LCD monitor's Inverter bad ?

Rajiv: I just repaired an inverter for a 15" Viewsonic LCD. It was a dual lamp one like yours. Two transistors that drive the transformer for one of the lamps were bad. I used the diode position on my DMM to check the SMD transistors. Replacing the transistors with regular larger TO-92 case style ones fixed the problem. I found that if you hold a small NE-2 bulb near the transformer, the gas in the bulb will glow if there is high-voltage AC close to it. If the bulb glows, the inverter for that section is working OK.

Most inverters will shut down if there are problems on the board. The bulb would glow for just a second or two, then go out. It would not glow near the transformer with the bad driver transistors. The 4 wires going to the inverter on mine were 12 Volts, ground, enable, and dimming (some will change the voltage on the lamps to control brightness). It helps having a dual-bulb inverter in that you can check back and forth between the parts on the two ends of the inverter board. I have seen places on the Internet that sell the inverters for between $70 and $130 (plus shipping). Hope this helps... John

John, Thanks for your response. I checked the transistors using my DMM but they seem all right to me. One thing is interesting, though. My inverter has two transistors for each lamp pair. It appears that the transistors are connected in parallel (their EBC legs are shorted with the paired transistor) But I guess they have it that way because they just want more current handling capability.

I have a few questions for you:-

1. About your Viewsonic monitor, a. Do you have a pair of lamps at the top and bottom (a total of 4 lamps)? b. Do the lamps at the top and bottom light up simultaneously?

1. If the lamps connected to one side of the inverter do not light up (or are faulty) does the other side's lamps also turn off ?

2. Did you actually measure a +12 on one of the 4 wires going to the inverter? Is that DC or AC ? Is that voltage stable at 12 v or does it varies with the brightness? (although I understand that there is a separate dimming control voltage)

1. When the inverter shuts down (due to a fault or something) does the

off the inverter board (maybe on the motherboard)?

it. I have GND on two of them and one has +5v (which is display on signal) and the 4th is kinda floating !

I used my DMM to measure DC on the 4th wire, unless it has 12 v AC my measurements are valid.

Still I do not understand how come one of the CCFL flashes on when switching on when the inverter is not getting 12 volts. My inverter sure does need 12 volts, as evident from this page

Unless it gets 12 volts for half a second !

Any input is highly appreciated!

-Rajiv

AFAIK All inverters use 12V as the supply. All TFT screens I've seen have all the lamps switched on all the time - the brightness signal alters the intensity of all lamps, not switches some of them off. Of the four connection wires, one should be 12V, one Ground, one switches the tubes on if there's a valid input signal, and the other controls the brightness.

In your case the inverter is faulty - if one of the outputs goes bad, a fault signal switches off all the inverter outputs. It is completely normal for the display to flash on when you switch it on or apply a video signal, it takes a second or so for the inverter fault circuit to detect the fault and switch off the outputs.

The fault could be an output transistor or a faulty transformer. I've found the only way to repair these is to fit a new inverter. I have had some success using the cheap inverters sold with CCFLs for computer case modding - you might find these at Radio Shack or similar. You will lose the brightness control though.

Hope this helps, Richard

Hi Richard,

Thanks for the response.

You seem to be pretty knowledgeable so can I ask you some questions?.

First

What if my CCFLs are faulty? Don't you think in that case too the inverter will shutdown?

I just want to be sure before I put some money in a new inverter (some $60 but even then the exact same part is not available anywhere. They do have similar inverters which can make 2 CCFLs light up, instead of

I did see some $7 inverters on All electronics. They might be my only solution.

Second

Why does only the bottom pair of CCFLs in my monitor have black ends?

I am guessing that only those ccfls were switched on when the monitor was all right, am I wrong ?

Last:

Do I really have to light up all 4 CCFLs? What if I get 2 of those cheap inverters and light up two CCFls , one at top and other at bottom. I do understand that the display's brightness might suffer but having 4 CCFLs sounds like overkill to me. Maybe that is why the CCFLs at the top never switched on.

Thanks for your time!

Regards,

Rajiv Mehra

PS: BTW there are inverters out there which take 5 volts (as the supply) instead of 12 vdc. I saw them on some websites. One can be seen here

If the tubes light up even briefly they are not faulty - there isn't really anything to go wrong with a CCFL - it's just a tube with an electrode at each end filled with gas.

Buy one and use it to test the tubes if you are not sure!

I have no idea - I have a set of tubes I took out of an old 18" screen

- some have black ends, some don't, and they were all switched on when the screen was in use.

Different screens have different requirements for backlighting - Some use a tube at the top and bottom, mostly laptops I think, some have tubes equi-spaced behind the panel. You could try using just two tubes and see if the result is acceptable.

HTH Richard

That's not entirely true, as they age, the emissive coatings on the electrodes wear out and the voltage required to sustain an arc rises. It's possible for it to reach a point where the inverter has enough kick to strike an arc but can't sustain a high enough voltage to keep it going. I would certainly suspect the inverter first though.

First, sorry for bringing such an old thread back to life but I wonde

if anyone has any other ideas about this. I have the same problem wit a KDS PixelTouch monitor. I have found an interesting twist to thi problem though. If I leave the inverter plugged in but remove th screws holding it and position it outside the monitor, it work perfectly.

Also, if I place the inverter board underneath the LCD and assemble i while it is working, it will continue to work until power is lost. Normally I would just replace the inverter but PixelTouch no longer ha any of these. In addition, the original company that made thes monitors (gvision, who sold them to KDS for rebranding) has th inverters for $25 but will not sell them. They state they are fo internal use only and I'd have to send it in for a $50 repair plus th $25 part.

I initially thought that it was the positioning of the cables but I'v tried to reposition the connecting cables to no avail. As far as I ca tell, the inverter looks perfect and performs perfect until it i installed properly.

Any ideas

On Sat, 13 Jan 2007 19:46:47 +0000, stevezemlicka Has Frothed:

Maybe the screw apply just enough pressure on the inverter that it causes it to fail?

Maybe there is a defect in the circuit board, or in some part, that when it is mounted, something is going opened. Another thing, is that maybe there is a component that has become temperature sensitive, and it is failing when there is a little more heat from being mounted.

Try using a heat gun on the inverter to see if it fails. If it stops working, you can then use some freeze spray on one component at a time until you find the failed one. Most of the time, the electrolytic capacitors go bad on these boards.

--

JANA _____

First, sorry for bringing such an old thread back to life but I wonder if anyone has any other ideas about this. I have the same problem with a KDS PixelTouch monitor. I have found an interesting twist to this problem though. If I leave the inverter plugged in but remove the screws holding it and position it outside the monitor, it works perfectly.

Also, if I place the inverter board underneath the LCD and assemble it while it is working, it will continue to work until power is lost. Normally I would just replace the inverter but PixelTouch no longer has any of these. In addition, the original company that made these monitors (gvision, who sold them to KDS for rebranding) has the inverters for $25 but will not sell them. They state they are for internal use only and I'd have to send it in for a $50 repair plus the $25 part.

I initially thought that it was the positioning of the cables but I've tried to reposition the connecting cables to no avail. As far as I can tell, the inverter looks perfect and performs perfect until it is installed properly.

Any ideas?

-- stevezemlicka