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.
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.