of the cap, I quickly put the sheild back on and gave it a quick wheirl. Fixed!!!
Reassembled the casing and left the lcd running over night with a  screensaver. By morning, still running and has

been for over 3 weeks now.

Net Result? Dual Monitors!!!

So, whos up for a cheap LCD? 🙂

Brand: AOC
Product name: LM722
Model Number: TFT1780ps+