[Hps] ECal high-current preamps resolved

[Maurik Holtrop]

Hello Nathan, Emmanuel, Gabriel and Mathieu,

Excellent news and some good investigative work!

Thanks,
	Maurik


> On Apr 9, 2015, at 10:50 AM, Nathan Baltzell <baltzell at jlab.org> wrote:
> 
> 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
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