[Hps] ECal high-current preamps resolved

[Nathan Baltzell]

Hello Everyone,

The cause of ECal's high-current HV groups has been found to be the glue of a label on the
preamps' circuit boards.  Apparently it has become conductive over time, resulting in short
circuiting the HV trace for some preamps.  Thanks to Emmanuel for diagnosing this with the
“faulty” preamps we had replaced and sent home.

This week, after only removing the labels, the current on our two existing high-current
channels went down to zero.  Thanks to Gabriel and Mathieu, all 442 labels have been
relocated and all channels retested with LEDs.

This is important because, over the previous few months, we've seen a dozen preamps develop
this problem and cause their entire group of 10 channels to draw tens of uA (some up to 100 uA,
and vary significantly over time).  And since +10 uA results in a ~3% gain reduction unless voltages
are manually adjusted, this is a big improvement for gain stability.

-Nathan