ViewSonic VX910 monitor power supply problems

[kpatz]

My 6 year old Viewsonic 19" LCD monitor stopped working a few weeks ago. It powers on for a second or so (power LED green), flashes, then goes off (LED goes out), for about a second, then repeats ad infinitum, with a slight click coming from the power supply each time.

I opened it up and found bulged capacitors on the SMPS/inverter board, so I ordered replacements. They arrived today, and I put them in, but the problem didn't go away. The PSU still cycles on and off. I tested the MBR10100CT dual Shottky out of circuit and it is good, as is the CEF04N8 MOSFET. I also tested the other diodes on the board in-circuit and they are good. I don't see any burned resistors or anything.

So, those of you more seasoned in troubleshooting SMPSes, what other things should I check? If the capacitors go bad, what other parts can they take out with them?

Here's a pic of the board after I installed the new caps.

 

[kpatz]

Bump after 3 days... anyone?

 

[Sceadwian]

They arrived today, and I put them in, but the problem didn't go away

Lets invoke some basic logic here, if you think it was the power supply because of what you perceived to be bulging caps and you replace them and the issue remains then it's likely that your assumption that it's the power supply might also be wrong, or if in fact the power supply caps were the problem but it damaged something else outside of the power supply.

Can you power the supply up by itself outside of the monitor? If you can then you can test the power supply directly with dummy loads. The supply there looks pretty simple so I don't see why you can't power it up isolated from the monitor to test it, that'd be the next step, just don't fry yourself =). Check all the main rails as close to their max current ability as you can to see what happens and monitor the voltage for dips and if you have a scope for ripple.

 

[kpatz]

Ok, I tried the following:

1. I tried the PSU disconnected from the monitor board and I got a nice steady 12V (the only "rail" the PSU has, other than two CCFL inverters). Connecting it to the monitor, it would give me 12V but would then dip once per second with an audible "tick", like it was shutting down due to overcurrent or something.

2. I tried powering up the monitor with a different 12V supply. It powered up fine, at least to the extent where the power LED came on and stayed on, and then went to power-save mode after several seconds since it had no input signal (LED went yellow). This leads me to believe the logic board is ok.

3. I then hooked the PSU back up to the monitor and hooked a scope up. The power has minimal ripple (.1V at the most, and it's not AC ripple, more like switching PSU ripple at a higher frequency). But it would cut out and cut back in once per second as before.

I don't have any simple way to put a dummy load on the PSU. The only high-power resistors I have handy are 1-ohm which would draw too much current! I did measure the monitor's current draw with my other 12V supply and it was around 300 mA, which is well below the 1.75A maximum shown on the PSU board. So maybe the problem is in the overcurrent sensing circuit perhaps?

 

[kpatz]

Update: On another forum I found a thread where others have had the same issues with this monitor. After reading that, I tried removing a jumper that supplied power to the inverters. Doing this enabled the monitor to power up successfully. Doing some more digging I found some bad transistors in the inverter circuits so I ordered replacements. Hopefully that's all...

EDIT: I removed the bad transistors from one of the inverter circuits, leaving the 2nd inverter intact. Now the monitor powers up and stays powered. I connected it to a computer and the 2nd inverter only fires the backlight for about a second, then it goes dark. I don't know if that inverter is faulty as well, or if the lack of output from the other inverter is causing it to shut down.

 

[Sceadwian]

The backlight drivers are probably the first thing you should have checked, they're notorious for going bad.

Start replacing caps on the inverter boards and looking for other possible parts that went with it. You could have bad caps without them being bulging, only way to check is with an ESR meter, it's generally easier to just replace them unless you have one.

 

[kpatz]

The inverters are on the power supply board, they're visible toward the bottom of the pic I posted.

There are SMT parts on the other side of the board. This is where I found the bad transistors in one of the inverters. They probably fried when the power supply caps got too weak to deliver clean power.

The logic board in the monitor seems to be fine, it powered up no problem with a different supply. Plus the caps on that board are a different brand, probably one less prone to premature failure. The power supply's caps were all CapXCon, better known as CrapXcon.

 

[vedo35]

look i chacked of your inverter controller ic BI3101A if you read of datasheet first page of datasheet says :

" Dual independent dual lamp protection !!!!"

I had same problem. Dell 19" monitor. 2 sec later was going to standby. I changed all caps but still was continued. I found problem and repaired. Before I explain let me tell u explain everything. If your smps controller ic supports green mode (like SG6841) than u get this problem. Because some kind of smps controller ics if feels no load goes to sleep. U can test it of your smps. Just take it and connect to 12 and 5 volt outs something like DC motors or lamps. If smps works green mode style probably will work on load. i had a smps from sat-receiver. sg6841 was dead. i changed was doing tic tic sound i changed all caps and also again sg6841 and rectifiers still was doing.i connect to sat-receiver started work.. Strange but was like that. ( also dont forget some kind of smps/inverter boards controls by the monitor mcu for on and off ) or you will make on /off with logic 5v. you have to find whic is on/off pinout.
But i am almost sure u dont have any problem with your smps like you told u changed all caps and controled all fast rectifiers. So if we think no problem remains of your CCFL back light bulbs. Because my problem was bad CCFL bulbs both lamps both sides was black!!!! so inverter controller ic cant find (or open outs) florecent lamps and going to sleeps. my inverter controller ic was OZ9938GN i read of datasheet i learned. your inverter controller ic spectification also same. I dont know since 6 years totally how money hours you were using every day. Because of long time use florecent lamps died. You have to open gently and be very careful never touch lcd controller and lcd driver board or use esd protection arm band and just open all screws and open gently up and down uses double very thin florecent lamps. u will see black flamans. and dont forget before do this take a photo of your ccfl lamp socket because if you connect wrong ccfl sockets will not work and u can burn somethin on inverter board. ( i did i heard strange sounds ) If you any question just ask and good luck.

 

[sanjubluerock3]

change the transistor which is next to optocoupler

 

[vedo35]

that is not transistor TL431

 

[kpatz]

that is not transistor TL431

If you know that's a TL431 you have better eyeballs than I do. Or you have the same PSU in front of you, or a schematic (I could use one if it exists...).

I know it's marked IC330 or something, so it's not a transistor. I'm at work so I can't look at the board itself.

AFAIK, my remaining issues are in the inverter section, since with that disconnected I get a steady 12V that will power the monitor no problem (other than the backlights). Once I get my 2SC4672T100QCT-NDs from Digikey I can repair the faulty inverter.

 

[vedo35]

If you know that's a TL431 you have better eyeballs than I do. Or you have the same PSU in front of you, or a schematic (I could use one if it exists...).

I know it's marked IC330 or something, so it's not a transistor. I'm at work so I can't look at the board itself.

AFAIK, my remaining issues are in the inverter section, since with that disconnected I get a steady 12V that will power the monitor no problem (other than the backlights). Once I get my 2SC4672T100QCT-NDs from Digikey I can repair the faulty inverter.

Okkkkkk... arrogant Professsor okkkkkk what the hell u want to do u can do . just do it. did you read of my before answer? i just answered for help. if u will behave like this what the hell you r posting help on this forum. Do you know what is tl431? do you think that is transsistor or ic? if u dont know search datasheet.( just a tip its a ic )

 

[kpatz]

I got the transistors from Digi-Key and finally got around to finishing the repair.

Here's the PSU/inverter board after recapping.

**broken link removed**

I replaced the driver transistors in one of the backlight inverters. This solved the power cycling issue.
**broken link removed**

And she works again!


Thanks to those who helped.