Based on some histograms made during recent WFD running, we have some
repairs we would like someone at Gran Sasso to make. There is FADC
oscillation on two channels. Ash Sanzgiri first noticed this more than a
year ago and called out a smaller value for the bypass caps on the FADC
reference ladder. It seems this works *most* but not *all* of the
time. The fix is to remove these two caps.
Please be gentle with the boards. Try not to let anything get near the
ASIC pins (very fine leads). Work on a clean table.
The WFD channels you modify are:
1) SM-2 channel 28 (connected to E08/E10/E12/E14)
2) SM-2 channel 32 (connected to T01/T03/T05/T07)
The caps you should remove are: C180 and C92. These are small surface
mount caps on the back side of the motherboard. You will find them
underneath the FADC of the first channel.
These caps may possibly be glued in. I suggest the following technique:
heat one side of the of the capacitor with a soldering iron. Using sharp
snippers, cut the middle of the cap. The hot side will come off
immediately. Now heat the other side of the cap and clean up the site.
If there is no glue, you may be able to flick the capacitor off by just
moving the soldering iron back and forth from one pad to the other.
Since this is a very minor modification, you can just plug the boards
back in and turn the power back on and recable the signal cables.