[Halld-tagger] wrong component

[Richard Jones]

Dear colleagues,

Kudos to Alex Barnes, who has been struggling for almost two weeks now to
understand the reason why our preamplifier boards have an ugly oscillation
in the signal baseline at 100 kHz, as soon as we set them up and bias them
for readout of fibers.  We traced the problem down to one malfunctioning
component which is a voltage regulator on the preamp board that generates
one of the critical dc voltages for the amplifier.  That part was
generating 100kHz oscillations on our board, but they would go away under
certain weird conditions, which are not compatible with actual running,
unfortunately.  Chasing down the particulars of those weird conditions
turned out to be a red herring.

Finally Alex decided that either all of the parts are burned out and
malfunctioning in the same way on all of the boards, or else it is actually
the wrong part that was installed by the manufacturer.  The second turned
out to be correct.  We ordered some of the correct part ourselves from
Digikey and swapped it out, and voila the oscillations are gone.  After
extensive searching online, we have determined that the manufacturer
installed the TLV431BSNT1G instead of the NCP100SNT1G that we specified in
our parts list. This is confirmed both by the markings on the chip and by
the operating characteristics. Both of these are shunt regulators, but they
have reference voltages that are different by almost a factor 2.  Now that
it is understood, we are back on track with setting up for QA of fibers.

-Richard Jones
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