scorch mark on board around resistor

[jsjoens]

I've got a monitor that a guy gave me that wouldn't come on. All the Caps look good, but there's a nice black scorch mark surrounding a resistor. Friend suggested I use a multimeter to check the rest of the inverter board, but I must confess that I am a n00b when it comes to that. replacing obvious bad caps? Not a problem...but the multimeter...What is it that I am supposed to check? How do I determine what fried the resistor?

[999999999]

Re: scorch mark on board around resistor

A picture or two might help.

If the inverter board (or that subsection of the power supply board) has a problem, it won't keep the monitor from turning on (usually), it'll just keep the backlight from coming on and if you shine a flashlight at the screen at an angle you should see text or graphics (unless with no video signal it goes into sleep mode).

Generally speaking, measure the resistance of the resistor, on its pins and on the solder portion of the solder joints for it, there is a chance it has broken them or damaged the traces there.

Generally speaking you'd measure the power supply's outputs to the video board to see if it is working. Also if it has more than one input (analog vs DVI for example) try the other input and the menu screens "if" what you mean by won't come on is the power light comes on but it just never displays any video.

Might not hurt to mention the monitor model too and inspect the video board for any visual indicators of problems.

[jsjoens]

Re: scorch mark on board around resistor

Apologies. Was half asleep and frustrated. Monitor will turn on, but backlight will not. Monitor is HP HSTND-2L04. It looks like it's been repaired before. There's some writing and an arrow pointing to the fried resistor. Attaching a photo like you suggested. It was sort of a "where here's your problem!"

[ratdude747]

Re: scorch mark on board around resistor

could a more general board pic be uploaded? sounds like something shorted and sent a large amount of current to a low wattage resitor, causing overheat that caused the board damage.

w/o a pic of the whole board, both front and back (to follow the traces), I cannot say much else... although my gut feeling is a shorted transistor/diode, possibly caused by poor transformer soldering.

as for being fixed before, if it is the transistor/diode bug, they may have only taken care of the resistors and transistors/diodes, but not the transformer soldering issues, which would have promptly killed the replacement parts. this also is a common occurance...

[jsjoens]

Re: scorch mark on board around resistor

Pics...INCOMING

There's one of the front and two of the back with different light sources. Unfortunatly best camera I got is the one in my phone. If these don't work I can get some close ups after I get back from work tonight.

EDIT:wow those were blurry. Will post better tonight.

[jsjoens]

Re: scorch mark on board around resistor

Better pictures...sorry about last time

[ratdude747]

Re: scorch mark on board around resistor

let's see:

it's in the inverter section alright... I also see heat damage near the transformer and questionable soldering...

maybe plainbill on one of the other lcd experts could chime in, but it looks to be a case of bad transformer soldering that shorted a transistor and overheated the resistor,

[tom66]

Re: scorch mark on board around resistor

These cheap phenolic PCBs burn around heat, the resistor may be okay but it's definitely under considerable thermal stress. The resistor has clearly got hot and damaged the board. Similar to a lot of monitors, it's cheap design. Question is, is the resistor damaged? Use the ohms mode on a multimeter while the board is off. If it corresponds with the colour bands (yellow violet yellow = 470kOhm) then it may be okay.

[paslug]

Re: scorch mark on board around resistor

Even if the resistor is the correct value, the leads have probably heated enough that you will have a hard time soldering it.

I would replace the resistor, use a dremmel tool and some epoxy to repair the burned board, and run jumpers to replace any bad traces.

It could have been originally a bad solder connection on the resistor which caused the heating and charring.

Ray

[999999999]

Re: scorch mark on board around resistor

You guys are making this harder than it needs to be.

Resistor body looks fine still, thermal stress was not significant enough to bother hunting down a new resistor (though if you have a spare lying around anyway...), just pull it off the board, clean any residue off the leads with metal polish or a steel wool pad, etc, tin them with solder, and redo the board traces if they are lifted off the PCB.

Check continuity & resistance from the next solder joint on both sides of the resistor and if everything looks ok, apply power and see if it works. If it doesn't, follow the circuit backwards testing next component and so on. That other areas are discolored could mean nothing more than that the monitor had insufficient chassis cooling slits or was operated in a warm-hot area for a significant period.

On the other hand if you do end up replacing the resistor, I'd pick one with higher wattage rating like 1W, AND double-kink the leads so it stands up off the PCB for better cooling, with the lead bends holding it mechanically in addition to the solder joint.

[popeye]

Re: scorch mark on board around resistor

Apologies. Was half asleep and frustrated. Monitor will turn on, but backlight will not. Monitor is HP HSTND-2L04. It looks like it's been repaired before. There's some writing and an arrow pointing to the fried resistor. Attaching a photo like you suggested. It was sort of a "where here's your problem!"

Looking at the top and bottom view of the resister in that position I would be surprised if that is the original resister from experience that sort of heat generation should have had an effect on the resister that one looks to good also I would have expected a safety resister there to take the heat or even a wire wound one it has definitely been dry jointed hats what has carbonized the board in that area the carbon will be conductive so needs to be removed as the post above says take pictures first so you know where the track's go take care go carefully make the area clean after you have removed all carbon then join the resister legs to the track that is left with insulated wire so that only the tips are poking out to be soldered when that done fault finding can be continued.