Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

I took apart my Dell 17" to try to repair it. I expected a burned out capacitor or something else obvious but the only thing I found was this white paste touching several components. Could this be the cause of the problems or is it generally non-conducting?enter image description here

enter image description here

share|improve this question

closed as off-topic by Matt Young, Nick Alexeev, Leon Heller, Joe Hass, Daniel Grillo Mar 10 at 11:31

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Matt Young, Nick Alexeev, Leon Heller, Joe Hass, Daniel Grillo
If this question can be reworded to fit the rules in the help center, please edit the question.

    
That white gunk you see is sometimes a silicone material as Gonzik has suggested. You can generally tell if it is silicon if it has retained a rubbery texture. However note that many far east manufacturers use other types of materials that can get very hard after some years. This white gunk is generally OK. There are other types that are a nasty yellow color that sometimes contain chemicals that have been known to slowly eat away copper traces on a circuit board or interact with the seals on the bottoms of elecrolytic capacitors turning it into a gooey black mush. –  Michael Karas Mar 10 at 4:54
2  
1  
This is a perfectly fine, specific EE question; that it arrises in a repair context does not change that. Voting to re-open so that it can be properly closed as a duplicate instead. –  Chris Stratton Mar 10 at 14:29
add comment

3 Answers 3

up vote 8 down vote accepted

What you have there is the dreaded BenQ-designed CCFL inverter design that permeated all 17-19" TFTs in the mid-00's. You will likely find many more of the exact same failures in almost all brands of CCFL-backlit displays of that era, because with very few exceptions all commodity displays used the same CCFL inverter design. The biggest problem with this design is that BenQ designed in freestanding 2SC5707 transistors for the (bipolar) power switching, which are designed for 9-W (peak) 15" display backlights. Then they reused the design for 17 and 19 inch displays at higher brightnesses, driving the transistors beyond their design limits. This caused them to overheat (you will see a bit of scorching near those transistors) and eventually fail.

The problem was compounded by a bad default choice of capacitors, which are often faulty as well. These capacitors would get higher ESR which caused the primary power stage to ripple more and exaggerate the overloading of the 2SC5707 transistors. These problems are technically unrelated, but one often leads to the other.

I can't really read your brand and type of capacitor from the photos, but at first glance they look like quality brand and my experience with Dell monitors is that they usually replace the default BenQ choice with better caps. That means that the repair is probably as easy as getting a couple 2SC5707's from eBay and replacing them on the board. The fundamental problem still persists though, and it is advisable to improve the heat dissipation capacity of the transistors by e.g. attaching a bit of copper clad board on the flange.

share|improve this answer
    
Wow thanks for the info this is very interesting –  stormist Mar 10 at 8:53
add comment

No. This is a silicone adhesive used to ensure that large capacitors do not break off from the board due to vibrations. It is non-conductive and widely used in industry for DIP components.

Look for shorts or capacitors that look like they have leaked something out. If you have a multi-meter begin by checking that the power supplies are correct. Also try cleaning the dust off, sometimes it can be the cause of problems.

share|improve this answer
2  
Look also for capacitors in the power supply section that look like the tops have bulged out. This is clear sign of capacitor failure or pending failure. Note that it a commonly known fact that LCD monitors often fail after 2-4 years of use due to capacitor failures. Check on the web. There are whole web sites that discuss this subject. Some even offer kits of replacement capacitors for the DIY hobbiest to be able to easily repair their monitors. I am writing this comment right now on a pair of SyncMaster monitors that needed capacitor replacements 3 years into their 5 year life. –  Michael Karas Mar 10 at 4:47
    
I added another picture of the whole board. Is it common for a capacitor to go bad with no outward signs? How should I troubleshoot from here? –  stormist Mar 10 at 5:05
add comment

That white gunk you see is sometimes a silicone material as Gonzik has suggested. You can generally tell if it is silicon if it has retained a rubbery texture. However note that many far east manufacturers use other types of materials that can get very hard after some years. This white gunk is generally OK. There are other types that are a nasty yellow color that sometimes contain chemicals that have been known to slowly eat away copper traces on a circuit board or interact with the seals on the bottoms of electrolytic capacitors turning it into a gooey black mush.

share|improve this answer
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.