Can't find the origin of a burn mark on a PCB

Thread Starter

Alois Luong 1

Joined Aug 15, 2019
5
Hi,

So I was trying to repair an old shredder (Comix S310) I found in my dad's office and while I was doing so, there was a big spark and poping sound on PCB.

When I evaluated the damage, I noticed 2 things:

1. The fuse (5A 250V) was completely fried (see image one) and needed to be replaced. In the third image, the fuse has been desoldered.

2. There is also a burn mark near the fuse which I don't know the origin of (see image two). I don't know if there use to be a small electronic component there or if the spark of the fuse created this burn.

Could you help me identify the cause of this burn?

Thank you in advance! :)

IMG_6067 2.jpg IMG_6072 2.jpg IMG_6073 2.jpg
 

LesJones

Joined Jan 8, 2017
2,352
There would have been no component fitted at the point where the track has melted. this is just a short section of track between the neutral input wire and the links that have been fitted where the common mode filter would be if it was fitted. I think you must have shorted live and neutral at some point after the fuse and the place where the common mode filter would have been if fitted. The fault current was high enough to melt the track and blow the fuse.

Les.
 

Thread Starter

Alois Luong 1

Joined Aug 15, 2019
5
There would have been no component fitted at the point where the track has melted. this is just a short section of track between the neutral input wire and the links that have been fitted where the common mode filter would be if it was fitted. I think you must have shorted live and neutral at some point after the fuse and the place where the common mode filter would have been if fitted. The fault current was high enough to melt the track and blow the fuse.

Les.
Thank you for you reply.

Do I need to do anything (apart from changing the fuse) before I start testing again?
 

LesJones

Joined Jan 8, 2017
2,352
You need to repair the melted track. You have not said what the original fault was that you were trying to repair. The way I would approach the fault would depend on the nature of the fault. It might involve tracing out part or all of the schematic and working out from that which area of the unit is likely to cause that sort of fault. It would NOT involve just powering the unit up with no plan as to what tests you are about to do. When you get to the stage of powering it up I suggest connecting something such as a 60 tungsten filament bulb in series with the supply to limit any fault current. If it is a mechanical fault I would look for the cause WITHOUT power by turning the mechanism by hand.

Les.
 

Dodgydave

Joined Jun 22, 2012
8,466
Looks like a Transformerless psu, and more than likely a piece of metal or the case has shorted to Earth or shorted the Live and Neutral together and blown the fuse.

Test for shorts on the input side and after the fuse input.
 
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Thread Starter

Alois Luong 1

Joined Aug 15, 2019
5
Thank you everyone for your help! I learnt a great deal through your input.

For a closure, I ended up scavenging some of the components like the shredder motor as some of the electronics were either too damage (and beyond repair with my knowledge) or simply missing altogether.
 
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