Two second then Black..What is actually happening?

[bobdee47]

Hi guys, I have read, I think, just about every thread re the 2 second problem. Some folks have recapped virtually everything in sight and cured the problem, some have done the same and the problem is still there. Virtually all the symptoms point to the inverter section shutting down. It is doing this because it dosnt like the CCFL's but is it being oversensitive and if so how does replacing the caps cure this?. Without doubt the bad caps are not as "efficient" as they were, and this I guess, means that power supply lines now have excess ripple or noise on them and maybe this increases the chance of the inverter circuitry thinking the CCFL's are flaky. An interesting poster cured the problem by applying en extra decoupling cap directly across the IC responsible for brightness and general inverter operation. Getting to the point then, is it fair to say that the purpose of recapping is really to prevent the inverter from shutting down prematurely on detecting a possibly marginal CCFL that could perhaps last a few more years? I may have missed it but it would be good to see some power rail Oscilloscope pics, of before and after recapping. I have currently a packard Bell Callisto on the bench in pieces awaiting the arrival of new capacitors. The 2 second till dead problem was initailly "fixed" by reducing brilliance levels, but that lasted just a couple of days. Anyway, really interesting and thought provoking stuff on this site..

[PlainBill]

Re: Two second then Black..What is actually happening?

The Cliff's Notes version of 'Two Seconds to Black is there are multiple possible causes.

You mentioned one - excessive noise on the power supply rails caused by deteriorating filter caps.

A second cause can be an aging CCFL. As they age the current increases; the inverter controller regulates the total current through the CCFLs and monitors the current through each CCFL.

A third cause is a broken CCFL. The inverter controller detects the excess voltage and shuts down.

A broken lead to the CCFL can trip the protection by two methods - if the current is unable to find a path, the voltage will rise and trip the overvoltage protection. If an arc to the metal occurs, the return current for that lamp is lost, so the controller responds by increasing the drive, thus tripping either the over voltage or over current protection.

Another cause is a bad (shorted or open) transformer.

The final cause can be a bad component on the protection circuit - for example, a leaky diode.

Adding a bypass cap at the inverter controller won't really interfere with the protection circuits - rather it prevents a false indication of an error.

The recommended troubleshooting method for 'Two Seconds to Black' is fairly simple.

Look for a reddish hue on the panel when the backlights first come on.

If the capacitors are obviously bad -either bulging, or show excessive ESR with a tester, replace them.

If the caps appear to be good, substitute known good CCFLs for those in the monitor (just hook them up externally).

If test CCFLs are not available, check the resistance of the transformer secondaries.

PlainBill

[bobdee47]

Re: Two second then Black..What is actually happening?

Thanks Bill, I have seen references to the reddish hue in one of your posts before. Are you saying that that indicates a bad CCFL or one that is about to go that way. The monitor I am working on at present did show that effect some time back but seemed OK after about 10 mins of being switched on.

[PlainBill]

Re: Two second then Black..What is actually happening?

Thanks Bill, I have seen references to the reddish hue in one of your posts before. Are you saying that that indicates a bad CCFL or one that is about to go that way. The monitor I am working on at present did show that effect some time back but seemed OK after about 10 mins of being switched on.

That indicates a CCFL that is approaching 'end of life'. The monitor I am currently using showed a pink hue when first turned on for several months before it became necessary to replace the CCFLs. In this case, the problem lasted less than 10 seconds. Another one currently waiting a replacement CCFL had the same problem, but the pink hue only lasted a second or two.

PlainBill

[ratdude747]

Re: Two second then Black..What is actually happening?

I will toss in another possibility that I ran into once.

it is possible a failing transistor will do the 2 second thing... I recently did a benq made dell with the problem... the following fixed it:

1. resoldering the inverter transformers
2. recapping (it had capxons, so it was a "by default" thing to me)
3. replacing the inverter transistors.

the first two didn't fix it... the third did, but the first and second will help ensure a long term fix (bad transistors is often a symptom not a cause per-se).

every case is different, so good luck!

[bobdee47]

Re: Two second then Black..What is actually happening?

Thanks for the responses. Some major progress this end. Based on the assumption that the inverter shuts down when it sees excessive CCfL current, I thought about reducing this and at present I have the monitor going again just by inserting some high voltage disk ceramics in SERIES with the CCFL leads. The monitor has been running for about 2 hours now. Nothing gets hot, all seems stable. I will write this up properly if the "fix" seems good after say a few days.

[PlainBill]

Re: Two second then Black..What is actually happening?

Thanks for the responses. Some major progress this end. Based on the assumption that the inverter shuts down when it sees excessive CCfL current, I thought about reducing this and at present I have the monitor going again just by inserting some high voltage disk ceramics in SERIES with the CCFL leads. The monitor has been running for about 2 hours now. Nothing gets hot, all seems stable. I will write this up properly if the "fix" seems good after say a few days.

Back in the 40's and 50's a common cure for a noisy transmission or rear end was to add sawdust to the oil. While this DID quiet the noise, it didn't fix the problem; it simply allowed a crooked used car dealer to unload a lemon. Your 'fix' is in the same category.

PlainBill

[bobdee47]

Re: Two second then Black..What is actually happening?

Of course William, hence the " " but I still thought it might be of interest to someone. When my C's arrive I will do the job properly but at the moment just to get the monitor back on line is a plus.