reverse test repair circuit boards?

Need  good procedures on repair (locate defective parts) without board schematics.....Im a welder tech and many board schematics are withheld by oem's...i need a good tutoral,thanks,joe

jeff-o6 years ago
The first thing to look for is a busted fuse. Test them with a multimeter to see if they're open.

Next, and before applying power, check the rest of the PCB for components that look damaged. Capacitors may be bulged. Resistors may discolor. Semiconductors, like ICs and transistors, may be discolored, cracked, or blackened. Also check the PCB itself for damage, like burnt traces or discoloration (which indicated excess heat due to shorts). Replace everything that looks like it's gone bad.

Test for shorts and opens using a multimeter. The best way, if possible, is to have a working unit to compare to the bad one. Other times parts of the circuit may be duplicated or in parallel, so you can compare a branch that is working to one that isn't. Check cables for broken wires or contacts that have arced and are no longer making a connection.

Look for things like solder bridges and debris. Sometimes conductive objects will fall into the chassis and short stuff out. Doubly so if you're working in an environment with lots of little bits of metal flying about.

Hope that helps!
Its a difficult thing to do.
The approach I'd take would require a deep understanding of what the board is supposed to be doing, and the circuit blocks required to do it - typically, a welder needs current monitoring, variable frequency, variable duty cycle oscillations, power translation from the logic to the power electronics etc.

Tracking those functions could be done from known good boards.

You won't be able to attempt this without an oscilloscope either - I don't think you'd need anything particularly fast or fancy, but you HAVE to be able to see the waveforms on the board.

Steve
iceng steveastrouk6 years ago
+1
And the Best Tool !
A Keen Mind like Steve's

A