A lesson on LVDS, impedance and cheap cables from China

A handful of months ago, my laptop (Lenovo Ideapad Y500 for those wondering) started suffering with screen issues. Specifically the ones that result from the tiny wires in the LCD cables shearing in the hinges. Taking it upon myself to go to eBay to find a replacement, I found a single seller selling the part for the grand total of $15, all the way from some hole in China. Estimated delivery 3-4 weeks. *sigh*.

When it showed up, I did the obvious thing and replaced it there and then. Upon re-assembling, things were looking good. The red jittering/tinge to my screen vanished, and all seemed ok.

Except when they weren’t. Shortly after replacing it, the problem started to get worse, not better. Instead of just the red jittering, the entire picture was. At this point I’m starting to wonder what the problem actually was, and that it wasn’t the cable after all (in the way that replacing the cable didn’t solve the problem, but made it worse). Now I’m starting to suspect all sorts of other components. Except there’s not a lot involved in the transport of the signal between GPU and LCD. There’s the GPU, obviously. The cable. And the LCD’s controller. Though upon disassembling the machine again, I notice that I’ve routed the new cable adjacent to the WiFi RF cables. Figuring interference from the RF is what’s causing the jittering, I re-route both cables as best I can within the confines of the inner chassis.

No avail.

I start investigating other options. Those that talk about the solder joints between GPU and mainboard breaking down under the heat. A legitimate resolution for some display issues is to bake your motherboard in your oven. Yes, that’s right. Strip the machine down, yank the board out, strip it of all heatsink paste, stickers, batteries, and shove it in the oven at 200 degrees celcius for 15-20 minutes. This is a $1200 laptop, so you can imagine my concern when I’m putting the most expensive part to replace in the oven. The longest 15 minutes in the world pass by and I remove the board. Let it cool, then re-assemble.

No avail.

So now I’m wondering either the reflow didn’t work as I expected it to, or that something else is bad. Perhaps it’s the LCD controller. I spend some time reading up on LVDS (the signal protocol that’s used to transmit the high-speed video data between the GPU and the LCD. I find that LVDS has termination resistors on the receiving end, and that these resistor values are matches to the impedance of the cable. If they don’t match, the signal ends up “bouncing” when it gets there, and as it clashes with the new data coming in from the opposite direction, the data’s integrity is lost and the picture ends up flickering as the LVDS buffer “resets” to get a good data stream back. Rinse and repeat.

Ok, so maybe the $15 cable from eBay is bad. I go to Lenovo and find a genuine factory replacement (which was somewhat over the top for what is essentially wire, €25 + €10 shipping, around $50 in total). Ships from Germany, takes 2-3 weeks to turn up. Whatever, if it’s going to fix the problem, right? The new cable turns up, same process, strip machine down to its bare bones and replace the LCD cable. Again.

Success. 2 and a half weeks on and the problem hasn’t re-occurred.

Lesson: don’t buy replacement LCD cables from eBay/China. They’re no good. My guess is that the wires are made up of a cheaper copper, or are thinner. Something different that affects the impedance of the cable. It can even go so far as being a couple of millimeters too long/short to be enough to affect it this much. Go buy a genuine one from the manufacturer. Your laptop will thank you for it, and you won’t end up in a situation where you just want to throw it out of the window and buy a new one. A $50 genuine cable saved me $1500 on buying a new laptop.

Edit: for those wondering where I got the replacement cable for the Y500 from, I got it from the MEDION Service Shop. Product number is 35008883, priced (on December 17th 2014) at €20.12 + shipping.