Re: What fails in CCFL inverters?

[PlainBill]

What fails most commonly in inverter circuits in, for example, 18 inch LCD
displays?

<http://www.lcdrepair.us/1800inverterKUBNKM045A.html>

I know anything *can* fail. But is there a common, high-frequency failure
component in these inverters?

Thanks.

One common failure in some designs is a chain reaction failure. Poor
quality caps are used in the power supply and the inverter. As the
ESR rises the drive transistors in the inverter tend to overheat,
eventually shorting and blowing the input fuse on the inverter.
Replaceing the low quality caps, the transistors, and the fuse results
in a working monitor.

The Westinghouse Westinghouse L1975NW (and equivalent Acer model) -
identical monitor except for the plastic - have this problem.

PlainBill

 

[PlainBill]

-=-=-=-

In case anyone watching was wondering, this is what I was looking for.
Knowing what *might* be the modus operandi of this monitor is a huge leg up
on the attack for a non-pro such as myself.

Thanks, PB! And to all the others who pitched in.

Paying it forward, one technique I have been known to use is to look
up the data sheet and any application notes of the major components on
an electronic assembly. In the case of an inverter, the PWM
controller IC is of major interest. The application notes usually
include a full schematic of an inverter. Quite often they also
include the calculations used to design the inverter.

One problem with the typical Chinese made monitors is no rigorous
analysis of the design has been made. One popular inverter design
included a pair of FETs in series from the positive power rail to
ground, with the primary of the transformer connected to the junction
between the two. Someone smarter than I dissected the circuit and
realized there was no provision to limit the current through the two
FETs if both were gated on at the same time. In addition, the circuit
driving the gates featured a resistor which (due to the capacitance of
the gates) would result in both FETs being on for a significant time
on every switching cycle. One could say the design was designed to
fail.

PlainBill

 

[Steve Sousa]

Re: this Gateway FPD-1830 (a.k.a. Westinghouse Westinghouse L1975NW, or
equivalent Acer model.)

Update on the inverter PCB repair:
Checked electrolytic caps for ESR; all were good, better than suspected.
Checked transistors for shorts and found none. SMD fuse was blow so I
replaced it.

Now when the display is powered on, the backlight comes on for 1 second
then
goes off. The Power indicator (in the On switch) acts normal (no blinking
error codes, etc.).

Any idea what might cause this symptom?

Thanks,

The HV transformer. Power it in a dark room, you'll probably see it spark.

 

[Ken]

So I just kept cycling power and... it has been powered on for over 2 hours
now with no problems.

I'm beginning to suspect the electrolytic caps. Being el-cheepo (no-name)
brand, they probably went intermittent short or low ESR and blew the fuse.

High ESR !

 

[Bob Larter]

Just to follow up...

Replaced all the caps (using Panasonic FM series) in this monitor and it
works like new. And should last quite a bit longer.

Thanks for passing on your results, John. It's always nice to know how
these things turn out.

 

[Rich Grise]

I concur. It's all-too-common to follow such a thread that is years old only
to find no-one cared to post the results. I like to think about the guy or
gal who is Googling a few years from now and comes across this thread. (Hi!)
;-)

Speaking of that, what ever happened to the self-tuning piano? Was that
guy ever going to post a .WAV file or something, of the piano tuning
itself?

Thanks,
Rich

 

[Bob Myers]

Speaking of that, what ever happened to the self-tuning piano? Was
that guy ever going to post a .WAV file or something, of the piano
tuning itself?

I believe it turned out that the only existing documentation
of the self-tuning piano in action was in the form of miniature
photographs stored as multiple images on a common sheet
of film, and these proved impossible to correctly adjust for
conversion into any electronic media. For as we all know,
while you CAN tune a piano, you can't tune a fiche.

Bob M.

 

[Rich Webb]

I believe it turned out that the only existing documentation
of the self-tuning piano in action was in the form of miniature
photographs stored as multiple images on a common sheet
of film, and these proved impossible to correctly adjust for
conversion into any electronic media. For as we all know,
while you CAN tune a piano, you can't tune a fiche.

Well played, sir!

 

[Spehro Pefhany]

I believe it turned out that the only existing documentation
of the self-tuning piano in action was in the form of miniature
photographs stored as multiple images on a common sheet
of film, and these proved impossible to correctly adjust for
conversion into any electronic media. For as we all know,
while you CAN tune a piano, you can't tune a fiche.

Bob M.

Not *alone*, certainly, but perhaps if you had a tuner helper...


Best regards,
Spehro Pefhany

 

[Rich Grise]

I believe it turned out that the only existing documentation
of the self-tuning piano in action was in the form of miniature
photographs stored as multiple images on a common sheet
of film, and these proved impossible to correctly adjust for
conversion into any electronic media. For as we all know,
while you CAN tune a piano, you can't tune a fiche.

Not *alone*, certainly, but perhaps if you had a tuner helper...[/QUOTE]

Don't hire Oppornockity!

For, as we all know, Oppornickity tunes but once. ;-)

Cheers!
Rich