Repair

[johndon2000]

Hi, if you have a circuit board that you know is faulty but you have no other diagnostics, and the only way of repairing it was to swap out devices from a good board until it eventually works, would there be a specific order of devices to swap out first... i.e, in what order would you swap out the following devices going on prior knowledge of device failure rates:

Processor/ DRAMs, SRAMs, PROMS, ASICs, FPGA's, resistors, capacitors etc etc???

Many thanks,

John.

[Paul Price]

The fault in the board may not be due to component failure at all but do a bad soldering joint or contamination of the circuit board, so swapping parts is not your first option.

The least reliable parts would be capacitors, electrolytic types, so this would be my first choice, but your question begs a common answer to a complex problem that requires a different judgement in each case.

In general, power handling components, things on the circuit board that get hot, or handle the most current, such as filter capacitors and power resistors, all things running hot, are the most prone to failure. Also the interconnection technique, the connectors connecting this board to its host are more likely to become damaged than other components on the board.

A careful visual inspection is a must before reaching for the soldering iron.

[johndon2000]

Hi Paul, thanks for the reply. I agree swapping devices out is not ideal but it does (eventually!) work... I guess I'm looking for a quicker & more technical method of repair than just swapping devices!

[Paul Price]

Maybe I or someone else could help if you gave a schematic, picture of the board and the type of failure and the purpose of the board in question.

[AndyC_772]

There's plenty of better ways to fix a board, but you'll need to know more about it than simply "does it work? Y/N".

You'll need, at the very least, to know what it's failing to do a working board would do correctly. Is it, for example, getting hot? Generating incorrect data? Is there a signal that's going out of range? Do the correct lights come on? What criterion exactly is being used to define the card as "not working"?

Without that information, you're stuck. You could swap the parts that have a finite design lifetime (eg. the caps), or perhaps those which connect to external equipment (most likely to have been damaged by outside influences), or those which are most difficult to solder correctly and reliably. Without knowing what fault you're trying to fix, all are equally valid and all are equally likely to be a waste of time.

[johndon2000]

Thanks guys.

Paul - unfortunately no schematics... not too sure how to upload pictures but if you google NT9X35BA it will bring up some images; it is a telecomms ENET crosspoint card, and there is no fault information - it just shows as FAIL, no other diagnostics! Many thanks!


Andy - it's not getting hot. There is no other information other than it fails; no LED's. It is put into a telecomms system as is flagged up as Faulty (no other info!). I'll check what you said; are there any other finite design lifetime devices other than caps? And I guess PLCC's etc woudl be the difficult devices to solder?

Thanks again guys, much appreciated.

[MacAttak]

This board? 

Randomly replacing parts on that thing is going to be a hell of an expensive way to fix it. And the problems aren't always with the parts on the board! So there is no guarantee that if you replaced every part that it would work again.

It could very well be a broken or bent connector pin. Or a dead config battery.



There must be a way to get more information about the failure mode.

[dfmischler]

Could you repair it for $50 worth of time and energy?  Because it looks like maybe you could replace it for that.

[Galaxyrise]

Lever those socketed DIPs about half way out and then push them back in.  If that doesn't fix it, $50 is sounding like the winner to me...

[Stonent]

My guess is either chip creep or a solder joint.  The solder joint because of the sheer size of the board means it could flex a bit.

[Ferroto]

I usually start with checking voltages. 

[SeanB]

Voltages and then clocks.

[johndon2000]

oK, so assuming that there is no option of scrapping the board, and it is visaually ok, and the voltages, clocks and resets are all ok where would you go next?

[dfmischler]

I would put it in the parts bin and buy a spare board on ebay.

Your chances of figuring out what is wrong with this board without any information or serious diagnostic equipment is about the same as me winning $100 million in the lottery (I didn't buy a ticket).  Unless this is your hobby you are spending more money messing with it than it would cost to replace.

[AndyC_772]

Next random, arbitrary, uninformed guess would be that maybe a programmed device has lost its contents. In the photo I see a couple of socketed chips with labels on them. Try swapping these between the good board and bad one and see if the fault moves with the chips.

It bears repeating what others have said, though - without schematics and some really specific details on the precise failure mode, you're going to be swapping bits on this PCB for hours and hours, and will more than likely end up damaging something else in the process.

[digsys]

... more than likely end up damaging something else in the process. 

A VERY likely scenario.
IF I* had no other choice, I'd check all connectors for cracks / dry joints, maybe wiggle them a bit.
Then I'd a hot air gun over all the solder joints. IDEALLY, one with a narrow nozzle, so you can target ONLY the solder joints.
Then I'd do the freeze deal, while it is running. Again a narrow nozzle.
IF you don't have at least these minimal tools, get them !! Don't try to do it with solder wick / hair dryers etc !!!!
It looks like it was an expensive board, so I wouldn't risk too many attempts !!

[brainwash]

Guide to troubleshooting

First things are the senses: sight, smell (if it's a fresh failure) and touch. Use a strong light and loupe to check for cracked chips, black smoke marks or cold joints.
Press on the board with the back of the screwdriver, flex the board, press each corner of each large chip, especially BGAs.
Use a can of compressed air turned upside down and spray shortly on chips and see if it does something.

Check shorts between ground and power pins.
Next, for component checking: check diodes and caps for shorts, check [SMT] fuses for open. Check transistors and other 3 pin devices for shorts.
Check ground continuity, though it's not usually a problem with production boards.
Power up the board and check for voltages, especially on the controlling part, if there is any. I.e. the part that should have a standby voltage like buttons, LCD, I/O. Check for voltage correctness, 0.2V lower than design could mean non-functioning board.
Buy a new board.
If acquiring a new board is out of reach, try to read socketed devices with a universal programmer. Try to use JTAG or other protocols on the board.
Start replacing chips on the old board. Start with the cheap and accessible ones (power regulators, opamps and comparators, dil, tssop and sot-23 packages) and work your way up to BGAs.

Unless it's really specialized equipment no one working for money would try the last step.