Weird power issue - replaced caps

[daveB_303]

Monitor Brand: Starlogic
Power Board: EMAX

Symptoms:
Monitor sometimes works and then after a while it will cycle on and off. One second on, shuts off, then one second on, shuts off .... continues as long as it's plugged in or I time the pressing of the power button to turn it off. Sometimes, disassembling it (unplugging the lcd from the power supply) it will work - coincidence?

Now, I mean it turns off. It doesn't go into sleep mode. The power indicator led goes from green directly to off - do not pass orange standby.

When the monitor is briefly on, all I see is the multicolored "DVI" on the top left hand side of the screen. When I shine a flashlight on the screen, I don't see the desktop.

What I have done:
Replaced just about all the caps, except a 4.7uF that is glued in and a very large one (400V+) that looked in great shape. After removing a cap that was nestled in the middle of other parts, I saw that it was a bit buggered on the side that wasn't visible. I thought that was it!

When I first fired it up, it proceeded to act like it did above and then when I hit the power switch, it was working fine. I was able to turn it on and off and I thought I fixed it.
Until, I remembered that I forgot to put the screw hex mounts on - the ones that you use to screw in the VGA cable. Plugged it back in and back to the power cycling.


Thanks for any ideas! If anything, it has gotten me reviewing my electronic s.
😀

 

[Guest]

Of course it may be something else but perhaps the cable is the problem.

On the capacitors -- I have fixed three monitors that had failed due to cap problems. I ignored the high voltage cap but replaced all the 20 volt 680 and 16volt 1000 ones regardless of appearance.

In fact on one monitor none of the caps were bulging or leaking but replacement did the trick. Most were glued in. Capacitors are cheap and removal is simple with a solder pump.

The cause of cap failure seems to be heat -- I've mounted a large heatsink on one of my screens but just cutting a hole in the plastic case might help.

 

[daveB_303]

The cable doesn't seem to be the problem - the monitor I'm using now is plugged into the same video and power cables that I used on the broken one.

I might disassemble it again and replace the 4.7uF just for giggles.

I wonder if the mica (I think they're mica - they're these blue blobby looking things) capacitors broke down too - is it possible?

I looked at the PB and there isn't any obvious heat damage.

 

[Guest]

If the blue things are cheap and easy to replace, might as well buy some when you're at the store. I haven't had to replace any so far.

Shouldn't have to though as the problem with the regular capacitors is they contain an electrolyte fluid which can dry out or leak.

The fluid itself was at the centre of a scandal a few years back when Taiwanese engineers employed on making caps for a Japanese brand stole the formula for electrolyte -- but wrote it down wrong. Result; Taiwanese motherboards going belly up for a few years.

Now this rash of display failures -- attributed to makers underestimating the amount of heat generated in a small space. This wouldn't necessarily produce burn marks or any other evidence of heat.

 

[daveB_303]

Replaced the 2.2uF 50v cap - it wasn't 4.7uF like I thought. No change.

Yeah, all those little blue caps - I can't even read the capacitance or the voltage - and there's a couple of large ones that almost look like fat yellow bridge rectifiers only they have 0.47uF 100v on the side.

I'm going to let it sit for a while as is and maybe I'll stumble across something. I would hate to spend all that time replacing caps and not have it work. There's a ton of micro electronics on the board and as it is, it was a real pain getting the 2.2 in because it was a very tight squeeze - had to bring out the loupe to fix the solder bridges. I may have even cooked a couple of things fixing the bridges.....

Thanks for the input, though.

 

[daveB_303]

Here's a link to the PB:
http://www.emax-co.com/english/products_show.php?queryrecno=7472&sup_no=1714

It's revision 7 and it's a little different - I don't see the little 2.2uF cap laying glued on its side in the photo and the toroidal by the 120V female connector is turned along with the yellow cap (0.47uF 100V). But that's basically it.