Why does applying light onto a mylar ribbon "fix" my TV? - Page 1
Forum section: electronics / repair

author: dyril
author_rank: Newbie
timestamp: January 20, 2016, 04:07:25 am
content: My backlit Samsung UN32EH500 LED TV got crippled, probably due to excess indoor humidity from El Nino late last year.One third of one half of the screen was glitchy.Visuals explain it quicker:https://youtu.be/z-3m7UqoIBgPictures of me trying to fix it:http://imgur.com/a/lOCeSSo shining a light on the chip on a rotted ribbon leading to the panel corrects glitchiness enough to call it a fix.How and why does this correct the picture?And can I expect this to only be a matter of time before it fails?

author: Brumby
author_rank: Unknown
timestamp: January 20, 2016, 05:10:04 am
content: Neat trick.No idea - but I'd be interested to find out.

author: cdev
author_rank: Super Contributor
timestamp: January 20, 2016, 05:16:55 am
content: Does your TV adjust its brightness based on the ambient light level?

author: Monkeh
author_rank: Super Contributor
timestamp: January 20, 2016, 05:39:44 am
content: There is actually a chip on that FFC, which explains it being photosensitive. Couldn't tell you why it behaves that way, though.

author: helius
author_rank: Super Contributor
timestamp: January 20, 2016, 05:43:04 am
content: Very interesting question that requires a dissertation on quantum physics to fully answer.The short of it is that light is affecting some device it is shining on, that is not designed as a photocell. This is possible because the device is not encapsulated but is mounted as a CSP (chip scale package). See how light can upset the behavior of CSP causing a product to crash:https://www.eevblog.com/forum/blog/eevblog-716-raspberry-pi-2-xenon-flash-problem-explained/Now, in that thread the problem is that bright light upsets the chip and causes a crash. In your problem light is required for correct operation. These are both consistent with the behavior of silicon ICs: light is trapped in the silicon and creates charge carriers (electrons and holes) which then diffuse through the silicon. In a very bright flash, the amount of charges is huge, and can upset the state of the device making it lock up from sudden voltage fluctuation. In a dim light, the amount of charges is low, but they do move around in the silicon and interact with other charge carriers. If a transistor is injecting a lot of charge because it is damaged, the charges generated by the light can recombine with them before they upset something else.You can look at it as another way of diagnosing a faulty chip, similar to using freeze spray. You're creating a condition that makes a damaged chip work a little bit better by reducing the amount of effective leakage current that is screwing it up. The chip will need to be replaced.

author: Rerouter
author_rank: Super Contributor
timestamp: January 20, 2016, 07:11:37 am
content: I also agree on a photosensitive chip, I would just bodge a white smd led next to the ribbon and call it a day,

author: voltz
author_rank: Frequent Contributor
timestamp: January 20, 2016, 11:23:45 am
content: You could try using a much smaller point of light, say an LED (white if possible, but others may work) and really home in on the exact point where the fault (fix) occurs. You have a great opportunity while its in this state to locate the exact component and fix it.If it turns out to be the ribbon cable then look for printed components on the cable itself and just replace the cable assembly. If its not populated with any components then that is a very strange photoelectric effect worthy of physics interest but still just replace the cable.

author: EPTech
author_rank: Regular Contributor
timestamp: January 20, 2016, 06:23:00 pm
content: Quote from: Rerouter on January 20, 2016, 07:11:37 amThis has made me laugh out into tears.Who dares to say us techs do not have a sense of humor. But then again only us techs would laugh at a joke like that.Anyway. It would be nice to know what kind of light it reacts to best. I would expect the higher the energy, the better. Maybe try some UV and some infrared if you are up for an experiment of course.

author: Fred27
author_rank: Unknown
timestamp: January 20, 2016, 06:34:07 pm
content: Quote from: EPTech on January 20, 2016, 06:23:00 pmGenius. If the light isn't visible nobody will know that you bodged it!

author: dyril
author_rank: Newbie
timestamp: January 20, 2016, 11:10:58 pm
content: Thanks for the insights!I can only add that it seems 5 of those 10 broken traces seemed to be common (visually at least) and there are a couple of smd caps where they originally lead to.I had already bodged it in a most conspicuous manner (see album link in OP); really I was just curious as to why this was happening.The ribbons are integrated with the panel so they are unfortunately not user-replaceable. They are also pressed onto the controller PCB in a permanent, one-time fashion.Aside from better climate control I also wonder if one could preemptively protect mylar ribbons of this nature by say, slathering them in mineral oil to prohibit oxidation. Probably not worth the effort in most cases to go against planned obsolescence where entertainment hardware is concerned.

author: helius
author_rank: Super Contributor
timestamp: January 21, 2016, 12:05:19 am
content: If the device has multiple bond-outs for its common net, and most of them are no longer conducting because of cut traces on the FPC, then the light could be increasing the conduction to the remaining pins by supplying more charge carriers.

author: AndyC_772
author_rank: Super Contributor
timestamp: January 21, 2016, 06:54:56 am
content: Light shining on a silicon device tends to increase leakage current, by generating more electron-hole pairs. Perhaps what's happened is that one half of a differential pair has become flaky, but by changing the threshold voltage slightly on the other half, a correct signal is still being transferred via the working half alone?

author: Rasz
author_rank: Super Contributor
timestamp: January 21, 2016, 06:27:51 pm
content: brilliant bodge, this is hackaday worthy

author: EkriirkE
author_rank: Newbie
timestamp: January 22, 2016, 09:16:12 pm
content: Quote from: Rasz on January 21, 2016, 06:27:51 pmYou got your wish

author: Rasz
author_rank: Super Contributor
timestamp: January 23, 2016, 12:07:11 am
content: I did the leg workIv been doing LCDs for hmm 7 years now, and never seen anything like it. Heating, squeezing, reflowing (very gently), but never lighteven if its fake, it would be a brilliant prank

author: Delta
author_rank: Super Contributor
timestamp: January 23, 2016, 12:12:31 am
content: Superb work!It shows the difference between an engineer and a technician(proud technician here BTW)