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Tuesday, 05 February 2013

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Seriously? Their "techs" couldn't perform basic troubleshooting like checking resistor values? Sounds like there should be a few electronic technician slots opening up. Then again, what are they paying their techs? The answer may lie therein.

Wow, this just dumbfounds me.

I would have been okay with old design, new transistors with higher hfe than originally specced in the initial designs (actual story), intermittent issues in a film resistor with a crack in it and board flex due to vibration, heat, or due to board flex when tightened down, etc. Maybe even electrolytic caps reversed or electrolytic caps instead of ceramic.

Heh, they deserve to have to pay for competence.

Well... these are mainly power-electronics techs, and it's an intensely digital board. When the analog techs saw the firmware signing on with an error message of "FPGA initialization failed," they scratched their heads and passed it to the resident digital guy, who's quite junior (and, indeed, not being paid a lot at this point); he got as far as determining that the configuration memory seemed to be working, and the FPGA appeared to be accessing it, but that initialization wasn't completing. The obvious answer is "bad FPGA," but, liking to have a good look at failed boards before they're reworked, I insisted on having a look, and eventually determined that the FPGA was in the wrong mode due to the wrong resistors.
Had the FPGA been working, I expect the techs could have figured out the wrong resistors in the analog portions of the board. (Though that's a bit problematic, as the analog signal paths all lead through the FPGA, and have various firmware-defined and somewhat user-adjustable fudge factors applied before they're available for display, so debugging even the analog signal paths can get entertaining, and soon leads to trying to hit tiny SMT leads with scope probes.)
Come to think of it, I probably ought to have included some kind of "turn off all fudge factors and show me what's really going on at the analog level" command in the firmware. Maybe that gets added next time there's an intensive troubleshooting session.
(Just to make things even more fun, when the analog techs think "resistor" they think "1/4W axial" or bigger things; the wrong resistors here were 0402 parts, and the immediately problematic ones were on the back side of the board for good measure.)

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