We had the same affect when I was working on the prototype NMR scanners.
THe gradient coil perturbs the static magnetic field to determine
position information from the return signal. When you step the gradient
quickly , the aluminum tube surrounding the coil produces eddy currents
from the changing field and alters the field until it settles out.
THe transformer core is pretty good at capturing most of the flux but
some leaks out and rattles the steel next to it. Mu-metal might help.
---
Ron
BEAR wrote:
>
> IF it's the aluminum doing something that steel wouldn't do it is
> causing
> eddy currents to flow through it, shunting the magnetic field slightly...
> perhaps this is causing a change in the lams slapping themselves, or the
> lams against the chassis behaving like an electromagnet (buzzer).
>
> Usually, the lift from the chassis and isolate works well enough.
>
> I'm thinking that the eddy current thing might have merit, since
> there seems to be some effect upon audio output trannies when they
> are lifted off the chassis (no coupling).
>
> _-_-bear
>
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