DGM 1931WS Monitor repair Shorted

[jamesfreddie]

Hi,

I recently bought a faulty 19" monitor off ebay with the view to repairing it and using it in my office as my new workbench monitor as it has DVI which i now need for PS3 repairs.

I replaced 2 of the bulged capacitors then the monitor worked fine for days so i thought it will last a fair while so i went ahead to mount it on my wall mount.

That was where it all started to go down hill.

I mounted it and plugged it in, I then heard a small pop and all my computers etc. went down the RCD, and the room fuse went. The monitor would no longer turn on.

I looked inside the monitor and notised that the bolts where touching the back of the PCB. I have opened the monitor again and can't see anything obvious.

Could somebody please help me diagnoise what is wrong with it, hopefully its something easy and cheap to fix.



Thanks

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

I have just checked the fuse in the photo and it is open i would assume it has blown due to the board being shorted?

Hopefully the fuse has protected the circuit and i will just be able to replace it?

The fuse says t4.0ah250v

Would this be a suitable replacement? http://uk.farnell.com/multicomp/mst-...-2a/dp/1566103

Please let me know.

Thanks

[ben7]

Re: DGM 1931WS Monitor repair Shorted

Either it was just random luck, or there is a bad solder joint on the PSU that got cracked when you mounted the monitor on the wall.

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Either it was just random luck, or there is a bad solder joint on the PSU that got cracked when you mounted the monitor on the wall.

If i just shorted two leads on the back of the board would that fuse have blown?

Would the replacement fuse i put in my last post be sutiable?

[ben7]

Re: DGM 1931WS Monitor repair Shorted

If i just shorted two leads on the back of the board would that fuse have blown?

Would the replacement fuse i put in my last post be sutiable?

No, the fuse you picked is only 2A. You need a 4A fuse.

But, don't buy the fuse just yet!!!

Please check the resistance across the biiig high voltage filter capacitor. (Short it out just in case there is power left - don't want to hurt your DMM)

-Ben

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

The fuse was actually t2.0ah250v. It was a typo.

So would that fuse be OK now?

Do i need to remove the cap to test the restance?

Thanks James

[ben7]

Re: DGM 1931WS Monitor repair Shorted

The fuse was actually t2.0ah250v. It was a typo.

So would that fuse be OK now?

Do i need to remove the cap to test the restance?

Thanks James

Ah ok, it looks like that fuse will be a good replacement.

Nope, you don't need to remove the capacitor. Just make sure it is discharged (likely it is, if there is a short somewhere!) first.

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Ah ok, it looks like that fuse will be a good replacement.

Nope, you don't need to remove the capacitor. Just make sure it is discharged (likely it is, if there is a short somewhere!) first.

So i should go ahead and replace the fuse and see what happens?

[ben7]

Re: DGM 1931WS Monitor repair Shorted

So i should go ahead and replace the fuse and see what happens?

No, first check to see if there is still a short!

If the fuse blows, 95% of the time there is still a short, and replacing the fuse will only result in another blown fuse, and a few cents wasted.

If there is still a short, you should remove and check the bridge rectifier, and the switching MOSFET.

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

No, first check to see if there is still a short!

If the fuse blows, 95% of the time there is still a short, and replacing the fuse will only result in another blown fuse, and a few cents wasted.

If there is still a short, you should remove and check the bridge rectifier, and the switching MOSFET.

This is going to sound like a dumb queston, but how do i check for a short?

Is there a tutorial/thread?

Thanks James

[ben7]

Re: DGM 1931WS Monitor repair Shorted

This is going to sound like a dumb queston, but how do i check for a short?

Is there a tutorial/thread?

Thanks James

Get your digital multimeter out.
Also it is sometimes referred to as a DMM.

Unplug the monitor if it isn't already.

First, check to see if there is voltage on the big capacitor:
1. Turn your multimeter on to the DC VOLTAGE range.
2. Put the black probe on the negative of the capacitor (The lead/wire that is closest to the side of the capacitor with a big stripe printed on the plastic cover)
3. Put the red probe on the positive pin of the capacitor. (The remaining pin, not on the side with the stripe.)
4. If there is voltage on the capacitor, it will show on your multimeter.

If there is more than a few volts on it, take a screwdriver and bridge the two connections. If there is a large voltage still there (like 100v), you will get a big spark.

Most likely there will be no voltage - especially if there is a short somehere. But where?

Second, check the resistance from HV+ to HV-. (the big capacitor is connected across this)
1. Turn your multimeter on to the RESISTANCE range.
2. Resistance has no polarity - put one probe on the '-' of the capacitor, then put the other on the '+' of the capacitor.

If there isn't a short, you should get a reading of more than 100K ohms, up to infinity. If there is a short, it will probably read less than 100 ohms.

Let me know if your DMM detects a short!

-Ben

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Get your digital multimeter out.
Also it is sometimes referred to as a DMM.

Unplug the monitor if it isn't already.

First, check to see if there is voltage on the big capacitor:
1. Turn your multimeter on to the DC VOLTAGE range.
2. Put the black probe on the negative of the capacitor (The lead/wire that is closest to the side of the capacitor with a big stripe printed on the plastic cover)
3. Put the red probe on the positive pin of the capacitor. (The remaining pin, not on the side with the stripe.)
4. If there is voltage on the capacitor, it will show on your multimeter.

If there is more than a few volts on it, take a screwdriver and bridge the two connections. If there is a large voltage still there (like 100v), you will get a big spark.

Most likely there will be no voltage - especially if there is a short somehere. But where?

Second, check the resistance from HV+ to HV-. (the big capacitor is connected across this)
1. Turn your multimeter on to the RESISTANCE range.
2. Resistance has no polarity - put one probe on the '-' of the capacitor, then put the other on the '+' of the capacitor.

If there isn't a short, you should get a reading of more than 100K ohms, up to infinity. If there is a short, it will probably read less than 100 ohms.

Let me know if your DMM detects a short!

-Ben

The capacitor had 0 voltage.

I then did the second thing and it read 600K.

Thanks

[ben7]

Re: DGM 1931WS Monitor repair Shorted

Seems like there is no short there, might just be a shorted bridge rectifier.

Can you get a good picture of the back of the board, so I can show you which pins to check on the bridge rectifier?

[Lumberjack777]

Re: DGM 1931WS Monitor repair Shorted

If i just shorted two leads on the back of the board would that fuse have blown?

Yes, especially if one of those bolts shorted the backside of the board in the primary section. The good news is.....like Ben is showing you. This will probably be an easy fix. You might have a blown brige rectifier. Or it might even just be that fuse that blew. So what Ben is doing is making sure the primary circuit is intact before you replace that fuse. ~L-J-7 ~

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Seems like there is no short there, might just be a shorted bridge rectifier.

Can you get a good picture of the back of the board, so I can show you which pins to check on the bridge rectifier?

Here is the best picture i could get, i took it in front of a window to get the best light.



Is it good enough?


Thanks

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Seems like there is no short there, might just be a shorted bridge rectifier.

Can you get a good picture of the back of the board, so I can show you which pins to check on the bridge rectifier?

Should the numbers bounce around when i test the capacitor?

[ben7]

Re: DGM 1931WS Monitor repair Shorted

Test the bridge rectifier:

Turn your DMM on, in Diode check mode.

Each diode is tested two times. Once in the forward direction, once in reverse. Normal readings would be:
Forward - ~0.6v
Reverse - OL (No conduction!)

Make sure the PSU is unplugged!

1:
Forward
Red probe on 'A'
Black probe on 'B'
Report reading.

Reverse
Red probe on 'B'
Black probe on 'A'
Report reading.

2:
Forward
Red probe on 'A'
Black probe on 'C'
Report reading.

Reverse
Red probe on 'C'
Black probe on 'A'
Report reading.

3:
Forward
Black probe on 'D'
Red probe on 'B'
Report reading.

Reverse
Black probe on 'B'
Red probe on 'D'
Report reading.

4:
Forward
Black probe on 'D'
Red probe on 'C'
Report reading.

Reverse
Black probe on 'C'
Red probe on 'D'
Report reading.

That is how you check the 4 diodes in the bridge rectifier. If any reading is close to '0v' that means the diode is shorted. A good diode should measure about '0.6v'.



Test the NTC inrush current limiter:

Turn your DMM on, in resistance mode.

Make sure PSU is unplugged!

Put one probe on 'X', the other probe on 'Y'.
Report back the reading.

A good NTC will measure less than 10 ohms.

-Ben

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

When you say "Black Probe on A,B,C or D" where are these test points?

Thanks

[ben7]

Re: DGM 1931WS Monitor repair Shorted

When you say "Black Probe on A,B,C or D" where are these test points?

Thanks

DAMMIT!

I always forget to attach the picture

Sorry about that!

[jamesfreddie]

Re: DGM 1931WS Monitor repair Shorted

Test the bridge rectifier:

Turn your DMM on, in Diode check mode.

Each diode is tested two times. Once in the forward direction, once in reverse. Normal readings would be:
Forward - ~0.6v
Reverse - OL (No conduction!)

Make sure the PSU is unplugged!

1:
Forward
Red probe on 'A'
Black probe on 'B'
Report reading.

578

Reverse
Red probe on 'B'
Black probe on 'A'
Report reading.

OPEN

2:
Forward
Red probe on 'A'
Black probe on 'C'
Report reading.

516

Reverse
Red probe on 'C'
Black probe on 'A'
Report reading.

OPEN

3:
Forward
Black probe on 'D'
Red probe on 'B'
Report reading.

567

Reverse
Black probe on 'B'
Red probe on 'D'
Report reading.

OPEN

4:
Forward
Black probe on 'D'
Red probe on 'C'
Report reading.

002

Reverse
Black probe on 'C'
Red probe on 'D'
Report reading.

002

That is how you check the 4 diodes in the bridge rectifier. If any reading is close to '0v' that means the diode is shorted. A good diode should measure about '0.6v'.



Test the NTC inrush current limiter:

Turn your DMM on, in resistance mode.

Make sure PSU is unplugged!

Put one probe on 'X', the other probe on 'Y'.
Report back the reading.

A good NTC will measure less than 10 ohms.

4.2 Ohms

-Ben

Do those numbers make sense?

Thanks