Viewsonic vx2255wmb inverter help

[Snype32]

Hi

I just found this site and it looks to be a great resource for info about repairing lcd inverters. I'm hoping someone can help me out. I have a Viewsonic VX2255wmb LCD. The backlight is not working. I know the tubes are good because they light up fine with another inverter. I have replaced numerous caps already to no avail. What am I missing?

I have replaced:

2 25v 220uf these 2 were bulged
2 25v 1000uf
1 10v 1000uf

I tested the small surface mount fuse and it looks to be ok. I also checked the two surface mount transistors on the back of the board for shorts but don't know what to look for beyond that. Any help would be great.

Thanks!

[alexanna]

Re: Viewsonic vx2255wmb inverter help

I see 2 SMD fuses on the back of the board,F101 and F200

[sabre504]

Re: Viewsonic vx2255wmb inverter help

Have you tested the inverter transformer or do you no power at all?

[Snype32]

Re: Viewsonic vx2255wmb inverter help

Thanks for the quick reply. I didn't notice f200. I tested it, and it appears to be open. Would that be because of the blown caps earlier? I know it's not recommended, but can I bridge it for testing?

Thanks!

[Snype32]

Re: Viewsonic vx2255wmb inverter help

sabre504

The panel is getting power and the LED is on. Just the backlight is not working.

[sabre504]

Re: Viewsonic vx2255wmb inverter help

Check the resistance between the 2 pins circled in red
Do the same to the 2 circled in yellow and post result they should be within 3% of each other .
I wouldnt recommend jumping a fuse

[Snype32]

Re: Viewsonic vx2255wmb inverter help

just checked pins indicated in pic...exactly the same.

Thanks for the quick replies...wasn't expecting suck quick responses.

[Rtech]

Re: Viewsonic vx2255wmb inverter help

With everything plugged in,including a PC input,check the Voltage on the On/Off pin,on the right hand side of the board.should be between 3 and 5 volts DC as long as the Logic board has a signal from the PC and has recognised it.

[Snype32]

Re: Viewsonic vx2255wmb inverter help

checked on/off pin....3 volts

[alexanna]

Re: Viewsonic vx2255wmb inverter help

Let me suggest one more thing and then I'll let you guys have it,
F200 looks like it feeds the inverter, what comes to mind is the two MOSFETs possibly shorted.

[Rtech]

Re: Viewsonic vx2255wmb inverter help

You can try resoldering all joints from and including the Inverter Transformers to the lamp sockets..if that does not do it, then you are into tracing the on/off signal to the Control chip,and then to the Output Fets, for that you will need to find the Datasheets from the part numbers on the components.
When you checked the Backlights,, what other inverter did you use ??

[Snype32]

Re: Viewsonic vx2255wmb inverter help

I did resolder everything that look at all weak. I thought the on/off was working as I tested it and it was at 3 volts. I tested the backlight using an inverter from a different lcd I had that was cracked. I hooked the secondary inverter up to the tubes and left the original inverter conneted to the lcd. when everything was powered up, the lcd displayes an image normally. Tracing the on/off signal sounds like alot more work than I am willing to put into this LCD. If f200 is really a fuse, then I think I should look into that first. I'll see if the mosfets are shorted, and if not, I might be tempted to jump f200. I know it's not recommended, but if it works, I'll get the right fuse. If not, it's toast to me anyway.

Thanks for all the input.

[Snype32]

Re: Viewsonic vx2255wmb inverter help

So I took the plunge and jumped the f200. It worked! Monitor now works perfectly. I do think that I should put a fuse in tho. Could I de-solder one from an old motherboard I have and use that? I don't know how I would figure out the rating on it. The one on the lcd just has P on it. the one on the motherboard has p 110 on it.

Thanks again.

[momaka]

Re: Viewsonic vx2255wmb inverter help

Thanks for the quick reply. I didn't notice f200. I tested it, and it appears to be open. Would that be because of the blown caps earlier?

Yes, possibly.

I know it's not recommended, but can I bridge it for testing?

No.
The reason it's not recommended is because more parts can go bad if there is a fault, and then repairing this power supply may become impossible.

The P on F200 indicates it is rated for 3 Amps. Get a 3A fuse and replace it. If you don't have any SMD/picofuses of that size, just use a regular glass fuse and wire it with wires or hook it up with jumper cables.

*** EDIT ***
I see you beat me to it . You got lucky nothing else was bad, though.

Anyways, congrats on the fix! Now just get that fuse replaced and you're all set .

[Rtech]

Re: Viewsonic vx2255wmb inverter help

P may be 1.5 Amps, i would a 2 amp just in case.Note P could be 3 amp also...dependant on manufacturer

[Snype32]

Re: Viewsonic vx2255wmb inverter help

so a fuse marked p 110 from a motherboard would work then? It's a bit wider, but I can make it fit.

[momaka]

Re: Viewsonic vx2255wmb inverter help

P may be 1.5 Amps, i would a 2 amp just in case.Note P could be 3 amp also...dependant on manufacturer

Well, you may be right. A 3A fuse makes sense, though. After all, if the inverter is using a 12v power supply, then 3A at 12V is 36 Watts, which sounds reasonable for an LCD monitor of this size.

so a fuse marked p 110 from a motherboard would work then?

I don't know what the 110 indicates, so it may work or it may not work.

[PlainBill]

Re: Viewsonic vx2255wmb inverter help

so a fuse marked p 110 from a motherboard would work then? It's a bit wider, but I can make it fit.

A Modest Proposal. IF your DMM has a 10 Amp range (many do), use the DMM in ammeter mode to measure the current draw of the inverter. That will give you an indication of the current rating of a replacement fuse. Warning - don't let it run this way. Most DMMs with a 10 Amp range indicate a maximum of 10 seconds.

PlainBill

[Snype32]

Re: Viewsonic vx2255wmb inverter help

I just tried the p 110 fuse...no go. Fires up for abot 2-3 seconds and quits. The jump I did before works permanently. I do want a fuse in there tho. I did just notice that beside the fuse on the inverter it is printed 3amp 24volt. I'm betting the p 110 is 12 volt. Looks like I will need to order a fuse. Funny thing is that the p 110 would fire it up for a few seconds and quit. Turning the lcd off and back on repeats this. I thought the fuse would be a single use? I know it's the fuse causing this as when I do a direct jump, it works fine.

[momaka]

Re: Viewsonic vx2255wmb inverter help

Funny thing is that the p 110 would fire it up for a few seconds and quit. Turning the lcd off and back on repeats this. I thought the fuse would be a single use?

Not if it's a resettable fuse, which it likely is based on your experiments. Did you get it from near USB ports? If so, it's probably a 0.5A or 1A fuse.
Save it, those resettable fuses are nice to have sometimes.