LCD Monitor Problem - Turns on and off

When the screen goes "off" does it really goes off? It sounds like a bad inverter or a bad ccfl.

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mistermaniac

Yes, you need to establish this. It is very possible, if the monitor is genuinely not starting after a period of being totally unpowered, that the problem is actually the external power supply, which is pretty much certain to be a switch mode type. I have had a number of these giving similar symptoms to those you state, the cause being bad electrolytics, particularly on the primary side. In general, when switchers have this sort of problem, once you've persuaded them to come on, and the caps have warmed up a bit, they will stay on. Try measuring the output voltage of the power supply, after it's been off for a couple of hours. You may well find that there isn't any, or what there is, is pulsing. You could also try hanging a low wattage car bulb across the power supply output - say an interior light bulb - if you can figure a way of getting a connection to the output plug.

Arfa

It don`t sound like a bad inverter if it is running good when plugged directly to the wall outlet. If it only does this when plugged into the powersupply of the computer then this would be a problem of the computers powersupply. Did you try to plug it into another computer to see if you still have the same problem ? I would suggest to try this first and see what happens.

Bill Simps> I tried to post this yesterday, but it didn't seem to show up.

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When the screen goes "off" does it really goes off? It sounds like a bad inverter or a bad ccfl.

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mistermaniac[/quote:50075f7ac7]

Maybe I read it wrong, but I don't think that this is what the guy was saying. I read it as he was always using the external power unit, that came with the LCD monitor, and if he left it plugged in the wall, ie with power always available to this power supply, but with the monitor turned off, when he came to switch the monitor on, it came on ok. If, however, he plugged in to any power source that was turned off for periods of time eg his UPS, or unplugged from the wall for any length of time, then when he came to put power back on the PSU, and then tried immediately to switch on the monitor, he got the buzzing, spitting failure to start up, which is typical of a defective switching PSU, with bad electrolytics. Perhaps the OP can confirm ??

If this is actually the case, then all the stuff about Windows and rebooting etc is probably a red herring. It just relates to the length of time all this takes, which gives the power supply time to struggle to a startup.

Arfa

On Sat, 04 Mar 2006 18:57:30 GMT, "Arfa Daily" wrote: What you described is precisely what is happening. What you said about the boot process being a Red Herring, I believe is correct. Thanks to the rest of your for your help, apologies for the difficulty in description.

Do you suggest that I first test the output of the power supply?

Thanks again

plugged

was saying. I read it as he was always using the external power unit, that came with the LCD monitor, and if he left it plugged in the wall, ie with power always available to this power supply, but with the monitor turned off, when he came to switch the monitor on, it came on ok. If, however, he plugged in to any power source that was turned off for periods of time eg his UPS, or unplugged from the wall for any length of time, then when he came to put power back on the PSU, and then tried immediately to switch on the monitor, he got the buzzing, spitting failure to start up, which is typical of a defective switching PSU, with bad electrolytics. Perhaps the OP can confirm ??

If this is actually the case, then all the stuff about Windows and rebooting etc is probably a red herring. It just relates to the length of time all this takes, which gives the power supply time to struggle to a startup.

Arfa[/quote:733e2a47e2]

If that was what he was stating. I may have read his post wrong to what he was trying to say. Then I would have to say yes that he has something wrong with the external powersupply.

Yeah, I think that's what he's saying, tvguy - see his additional post above.

Bill.

I think me and tvguy are now both in agreement. Yes, go for power supply trouble. I had a FAX machine on my bench last week, which was powered from an external 24v 1500mA switch mode supply. If left on, it would work for ever. However, if left unpowered for a few hours, when put back on, the power supply would struggle for about a minute, to start up.

Once I got inside it, it was fairly obvious that the two series-connected

82k startup resistors were pretty distressed and high. I replaced both these ( they use two in series to double the voltage rating of these small film resistors ). Also, there was a single 220uF cap on the primary side. When checked on my ESR meter, this went 38 ohms, which is way, way too high, so this item was replaced as well. The supply then started up first time, every time, no matter how long it had been off. Both of the mentioned components are very common for causing startup trouble in any switcher.

Just a word of warning. If you are not used to switchers, be very very mindful of your personal safety. This type of supply is VERY dangerous, and could be lethal. Remember also, that the main smoothing cap will charge to almost one and a half times your local input voltage. If the supply doesn't start up, this cap will likely stay charged to very close on this figure for hours, as most switchers do not employ a leak resistor across it. I would always recommend having these supplies connected to a bench isolation transformer, when they are opened to be worked on.

Arfa

On Sat, 04 Mar 2006 18:57:30 GMT, "Arfa Daily" wrote: Tried posting this earlier, hope this isn't double posted, problems again with my ISP. Hope I haven't lost the thread.

What you described is precisely what is happening. What you said about the boot process being a Red Herring, I believe is correct. Thanks to the rest of your for your help, apologies for the difficulty in description.

Do you suggest that I first test the output of the power supply?

Thanks again

I see that my reply did get double posted. Thanks for the very precise information. With my rather low level of technical knowledge, do you think it more advisable that I just replace the power supply?

The power supply that I currently have is

Sunny Input :100-240 - 1.6A Max, 50-60Hz, 70-80VA Output: +12V - 4.0A

I contacted the manufacturer of the monitor and they want $75 for a replacement power supply (More on that later). I've found some others, more reasonably priced online. One is a 12V 5A (60W) and the other is a 12V 4.16A (50W). Am I correct in assuming that both should work? If the connector is different in these power supplies, would cutting and soldering the original connector to the new power supply be acceptable rather than the more complicated and dangerous course that professionals with your expertise would take? I know it's a crude solution, but it's something I would feel comfortable with.

The rest if off topic, but if you have some time, I would like your opinion. I had mentioned earlier that I contacted the manufacturer about a new power supply. I went to their home page and read the FAQ ahead of time. I found this in the FAQ:

"My screen comes on for a second and then goes black. How do I resolve this issue?

1. Please contact Customer Service for further assistance at

323-346-0888. There is a possible short in the ac adapter. "

It seems that the manufacturer knew about this problem for some time. I had registered my monitor and asked the representative when I called why they hadn't notified me of the problem (As the warranty on the montitor had just run out). I also asked if this wasn't a potential fire hazard. I got a typical rebuff. My question is: Could a short in the power supply cause a fire? I this a dangerous situation or would normal internal safeguards kept anything hazardous from happening?

Thanks again for all of the expert help. You all went above and beyond what I expected.

Bill

If you don't feel qualified to go inside the supply, then I would not be comfortable advising you to do so. The fault almost certainly will be one of the things I suggested, but it's not worth risking injury to try to repair it, if you're not absolutely confident of what you're doing.

Either of the two power supplies should be ok rating-wise. Any power supply with a rating of 4 amps, will be a switch mode type, so by very nature will be regulated to its declared 12v output. Cutting and joining on the output lead is fine, BUT make absolutely sure that you get the polarity the same as original. The monitor will not like having reverse polarity DC shoved up it ...

If ever I join wires in this way, I always use heatshrink sleeving, which you could probably get from your local Radio Shack. It shrinks easily with a small paint stripper gun on 'low', and does a nice neat job of re-insulating the cable, and reinforcing the join.

As far as the manufacturer's statement about a " short in the power supply " goes, it's probably a bit of generic techno-babble that means nothing more than ' a faulty power supply '. Joe public can equate with a " short ". It sounds just technical enough to be describing something real, but is basically meaningless in this context. It is unlikely that any fire would ensue from any of the typical ( and much more likely ) faults that the power supply would really be suffering from. It's probably just a typical switch mode power supply design issue, where whover laid out the board, decided, as they always seem to, to place an important cap right next to a resistor that gets hot. It probably just happens that the fallout from this has been particularly bad for this manufacturer, using this ( almost certainly bought-in ) power supply on his particular product.

Arfa

Thanks for all the help. Since you introduced the concerns about maintaining the proper polarity. Is there a simple way to verify this? I recently pick up a new Greenlee DM-810 True RMS Digital Multimeter at a garage sale for $5. Does this have a function that would test polarity?

Bill, I'm assuming that this is a digital meter - yes ? OK. Making sure that you have the red probe in the red socket, and the black in the black socket, then just go ahead and measure the output. If you have your red lead connected to the positive output wire, then the meter will just read " 12.00 " or whatever. If you have your red lead connected to the negative output wire, then the meter will read " - 12.00 " ie a minus sign at the front of the number.

As additional confirmation, the original power supply may well have a little picture of the output connector, indicating polarity, as might the monitor by its input socket. Additionally, as any replacement you buy is likely just a generic type, it will probably also have a picture of its plug, indicating polarity. For what it's worth, you very likely will not have to do any cutting and grafting. In general, there seems to be a loose " standard " on the plug size and polarity sense, employed by the current crop of externally powered LCD monitors and TVs. But that said, I emphasise, better to be safe than sorry ...

Arfa

Thanks again Arfa...I think we're done.

Bill

No probs, Bill. Glad we were able to help you.

Arfa