Monitor Goes Totally Black, 2-3 Seconds From Powerup - Power MOSFET Problem?

Most common failure on this type of power supplies is the drying out of low voltage rail capacitors caused by the heating from HF switching and lousy local cooling provisions that today's technology doesn't seem to be able to solve yet (LOL) so start with that. Of course, this is also a very convenient time bomb that always sets off after warranty expires. Capacitor swelling IS a quick diagnose sign, but that's not written in stone. Anyway, mosfet failure mode is much different, includes a short bang and an open fuse, and device doesn't come back. Now, letting the monitor work un-repaired for any length of time, WILL soon lead to more serious and possibly unrepairable damages. S.M.

Hi, SimpleMind - Thank you for your input!

Would you recommend this check for the capacitors:

https://www.youtube.com/watch?v=FDuN7QHn1-s?

I've used that procedure for checking cap's for many years. Keep in mind that it only works when the cap's are REMOVED from the circuit.

Hooker

That is a good ,fast,GO--NO GO test. If it is open or shorted,this will tell you,but it tells you nothing about the value of the capacitor.

Some caps become leaky,and change value over time.

This can make some circuits fail to operate properly.

If you have an old analog meter, set it to ohms,connect the leads to the capacitor and watch the resistance increase to infinity as the cap charges.

Then reverse the leads,and watch the capacitor"Kick" as it discharges.

An open cap will never charge,and of course,never discharge using this crude test.

There are meters made for testing capacitors but too expensive for the casual user.

Electrolytic caps can "die" over long storage periods,but can be "Reformed" using a controlled long term slowly increasing charge/discharge cycle.

I encounter this often on old tube type radios.

I use a "C" meter made by Sencore that gives value,leakage and other parameters,as well as having a "reform" setting that can bring most caps back from the grave,unless they are shorted or open internally.

It is old equipment, but reliable as gravity.(Newton's gravity".

I still suspect the fluorescent tubes or ballast(s) are your problem.

I have seen the capacitors fail on the ballasts without external evidence of failure.

Good luck.

I agree. For these small capacitors, once you remove them, you might as well put new ones in. Of course, you have the wait time to get them but the price is usually low. And low ESR is important on many of these circuits. I replaced a cap in a TV (Toshiba with fold-over) a few years ago with one from Radio Shack. It worked a couple of years, but I think the replacement has gone bad now - it has the same symptom. And I think you're right about the tubes or ballasts. I have a monitor apart now (and for several months ) with similar symptoms.

SM is probably right, but it could be your CFL tubes - or other things. Here's some good info.

Graphics cards sometimes give the same symptoms before failing. Don't automatically assume it's the monitor. Do you have another monitor or another place to drive the monitor to see if the problem follows the monitor?

Sorry, using another monitor now on same system (and everything's fine). That is why I am able to use my computer and post here!

Thanks for the reply!

Your monitor is probably back lit by fluorescent tubes about the size of an uncooked piece of spaghetti,and are driven by a ballast.

Some monitors have 2 ballasts,one for each half of the screen.

Either the tubes and/ or the ballast (s) are bad.

It may be more economical to replace the monitor than to repair it,unless the problem is a component on the ballast board,and you can do the repair yourself.

I have repaired them for myself,and subsequently had to replace the tubes also,in the final analysis, it cost almost as much for parts as a new monitor.

I have since learned that the bad tubes are probably the reason the ballasts failed.

Here is a link if you care to repair your own:

http://www.ccflwarehouse.com/

So if you repair the ballast,replace the tubes before powering it back up.

Good luck.

Hi HTRN,

I had ruled these out. According to one of the websites I visited, if only the backlights are out, you can still see an image by shining a flashlight directly into the screen, which I did not, and the power LED went to orange when the screen went black.

From everything that's been said, including your post (#9), I'm just going to replace all the capacitors on the power board.

I'll post the results when I get it done.

Thanks!

Mike

Most monitors will have an indicator LED, power on/standby/off, usually changes color. Today, monitors try to conserve power, and have a circuit to detect the video signal. If you turn off the computer, the monitor goes into standby and then powers off. The MOSFET is not the problem. An ESR meter is the best tool to check capacitors, in circuit, with power off. Another method which sometimes works, is freeze spray, directed one part or area at a time while the monitor is powered on. If it starts working, you have probably found the part or area that is bad. (Canned air, can turned upside down, is the same product.) You could also use a hair dryer to heat the circuit board, if it starts working, then use the freeze spray to make it fail.