What fails in CCFL inverters?

Speaking of failing TVs... 8-)

My old Zenith 25" TV has been doing the oddest thing. The menus are going crazy, like someone was randomly hitting the menu button on my remote. The menus pop up, start changing different values, go here and there throughout the menu tree. It drives me crazy to try and watch it! My wife doesn't mind, though... ;-)

Any ideas what could be causing it? At first, I thought it might be a remote problem, or even a remote interaction problem, but it doesn't look like it. I even to out the batteries to all the remotes, and covered the IR receiver window, but it still kept going crazy. My only thought is some sort of fault on the logic board, or something.

Charlie

Ask on news:sci.electronics.repair Give them the model number or chassis number.

Rule 1 (or is it 4...?) of news groups: do not hijack a thread.

If you want to ask another question, please start your own thread.

Cheers,

-=-=-=-

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.

Charlie E. wrote in news: snipped-for-privacy@4ax.com:

probably bad electrolytic caps. perhaps one or more of your LV supplies is getting out of spec,and affecting the logic ICs.

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

I'd look at the PCB & caps around the IR receiver. IR receivers are also very sensitive to RF, so you could get erroneous operation if there's something wrong with the shielding or grounding around the module. [sci.electronics.repair added]

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.

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. After replacing the fuse (remember it's been over a year since this thing has been powered on for more than a few seconds at a time) the caps needed a little time to reform.

Or maybe a power surge popped the fuse (and the cap re-form thing still applies).

I think I'll replace the caps -- for peace of mind -- and see if it continues to work.

High ESR !

Doh!!

Yes, of course.

(Homer answer: "Oh, so maybe I shouldn't have replaced all those good caps?" ;-)

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.

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

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.

Well played, sir!

Ngung, ngung, ngung!!