Damaged monitor, possible PSU fault.

Hi guys,

Could you kindly folk please help me try to fix my broken monitor? It's out of warranty and I've just gotten round to opening it up and taking a look inside.

It's a 30 inch LCD monitor that just stopped working as I was viewing it. The power LED isn't coming on.

I've checked the power cable and plug-top fuse, all ok. It appears to have an integral power supply to provide all the different voltages etc. Pictures attached. I found what I think may be a burnt out component, C98 and it doesn't look like any of the other 'Cxx' components on the board. The component Z19 next to it also looks a bit suspect. Is that a zener diode?

There is an onboard ceramic fuse that tests out ok although I've only checked continuity through it, with it still soldered to the board.

Any ideas what to look for or to check next? I have a multimeter.

Many thanks,
Ed.


PSU Top View.

http://webed.pwp.blueyonder.co.uk/pics/PSU/PSU_top.jpg





PSU bottom view. Small rectangle hilighted area.

http://webed.pwp.blueyonder.co.uk/pi...PSU_bottom.jpg



Damaged area (my assumption).

http://webed.pwp.blueyonder.co.uk/pics/PSU/damage.jpg

Very difficult to diagnose problems by picture I'm afraid, especially without a cct diagram. The components you have highlighted certainly look like they may be the culprits to me. The Z19 component could well be a zener but it could also be a transistor, difficult to see from the picture. It doesn't always follow that Z means zener unfortunately.

That component area isn't part of the PSU more likely a control area or a signal processing area for the screen. That doesn't mean to say that the PSU is not at fault, it could be the cause of the damage you see.

In the first picture do I see a possible 12v input next to the mains input? If so try powering the screen from there and see what happens. It may well give you some more clue as to where the problem may lie.

Good luck with it!

Can you take a hi-res photo of the other side of the board?

---------- Post added at 12:35 ---------- Previous post was at 12:23 ----------

C98 looks like a cap that quite possibly has gone bad and taken the (maybe) zener diode with it.
These 2 might both have been taken out by an IC gone bad in the area, i'd look at the 3 legged component just underneath the damaged area.

looks like something has arc'd over to the left of C105 and damaged a trace from here..
but then again its hard to tell with pictures

Thanks for the responses guys.

The connector that looks like a 12v input is a DC output, I think it's 9v that powers an external speaker array. I'll see if I can get a hi-res picture of the component side. I'll have to de-solder the heatsinks etc to do that. I'm wondering though, without a schematic, is this a lost cause if I don't know what values the suspected faulty components are?

@Amigafanboi, did you mean the 3 legged part Q29 ?

Thanks again,
Ed.

No look further south, the 3 solder points where there will be a 3 legged component on the other side of the board, it will be marked with IC then a number or Q then a number.

Ok, I couldn't desolder the heatsinks. But took a couple more pics.

The component side of the damaged area has 4 IC's, including the 3 legged one, a transistor?

http://webed.pwp.blueyonder.co.uk/pics/PSU/ics.jpg



I've added some overlays in the next image to show where the IC's are located. (click to enlarge). It looks like there has been a trace repair between C98 and R202.

http://webed.pwp.blueyonder.co.uk/pics/PSU/ics2.jpg

That now looks flux left over from the trace repair to me, you need to clean it off for a better view
Have you tested that Z19 diode?

The 3 4 legged IC's are optos and very rare that they go.

---------- Post added at 15:47 ---------- Previous post was at 15:47 ----------

The more i look at that bit the more i suspect it's just an old repair.

Quote:

---

Originally Posted by Amigafanboi

That now looks flux left over from the trace repair to me, you need to clean it off for a better view
Have you tested that Z19 diode?

The 3 4 legged IC's are optos and very rare that they go.

---------- Post added at 15:47 ---------- Previous post was at 15:47 ----------

The more i look at that bit the more i suspect it's just an old repair.

---

I think you're right mate. I scratched away the flux and the capacitor came up its normal beige colour.

I started looking at other stuff on the circuit, but tbh my electronics knowledge is very limited. Something I thought strange, the onboard fuse seems to be in the neutral path rather than the live path (I used to be an electrician). I was checking a rectifier on the primary input side of the board and was getting some strange voltages across it. I'd probably need to get it to a tech to have a look at.

Cheers anyway guys for your time and suggestions, really appreciate it.

Regards,
Ed.

Have you checked the transformer's secondary output, to see if any power is coming from the transformer? They are normally fitted with thermal fuses that aren't the most reliable things in the World and it may be that the thermal fuse is OC.

If the transformer is outputtinglow voltage AC, then we can start to look at any rectifier circuits to see if a regulator or bridge rectifier is at fault.

Follow the voltage....