Dim CCFL monitor repair

[MathWizard]

I have this old salvaged Acer LCD CCFL monitor w/ VGA connector, which I need sometimes. And it looks pretty dull and washed out.

So I took it apart, without probing or measuring anything when running, and mapped the whole PSU. I just wondering what to look for.

It runs of mains power, then has a flyback converter for 5V, Vaux, and a 40V or so rail. That rail feeds a half-bridge inverter for the high voltage.

There's only 5 electrolytic caps in total, and they have great ESR and DF.

I'm new to the half-bridge layout, in the primary side of it, I see the upper and lower cap's, that sit on 1 end of the primary inductor. They use 2 pair's of SMD, ceramic 2.4uF each. At least that's what they measure out of circuit at LV. I never tried their ESR, IDK what it should be anyways, or what rating these might have since they are SMD.

What's they chances they degraded, or the 2 mosfets. Also in the datasheets, the Nch had a way higher K value, like 10x, then the P-ch fet, I wonder why they are so different, like 6S vs 64S ?

The inverter controller IC is a INL816 , I can't find a DS, but at least I found a pinout that matches
https://www.datasheetq.com/INL816-doc-ETC

I see 3kV rated caps on the HV side, that pass the Vsen info to the IC. And I see the lamps are just sinked into a 4.7k+470 divder, and that is the current sensing.

So besides maybe SMD caps or the HV caps, what else could be going wrong ? I want to probe the flyback HV, and the Vsen and Isen of the inverter chip. But I don't have a data sheet for the inverter IC, so IDK.

Below is the basic layout of the PSU, then there's just a little VGA output board.

[BrokenYugo]

The CCFL tubes themselves do wear out.

[james_s]

Usually the problem is worn out CCFL tubes. I've also seen the light spreader optical parts turn yellow/brown with age, I took apart an LCD panel once that was so bad the sheets of diffuser material crumbled when I touched them.

[inse]

You can also try to squeeze the last of remaining life out of the tubes by tinkering with the current feedback.
This is how I revived or say extended the life of a TV some time ago.

[james_s]

Careful doing that in a monitor though, they are normally edgelit with the CCFL in contact with a plastic light spreader. If you crank up the current the cathodes will run hotter and you might discolor or even melt the optics. CCFL tubes are cheap, and LED upgrades are available these days. If the optics are ok it's probably worth replacing the worn out tubes.

[MathWizard]

Ok I have soldered on a bunch of wires, it's been a while since I probed all over a SMPS and I've learned a lot since then. I'm not paying for lamps, but yeah I'd consider trying to up the power a little bit, and see if it helps. I hoping it was just going to be weak electro. caps, draining away power.

IDK what mosfet's degrade like, I don't think I have anything similar, except way overrated stuff.

As for calculating what's going on, I thought I knew mosfet equations, but I kept tripping up over the factor of 1/2 in the saturation mode equation. Maybe it's in linear too. But I seem to remember in the derivation, something like integrating the E field along the DS channel, and basically the integral of the kVov= k(Vov^2)/2

But sometimes I see the 1/2 in with the parts that make k.  And I swear some pdf's have Id=k(Vov^2)/2, and other's just have Id=k(Vov^2).

I'd have to learn magnetization currents, reflected impedances, and maybe more to figure out the open loop gain of a simplified model. But I'm getting there.

[james_s]

I'm not sure what you're talking about, you're unwilling to pay a small amount for new lamps but you're willing to invest a bunch of time replacing capacitors or making other modifications? Mosfets generally don't degrade, they either work or they don't. It's almost certain that the tubes are shot and/or the light spreader is yellowed, messing with the electronics is a waste of time.

[MathWizard]

I haven't used this monitor since May, and I just used it a bit, and I forget I had taken it apart and checked it's caps and what not.

Today it was dim, and then after about 20min, it flickered, then went black, but the image is still there with an external lamp shinning on the screen. I was hoping for an easy cap replacement fix, then I remembered all the caps had great ESR.

I took it apart again, and there's no obvious problem, not without going de-soldering anyways.

So unless I want to try an LED conversion, from 2 other monitors I wanted to try fixing, I better order some new bulbs then. But then, there's people selling used VGA/DVI monitors, for probably the same as the bulb replacements.

[james_s]

Monitors that may be just as tired as this one. The LED kits are cheap, it's been ages since I bought one but at the time it was under $30. You need to check if the optics are yellowed though, if they are then it's not really worth messing with.

[CaptDon]

I should think you could find perfectly working monitors with as good or even better specs than your broken unit at any of the thrift stores or used computer refurbishing stores or even local ham radio flea markets. GE was throwing out hundreds of perfectly working Dell monitors with VGA/DVI dual mode monitors with auto-detect because everyone wanted 'bigger' monitors. Of course as a corporation GE was too f--king stupid to simply offer these perfectly working monitors to employees for free. They had to go to the crusher. Well, glad to say, several of these monitors fell off the trash cart and somehow landed in the trunk of employees cars!!!