I wrote to Charlie Fry last night:
Worked today (4th). Looked at shim boards and noticed that voltage
division is the same for all shims no matter how many there are on a
given board (4-8). In contrast to VXR's and probably Unitys, the
shim-DAC board is now subdivided into many DAC boards and, more
important, no longer just feeds a voltage to a current supply but
rather has the power transistors right there on the DAC boards
presumably to go directly to the RT shim stack.
My 6-yr-old Inova 500 has the same revisions of DAC boards (F) as my
new Inova 300, so I took a full (8-channel) board and started
swapping it in and out of 500 slots (there is a front edge
configurator that tells the board what slot it's in - you have to
change it with a probe screwdriver). When I hit slot #4, voila, big
homogeneity difference and stability seemed to be there (evaluating that now).
As you know, one bad gradient can implicate innocent ones. Since the
shim supply is designed to be versatile, I don't know if I can figure
out which DAC is assigned to which gradient. Even if I could, I would
not know whether it was the DAC, the register or the transistor -
although, registers rarely go bad, DACs usually irreversibly freeze
full neg or full pos, so I suspect one of the transistors.
This morning it seems that I have found the problem above. I also
have found that I own a board extender, so I can try to evaluate
which part of my bad board is bad - maybe even by feeling DACs for
heat. I've got to get into the schematics.
However, I have not had a day off since some time in June and I think
I'd like some rest first. The 500 user can wait until next week, so
I'll put my 300 back together, declare victory, and then see if I can
fix instead of replace the shim board. Right now, I'm really pooped
and hope to relax this afternoon.
Thanks to all who wrote.
Bill
William C. Stevens, Ph.D. Director
Nuclear Magnetic Resonance Facility
Southern Illinois University
Carbondale, IL 62901-4405
618-453-6498 voice -6408 fax 521-9892 cell
http://opie.nmr.siu.edu
Received on Wed Jul 05 2006 - 13:30:58 MST