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
Being 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).
Looking 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.
In 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…
So 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.
Putting the whole thing together and having a working product, Priceless 🙂