Viewsonic VX2450wm

[UpThyWazzoo]

Hello all, I'm currently attempting to repair an LED-backlit (edge-lit?) screen that appears to have broken LEDS on the right side (right side of screen is dim). If I'm guessing correctly, the panel uses surface-mount type diodes.

My question is: how do I go about identifying what "model" LEDs are being used?

A google search result suggested testing the voltage and current of a diode while it's on, but I'm afraid that the monitor's dimming/color profiles could skew the readings? Perhaps I'm over-thinking this.

Any help would be appreciated!

[PlainBill]

Re: Viewsonic VX2450wm

Hello all, I'm currently attempting to repair an LED-backlit (edge-lit?) screen that appears to have broken LEDS on the right side (right side of screen is dim). If I'm guessing correctly, the panel uses surface-mount type diodes.

My question is: how do I go about identifying what "model" LEDs are being used?

A google search result suggested testing the voltage and current of a diode while it's on, but I'm afraid that the monitor's dimming/color profiles could skew the readings? Perhaps I'm over-thinking this.

Any help would be appreciated!

I was going to suggest you ship the monitor to me, I'd do the testing and run it for an extended period of time to make sure it worked. But I doubt you are THAT gullible.

An LED is a constant voltage, variable current device. I would expect the LEDs were arranged in multiple series strings, with the strings either regulated individually, or each using a series resistor to avoid current hogging. By the appearance of the problem, I'd say there are 2 strings in this monitor with one not working (or there are an even number of strings, with half of them not working).

I have several approaches to suggest.

1. Measure the voltage drop across several individual LEDs in the working string. Measure the current through the string. Verify voltage is actually applied to the dead string.

2. If voltage is applied to to the dead string, it is a trivial problem to locate the open LED. Remove it and see if there is anything to help identifying it.

3. The service manual is available from Electrotanya. It's a somewhat long shot, but they might have a schematic or parts list that includes the LEDs.

PlainBill

[UpThyWazzoo]

Re: Viewsonic VX2450wm

Thanks for your reply, Bill!

Well, today was full of surprises. After sweating out the terrifying process of deconstructing the LCD panel, I found that there is actually only one strip of LEDs located at the bottom of the screen. And that one strip has at least 140 diodes or so.

Before I was able to test the strip, however, I found another problem. It seems that there is bad soldering involved. While attempting to lift a corner of the panel off of the monitor housing, I dropped it and the end of the ribbon cable connecting the LED board with the LED strip detached....along with the "socket" that it's supposed to be inserted into. Although the traces on the LED strip seemed to be in bad shape, I fiddled with the ribbon a bit with the power on (danger is my middle name), touching the two together briefly. Lo and behold, the LEDS on the right side lit up brightly, but then the LEDS on the left side were dimmed.

Long story short: broken solder/traces could be the real culprit--and given how minuscule the traces and the "socket" component is, fixing the LED strip could involve some more bad soldering... by me.

I think I may look into a circuit trace pen, but I read that the traces created by such a pen have higher resistance than wire, so I'm not sure if that will be helpful to use on something that's supposed to light up. I will go about testing for failed diodes on the LED strip tomorrow.

[PlainBill]

Re: Viewsonic VX2450wm

Thanks for your reply, Bill!

Well, today was full of surprises. After sweating out the terrifying process of deconstructing the LCD panel, I found that there is actually only one strip of LEDs located at the bottom of the screen. And that one strip has at least 140 diodes or so.

Before I was able to test the strip, however, I found another problem. It seems that there is bad soldering involved. While attempting to lift a corner of the panel off of the monitor housing, I dropped it and the end of the ribbon cable connecting the LED board with the LED strip detached....along with the "socket" that it's supposed to be inserted into. Although the traces on the LED strip seemed to be in bad shape, I fiddled with the ribbon a bit with the power on (danger is my middle name), touching the two together briefly. Lo and behold, the LEDS on the right side lit up brightly, but then the LEDS on the left side were dimmed.

Long story short: broken solder/traces could be the real culprit--and given how minuscule the traces and the "socket" component is, fixing the LED strip could involve some more bad soldering... by me.

I think I may look into a circuit trace pen, but I read that the traces created by such a pen have higher resistance than wire, so I'm not sure if that will be helpful to use on something that's supposed to light up. I will go about testing for failed diodes on the LED strip tomorrow.

I'd be interested in seeing pictures of this if you have the opportunity.

PlainBill

[momaka]

Re: Viewsonic VX2450wm

I'd be interested in seeing pictures of this if you have the opportunity.

Me too.
LED-backlit monitors are still fairly new technology. Would be interesting to see what's inside them (the LED strips, that is) and how they are constructed. Maybe it will also give us a glimpse of what to expect in these LED-backlit LCD displays and whether we'll have another wave of dead displays coming in (like the good-old CCFL backlit LCDs).

[PlainBill]

Re: Viewsonic VX2450wm

Wouldn't it be wonderful if we start seeing a rash of LED backlit monitors with bad soldering on the backlight connector?

PlainBill

[UpThyWazzoo]

Re: Viewsonic VX2450wm

Alrighty, well I'd edit my first post if I could find the edit button (sorry mods), but this is where I've gotten so far...

I tested the resistance of each LED, then switched leads to make sure it was open when I did. I finished about 1/4 of them, then got lazy and checked every 10th one or so. They all seem fine.

So here, we have a close-up of the section of LED strip in question and a normal shot.

As you can see, the traces/pads are really, really small. If the connection between one of the ground pads and the ribbon cable had been broken somewhere, it would easily explain why only half of the screen had light. But now I've gone and broken all the connections like a boss.

I agonized over getting that plastic connector back in place and fixing the pads, but ultimately that job requires tools I simply don't have. So, I think I will try a different solution. The plan is:
1. Use a sharp tool (exacto, or maybe something even smaller?) to expose a bit of the copper at the through-holes.
2. My student lab has this solder paste that has the consistency of thermal paste, and then becomes a bit like real solder once an iron is applied. By using a needle, I can deposit this paste accurately onto a through-hole without touching the other ones.
3. Since through-holes go through the PCB (lol.), I can give myself some extra room to work with by doing 5 on one side and 5 on the other, alternating the deposits.
4. Then, I insert the tip of a section of fine copper wire into the deposit, and secure it to the edge of the PCB with some glue.
5. Carefully heat each deposit with an iron (I've heard that using dental picks as soldering iron tip is doable)

Eventually, I should have 10 happy copper wire leads poking off the LED strip. Connecting these leads to the ribbon cable will be trivial, I think. If I could save myself some soldering work by using the liquid from a silver trace pen instead, please let me know. --Or any other advice

And here are some pics to make Bill happy:

Opening the monitor is a little weird. You have to dig your fingertips into the inside edge of the plastic trim, and start lifting carefully. Each hook and clasp should give once it's at the right angle.

Once the top, right, and left side of the trim are detached, flip the screen over and focus on the back panel at the bottom. It's another hook and clasp deal. The picture doesn't show it, but flipping the screen over is recommended because there are no screws holding motherboard/power section to anything, yet it's attached to the LCD panel and power button (in the clear acrylic thing, which is attached to that front trim). So any lifting of the top trim/LCD panel will probably put stress on the ribbon connection points, or make alarming *thunk* sounds when the motherboard section drops away (happened to me.)

After disconnecting the ribbon cables connecting the motherboard section to the LCD panel, I took the panel out and got to deconstructing (sorry I don't actually have pictures of that, but it did involve a serrated kitchen knife) Here is the panel unfolded clamshell style, and this is where the LED strip goes.

Really long post, sorry. I will try to take more pictures of everything during reassembly if it will help anyone. Cellphones are crap to take pictures with, unfortunately.

[momaka]

Re: Viewsonic VX2450wm

First and foremost, thank you for your pictures and explanations. Very informative and I'm sure this will be helpful to many other people who might have problems with their LED-LCD monitors in the future.

Now onto the questions...

I agonized over getting that plastic connector back in place and fixing the pads, but ultimately that job requires tools I simply don't have. So, I think I will try a different solution. The plan is:
1. Use a sharp tool (exacto, or maybe something even smaller?) to expose a bit of the copper at the through-holes.
2. My student lab has this solder paste that has the consistency of thermal paste, and then becomes a bit like real solder once an iron is applied. By using a needle, I can deposit this paste accurately onto a through-hole without touching the other ones.
3. Since through-holes go through the PCB (lol.), I can give myself some extra room to work with by doing 5 on one side and 5 on the other, alternating the deposits.
4. Then, I insert the tip of a section of fine copper wire into the deposit, and secure it to the edge of the PCB with some glue.
5. Carefully heat each deposit with an iron (I've heard that using dental picks as soldering iron tip is doable

Eventually, I should have 10 happy copper wire leads poking off the LED strip. Connecting these leads to the ribbon cable will be trivial, I think.)

Sounds reasonable to me, except for #5 - if you don't have a soldering iron, buy one, even if it's the cheap $2 variety from ebay. Not having the proper tools can sometimes really make a job frustrating and/or take very long to do.

If I could save myself some soldering work by using the liquid from a silver trace pen instead, please let me know.

Is that the solder paste that has the consistency of thermal paste you talked about above?
If yes, I've never used this product before so unfortunately I can't answer your question. My only suggestion would be to perhaps test it on something else before trying in on the monitor.

Really long post, sorry

No need to apologize . In fact, many of the members here, including myself, prefer (and to some extent even enjoy) long detailed posts with lots of information like yours.
So again, thank you for taking the time and posting these pictures.

[brunosmith]

Re: Viewsonic VX2450wm

Here you will find all you need for your DIY LED light strips installation, a full led strip kit that will satisfy the most demanding project.

[b700029]

Re: Viewsonic VX2450wm

If it's just connected as a series string of LEDs you don't need to bother with 10 tiny wires, if I were you I would just replace the ribbon cable with two large wires connected to + and - as appropriate, solder the ends to the right terminals on the boards and add an inline connector for convenient (dis)assembly.