[9659]

many years ago, i was working as an electronic technician. we had a stack of processor boards (from a Perkin Elmer 7/32) that were removed from service. broken. many different revisions, and only schematics for one revision of each board. i thought it was hopeless. an older wiser tech taught me how.

plug a good board on an extender. run a diagnostic that fails in a loop. using a scope, look at every pin on the connector. write down what you see. replace with a bad board. repeat.

which signals are different? chase them back. if the schematic does not match, get out a voltmeter and your eyes and draw a schematic that reflects how the board is wired.

he called this "good card - bad card".

and it worked. not going to make any claims about cost effectiveness, but we fixed every board. and my troubleshooting skills in digital electronics were greatly improved.

this was a 'fireman' kind of job. waiting for the system to break, so it didn't matter if 2 techs put a week into 1 circuit board.