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Message ID: 14261     Entry time: Thu Oct 18 00:27:37 2018
Author: Koji 
Type: Update 
Category: SUS 
Subject: SUS PD Whitening board inspection 

[Gautam, Koji]

As a part of the preparation for the replacement of c1susaux with Acromag, I made inspection of the coil-osem transfer function measurements for the vertex SUSs.

The TFs showed typical f^-2 with the whitening on except for ITMY UL (Attachment 1). Gautam told me that this is a known issue for ~5 years.
We made a thorough inspection/replacement of the components and identified the mechanism of the problem.
It turned out that the inputs to MAX333s are as listed below.

  Whitening ON Whitening OFF
UL ~12V ~8.6V
LL 0V 15V
UR 0V 15V
LR 0V 15V
SD 0V 15V

The switching voltage for UL is obviously incorrect. We thought this comes from the broken BIO board, and thus swapped the corresponding board. But the issue remained. There are 4 BIO boards in total on c1sus, so maybe we have replaced a wrong board?

Initially we thought that the BIO can't drive the pull-up resister  of 5KOhm form 15V to 0V (=3mA of current). So I have replaced the pull-up resisters to 30KOhm. But this did not help. These 30Ks are left on the board.
 

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