Friday, December 16, 2011

Flex cables - sometimes nightmare for DIY

I like to take appart anything that has electronics inside. I like seeing how it works. I also sometimes try to modifiy it, or just take out components that I need in my projects. That's how I end up with a large inventory of used electronic components and one of them is a big pile of flex cables. These are commonly found in any modern electronic system like scanners, printers and many other hardware.

You probably think, what on the world will I do with so much flex cables? In diy projects they are useless because of the limitations of our home tools in soldering. Well I do admit I rarely use them in my DIY projects, but when it comes to repairing some other hardware they do come in handy... 

Sometimes happens that flex cables ware out, because of constant streching and bending. One example of this is in the CD player where a flex cable is used to connect electronics with a laser and a pick up sensor. Now you could go and find a brand new one on ebay, but the most likely part is that you will not find it or it will be ridiculously expensive.

When it comes to that, I go and check my big pile of flex cables for the same or more number of wires and IMPORTANT - SAME PITCH SIZE.

I found one cable from Sony that met my application.

Now we have to cut this cable to meet our applications number of wires. I needed 16 wires, but as you can see from the picture bellow, there are 17 wires. This is becase the width of the cable must also meet our application and that means that we will have one extra wire that we will slightly trim to fit in our connector. This does not cause any problems in our system and so we can leave it the way it is.

Now we have to cut it to the same length on one end.

Now comes the tricky part. We must remove the isolaton to expose the wire. I found that this is best done with a fine wire mesh on my power tool. Tape the wire down with some duct tape and carefully grind down plastic isolation. 

When you are done it should end up looking like this:

Now all I we have to do next is glue a peace of hard plastic on to the lower part of the wire with some super glue and trim down the exces.

It does not look pretty I know that, but it gets the job done. I used it to repair a CD player from Teac and it worked perfectly. I also suggest that the end that we have made, is used in our application where it is least under stress. That is why I plugged my DIY end in the main board of CD player and the other industrial made endto the laser module, where it will undergo  a bit more stress in bending.

If someone knows a better way to remove the plastic isolation from a flex cable I would really appreciate it if you leave a comment. 

Tips and tricks on repairing LCD monitors

If you are a student like I am, sometimes it just too expensive to buy new hardware. And why should you? There are many companies, faculties that throw away used non-working equipment. Here I will show you how you can repair used LCD monitors practicly for free. 

In the past year or so I managed to repair about 50 LCD monitors out of 60, that I sourced around my town. From my experience I can tell you, that you can have a working LCD monitor in under 20 minutes practicly for free with no profesional tools required.

LCD monitors are not that complicated as some of you think. They are made from 4 parts: LCD module, main board, switched regulated power supply (SMPS) and button board. 

From my experience, the main faults are:
- 85% faulty capacitors
- 10% faulty inverters
- 5%   other issues (faulty buttons, main board, LCD module, CCFL, FETs)

Fixing this problems is very easy and very cheap. You can get capacitors and buttons in your local electronics store. Inverters are sometimes a problem to get, but ebay will probably have the one you are looking for and its price is around 5$. 

There are many different brands of LCD. The most common are Samsung, LG, HP and Dell. And they are all more or less the same. You should also know the fact that many LCD modules are compatible. For example, I replaced an LCD module from HP to Samsung and it was a perfect fit. This is because the modules are basicly same dimensions and same LVDS connector for all types. Sadly it is not the same for mainboards and SMPS. But hey, SMPS can be fixed and main boards and LCD modules are very rarely defective.
LCD issues can probably tell you which part is faulty. So lets go to the symptons:

1. LCD lights up for less then 5 seconds, then it goes blank (check if you can see picture):
       - it could be dead capacitors on the inverter low voltage line (high probability)
       - it could be a faulty inverter (low probability)
       - it could be a faulty cold cathod flourescent tube inside LCD module - CCFL (very rare)
2. LCD power LED blinks but monitor does not light up:
       - it could be dead capacitors on the 5V logic line to the main board (high probability)
       - it could be faulty main board (very rare)
       - it could be a dead LCD module (very rare)
3. LCD turns on, the picture is fuzzy:
       - it could be faulty capacitors on SMPS (high probability)
       - it could be defective main board  (low probability)
       - it could be defective LCD module (very rare)
       - flex cables in the LCD module have a weak connection (very rare)
       - LVDS cable is defective (very rare)
4. LCD turns on, backlight turns on, but gray or white screen appears:
       - LCD module is defective (high probability)
       - poor connection of LVDS cable (very rare)
5. LCD emmits strange buzz sounds:
       - dead capacitors or inverters (high probability)
       - faulty CCFL (low probability)
6. LCD works ok, but sometimes it seems like that buttons are pressed for no reason (OSD menu appears):
       - faulty buttons (high probability)
       - faulty main board (very rare)
7. LCD appears to be completly dead (no LED signal light)
       - it could be faulty capacitors on the 5V logic line (high probability)
       - it could be the fuse on SMPS (rare and probably for a reason)
       - it could be faulty main board (very rare)

These simptoms can tell you very much of your fault. Now lets beggin with our dissasembly... I had a 17" Samsung monitor, with an issue number 1. 

LCDs are encased in plastic housing which is hold together with few screws and a click sistem. This is probably the tricky part of disassembly. When you remove the housing, carefully remove the LCD module. But before you proceed, unplug the CCFL cables from the SMPS board that hide behind a metal shield.

Metal shield

Unplug these cables

When you are finished with CCFL cables, unscrew the LCD module and carefully unplug LVDS cable from the module. Now you have a full view of the main board and SMPS.


BE CAREFULL! The big capacitor on the mains voltage line on SMPS could still be charged. So make sure you discharge it before proceeding.

Now that you are done with that, unscrew the SMPS and look for faults. In most cases a dead capacitor is visible. But if there is no visual sign of a fault, replace all capacitors on the SMPS. This will fix all your problems in most cases. I also recomend that you replace all capacitors on voltage lines on SMPS even if they appear to be good.


Faulty capacitor here is visible

Next use your multimeter and measure the resistence of high voltage side of the inverter. In my case the high voltage side of the inverter shows about  620 Ohms on both of the inverters. Because values on the both inverters are the same, I can assume inverters are OK. But if the value varries between the two inverters, the one with lower value is faulty. So you will have to replace it. You will most likely find inverters on ebay. 

FETs can sometimes be faulty too, but it is very rare. If you can see cosmetic fault of overheating, try replacing them too.

SMPS botton

High voltage side of the inverters

Button board

When you are done, put everithing back together, except for the plastic housing. Now you can plug your LCD to the mains and see if you get a NO SIGNAL screen for more than 10 seconds on your LCD. 

LCD appears to work :)

Now that everything seems to be OK, put the plastic housing back on, plug your LCD to a computer an leave it running for an hour. If no problems will accour. Then the LCD is fixed :)

Now you have a new LCD monitor for 0.15$, the cost of a capacitor

If capacitors are the case, the success rate is allmost 100%. If you diagnosed other faults, like main board fault, LCD module fault or CCFL fault, the cost of the repair is too high and you will have to search for another LCD monitor. Don't forget to store your working parts from a busted LCD monitor, becase they might come in handy if you try to fix some other LCD monitors.

Here you can see some other pictures of faults:

Faulty capacitors
Inverter from an 19" Samsung, faulty high voltage side CD
Faulty main board - no visible sign of a fault. Tried to change voltage regulators but no luck...
If you end up with different working parts of different LCD monitors and all the same size, lets say 19". With a little knowledge and some spare time, you can put together a working LCD monitor. But you will have to make your own housing, becase the SMPS and the main board will probably not fit in the original housing. That's how I made a 19" LCD monitor in my workroom, made from a 19" Samsung LCD module and button board from Samsung, SMPS from Dell and main board from HP monitor. But this will require some more electronics skills, because SMPS and main board power connector is in most cases different.
I hope this post helped some of your problems out there. If you have any additional questions, fell free to contact me by e-mail.