Debugging PCB short - two traces short

[shashankbr]

Hi everyone,

I have an Altec Lansing FX5051 Home Theatre System.

Recently the unit stopped working and when I checked it, the fuse on PCB was burnt. After I replaced the fuse, I checked for PCB errors.

Design of PCB
- 240v to 18.5 V 5.4 A step down transformer which connects to a bridge rectifier IC on PCB through fuse.

The errors i found were - The +ve and -ve outputs of the rectifier were shorted. Hence the fuse blew up. So I removed the rectifier and found that its working fine.
Then when I checked on the PCB, the traces of +ve & -ve supply were shorted.

The PCB has lots of components on it. (Actually am amplifier + 5 channel sound card )

Question : Is there a way to find out which component of the PCB is burnt i.e the component which is causing the short?

Thanks all

 

[srizbf]

identifying the shorted component on power supply is a difficult task.

if you have the circuit diagram you can do it by elimation process.

even if you dont have one , just isolate each stage from power supply line and measure the resistance.

remove the stages one by one and repeatedly do the measuremnt.

if you have the current probe you can identify the trace which cause the short.

 

[FvM]

A short can be usually located by applying a small current limited voltage to the supply terminals (e.g. a few 100 mA) and measure trace voltage drops with a sensitive instrument. You'll notice voltage drops on the way between the supply terminal and the short, but not "behind" it or at traces carrying no current. Unfortunately, the method doesn't work well with power planes.

 

[shashankbr]

Thanks srizbf and FvM for replying.

@srizbf : The PCB is very complex and it is not possible to isolate each stage and put back again. I ll try out the current probe.

@FvM : I ll try the voltage drop method.

 

[srizbf]

isolating of each stage can done , by identifying the Vcc supply(+ve) line to that stage and cutting the track to that stage or removing any link component to that stage.

as said earlier , this is a difficult job but can be done if you have the patience.

 

[FvM]

isolating of each stage can done , by identifying the Vcc supply(+ve) line to that stage and cutting the track to that stage or removing any link component to that stage.

Surely, but I prefer a non-destructive method if ever possible.