Electronic Repair

[Guest 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.

[Relayer110]

Hello John,
Murphy's Law always states that no matter which end of the circuit you start at, the fault always lies at the other end...

When trouble shooting circuits its important to know a little bit about the circuit function to determine roughly in which section the fault may be. This will reduce the amount of components requiring swap over.
The most common faults are due to poor solder joints, and sometimes redoing suspect ones can solve the problem.
The second most common faults are due to dried out electrolytic capacitors. They deteriorate with age unfortunately.
If you've basically checked as much components as possible in a circuit and the only alternative is to swap them out one by one, then the obvious start is the easiest components to remove.
If there are any integrated circuits with sockets, they would be the first to swap out. But if they are soldered in, then they'll be the last.
Most passive components (i.e Resistors, Inductors, capacitors etc) should have been checked prior. Resistors need one leg to be lifted from circuit for accurate measurement. Inductors can be measured in circuit, and most should read extremely low ohms. Capacitors really require complete removal and checked with a capacitance meter and an ESR meter.
If you've swapped out what ever socketed integrated circuits there are, your next step will be electrolytic capacitors.
Replacing them would be a better option, but unfortunately that costs money.
By the way, quite often faults in circuits are usually due to dried out electro caps in the power supply. They can cause some weird symptoms due to excessive ripple on the voltage rails.
Next step is to swap out any transistors, FET's MOSFET's etc.
Next are the resistors.
Lastly, integrated circuits that are not socketed. Though if there are surface mount ones, you're going to need a specialized soldering station for the effective and safe removal of those devices.
Ball grid array IC's, forget it. You need the use of a robot to place those. You would need an extremely keen eye to place one of those accurately, plus you would need an oven to bake it on.
I hope the above helps.  ;)
Regards,
Relayer  ;D ;D ;D