Case of the Shorted Signal

I was working as a manufacturing engineer in the printing division of a company in Singapore. Our engineering team was called in to help diagnose a problem with a printer. Something was causing the printing of a magazine cover to come out completely black. This occurred so intermittently, that the folks on the production line were confused. If the printer was on the top shelf of the production line when the copy was made, the copy came out fine. When the printer was moved to a nearby table, the copy came out black.

We measured the source signal and found out that the scanner source was getting grounded whenever the printer was moved to the table. So we tried to figure out what could cause the signal to get grounded on the table and not on the shelf.

We finally realized that the power cord on the top shelf was short. It barely made it to the power connector. This short length of cord tugged and warped the PCA slightly higher than it was when the printer was on the table with an ample length of cord. The warping of the PCA created some distance between the source test point and the ground plane -- which was sitting under the PCA. That increased distance kept the printer from shorting, and thus the copy came out correctly.

When the printer was placed on the table, the PCA didn't warp, so the test point would touch the ground plane. That shorted the signal to ground, resulting in a completely black piece of paper.

This entry was submitted by Vignesh Sivaambihaibalan and edited by Rob Spiegel.

Vignesh Sivaambihaibalan is an operations engineering manager at Verigy Singapore.

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