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How to begin troubleshooting a pcb

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ohmslaw

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Hi all: I am kind of new to pcb troubleshooting and I am having the hardest time trying to figure out this card. The purpose of the card is to dispense flux to be used in conjunction with magnetism. The problem I am having is figuring what components tie in with each other. I do have one noticeably problem on the card but I can't tell if there is anything else wrong with it because I dont know what components are together. The one part that is surely damaged is the BD682 darlington transistor. Is there a better way to diagnose the board without supply power to it and without removing each component? In other words is there a way to breakdown the board if you cant see what all is connected together? Please see attached picture of the card!
 

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It is very difficult to diagnose a fault without turning the power on.

Hoever, there are a few initial checks you can do:-

do a visual inspection of the PCB looking for any obvious damage, eg. burnt components, cracks in the PCB tracks, dry joints, etc.

A good magnifying glass is essential as some problems are hard to see. For example, my TV had a dry joint that was only visible with a Mag Glass. There was fine crack in the solder around a wire.

After that, assuming you did not find anything, you need to turn the power on and use a multimeter.

Firstly measure the power supply voltage(s).

Then measure the voltage at various points in the circuit looking for one that is not what you would expect.

It is a big help if you have a circuit diagram. Otherwise you may have to draw one which is tedious & time consuming.
 
lj

I found one burnt darlingon transistor but I was unable to visually find anything else wrong, however, this does not mean anthing else didnt short circuit! I guess I'm trying to find a way to modularize the different components and see if any are or have been affected by the overcurrent that happened on the board. However, I cant figure out which capacitor goes with which resistor, or is the circuit utilizing an rc or is using a voltage divider or a filter or choke and it would probably take me a lifetime to trace the entire circuit without have the actual schematic of the board. So how can you figure out what components tie into each other without having the schematic of the board?
 
It is tedious. You can use a DDM that has a continuity option so you can trace where the tracks go.

eg. connect the DDM to say a resistor lead & then probe around the likely suspects looking for continuity.

I suggest you start at the damaged Darlington and work from there.

I assume that it has had either too much collector current or too much base current.

Electrolytic capacitors often fail.
 
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Use everything ljcox told you, and also a bright light behind your circuit board which can help; you look at the component side and the light reveals the traces.

Producing a complete schematic of your very dense board would take a long time (and like other things one becomes faster with practice,) but like Len says, start at the darlington and work from there. You may not have to do too much.

If the 'little card' is very expensive to replace, then it is worth spending lots of time on. On the other hand, if the little board can be replaced cheaply, better to do that way. What is that old saying? ah, choose your battles wisely.
 
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