Electronic Repair: Learn How to Fix PCBs

[stef3679]

TL;DR Summary

Does anyone know how to fix pcbs? I could use some help.

Hello,
I am a student in mechanical engineering and also like electronics ( repairs and things like that ),i know electronics and how to build circuits,even embedded systems,but have no clue how to repair a circuit.

I thought it'd be easy,but I soon found out that trying to fix pcbs isn't a step by step procedure. Does anyone know if there is some standard procedure or a book or something like that, that gives some insight on how people fix pcbs.

I asked my teachers and said that there is no standard procedure,you just try combinations and perform measurements and generally trying to find your way around. But i think that all those people that is their job to fix pcbs, and that their jobs depend on it know better than doing random moves trying to find what's wrong.

Sure if you know electronics you know where the fault probably is, but there has to be a more sophisticated method. Does anyone know? To be clear I don't want to fix something at the moment, just want to learn how to.

Thank you in advance!

 

[mfb]

It depends on what is damaged. Just a broken component that can be replaced? A broken trace that can be connected by a wire? A short that can be cut away? Something else?
Is the component part of a digital system or analog? Some high frequency system? How reliable does the repair have to be?

The repair method can depend on all these things, and many more that I forgot.

 

[Guineafowl]

This is a very broad question and so hard to answer; a bit like ‘how do you fix a car?’ In that it depends on the type of car you’re working on and what fault is present. Any fault-finding flow chart is likely to be specific to that board, and modern ones tend to be: Try this—-try that—-replace board.

If I have a step-by-step procedure, it is this:
1. Fully characterise the fault.
2. With (1) in mind, look over the board and identfy which areas do which. Decide where the fault is likely to be.
3. Try to find a schematic (usually fail). Check fuses.
4. Look over the board under magnification for physical damage to components or cracked solder joints.
5. Test the hard-working components - power MOSFETs, dropper resistors, electrolytic caps.
6. Flicking between resistance and diode check mode, screen resistors, diodes and transistors for shorts and opens.
7. Switch on and check key voltages, especially at ICs, and power supply voltages.
8. For microcontrollers, check power, ground, clock signal, reset signal.

Not exhaustive, but doing this will sometimes pick up anomalies that you can then trace through the board.

 

[alan123hk]

A few decades ago, when I was an electronic repairmen at the factory, I often used a less intelligent approach, but sometimes it was still feasible.

This basic technique is trial and error, repeated replacement of suspected faulty components until the PCBA resumes normal operation.

After accumulating experience, the success rate may increase and probably solve the problem faster.

 

[stef3679]

It depends on what is damaged. Just a broken component that can be replaced? A broken trace that can be connected by a wire? A short that can be cut away? Something else?
Is the component part of a digital system or analog? Some high frequency system? How reliable does the repair have to be?

The repair method can depend on all these things, and many more that I forgot.

I know,but when for example you have a laptop which has many systems inside and the pcbs are very complex,how do you know where to search,and even if you know which part is probably faulty, that section of the pcb still has too many components,how do you know which one is faulty.

 

[stef3679]

This is a very broad question and so hard to answer; a bit like ‘how do you fix a car?’ In that it depends on the type of car you’re working on and what fault is present. Any fault-finding flow chart is likely to be specific to that board, and modern ones tend to be: Try this—-try that—-replace board.

If I have a step-by-step procedure, it is this:
1. Fully characterise the fault.
2. With (1) in mind, look over the board and identfy which areas do which. Decide where the fault is likely to be.
3. Try to find a schematic (usually fail). Check fuses.
4. Look over the board under magnification for physical damage to components or cracked solder joints.
5. Test the hard-working components - power MOSFETs, dropper resistors, electrolytic caps.
6. Flicking between resistance and diode check mode, screen resistors, diodes and transistors for shorts and opens.
7. Switch on and check key voltages, especially at ICs, and power supply voltages.
8. For microcontrollers, check power, ground, clock signal, reset signal.

Not exhaustive, but doing this will sometimes pick up anomalies that you can then trace through the board.

Thank you very much, I understand your point,this procedure you provided is of great help.

 

[stef3679]

A few decades ago, when I was an electronic repairmen at the factory, I often used a less intelligent approach, but sometimes it was still feasible.

This basic technique is trial and error, repeated replacement of suspected faulty components until the PCBA resumes normal operation.

After accumulating experience, the success rate may increase and probably solve the problem faster.

I see, that's basically what my teachers told me,but what happens if you can't fix it ,or even find the problem. Worst case scenario of course must be the case where the technician makes it worse...but i see your point,thank you

 

[phyzguy]

As systems become more and more complex, the time and effort required to track down and repair a problem starts to cost way more than just replacing the failed board, or even the failed product. Trying to repair a failed IC, for example, is basically hopeless. You just throw it away and replace it.

 

[Guineafowl]

I know,but when for example you have a laptop which has many systems inside and the pcbs are very complex,how do you know where to search,and even if you know which part is probably faulty, that section of the pcb still has too many components,how do you know which one is faulty.

A laptop PCB is complex but if you know the topology you can identify rough areas where the fault might be. If you have a schematic, you can measure power supply rails and identify if one is down. You can even feed the suspect rail with current, and dribble something volatile on the components to see which is hot. As said above, sometimes substitution is the only way.

 

[stef3679]

I see, I agree that substitution is probably the only way, but there was a case a few months ago, I had a laptop which for some reason I couldn't turn on and when I could even plugged in there was no charging. I went to a laptop technician and told him to see it and that it was probably the battery because in the last months before this issue appeared wouldn't last more than 30 mins. He told me that the problem might be the loose charging connector or the battery, we replaced both and still nothing, the issue wasn't solved. I basically bought another laptop, in that case we couldn't replace the entire motherboard.

 

[davenn]

, in that case we couldn't replace the entire motherboard.

I don't understand why not ? unless a replacement board was
1) difficult to obtain
2) too time consuming to do the transplant
3) no access to the version of OS and drivers needed for that lappy.

I am routinely replacing motherboards in tablets and reinstalling the OS etc
I would never consider finding an actual fault on the motherboard
1) not worth the time
2) the service info is unlikely to be available
3) the mass integration and very fine SMD components make it pretty much impossible anywayDave

 

[stef3679]

I don't understand why not ? unless a replacement board was
1) difficult to obtain
2) too time consuming to do the transplant
3) no access to the version of OS and drivers needed for that lappy.

I am routinely replacing motherboards in tablets and reinstalling the OS etc
I would never consider finding an actual fault on the motherboard
1) not worth the time
2) the service info is unlikely to be available
3) the mass integration and very fine SMD components make it pretty much impossible anywayDave

So you say that its much. better to replace the motherboard, I agree. But on that laptop I had much data, could I just change the motherboard and take the internal hard drive from the faulty laptop and put it on the new one? Is there any compatibility issues I should know between internal hard drives and motherboards?

 

[davenn]

But on that laptop I had much data, could I just change the motherboard and take the internal hard drive from the faulty laptop and put it on the new one?

No, you don't use it as the main drive on the new laptop
You set it as an external slave ( isn't a hassle with SATA drives ) and access all your data
tis very easy to do

There are adaptor boards available in the market that convert between a SATA or IDE drive
and a USB connection to the laptop or desktop PC

 

[stef3679]

No, you don't use it as the main drive on the new laptop
You set it as an external slave ( isn't a hassle with SATA drives ) and access all your data
tis very easy to do

There are adaptor boards available in the market that convert between a SATA or IDE drive
and a USB connection to the laptop or desktop PC

Thank you, that's very helpful!

 

[rbelli1]

No, you don't use it as the main drive on the new laptop

This is a great idea for many reasons including speed, capacity, and reliability. You don't want a small, old, worn out, slow drive hampering the performance of a new laptop.

BoB