Beware of Engineers with Screwdrivers

Sometimes I really hate my job.

I'm an engineer by profession. I've got the State license and everything. I've got a customer's machine that refuses to behave. It's got a (Star Trek Technobabble Alert!) single-ended 1024ppr quadrature encoder on it that yeilds 129 pulses per inch - most of the time. Except occasionally when moving in one direction, it will drop some counts. Like several inches worth. And since we're trying to measure to 1/64" accuracy, that's not a good thing.

The encoder is connected to an Allen-Bradley high-speed counter card that just counts, 0-999, then rolls over. The counter card communicates with an Allen-Bradley PLC 5/20 processor that totalizes the count. There are three other IDENTICAL encoders on the machine, and they all work hunky-dory. We've changed the encoder, we've changed the high-speed counter card, today we changed the CABLE.

It's still thumbing its nose at us.

(Edited to add: It's not mechanical, either. Rack & pinion gearing, positive keyed zero slip coupling. Anybody got a chicken to sacrifice?)

UPDATE! Problem solved! Of course, after the fact it was something simple, and blindingly obvious - the 5VDC power supply was only putting out 3.5V - just barely enough to make the encoder card work - most of the time.

Engineers with screwdrivers? Should be engineers with voltmeters.