[morzh]

Brenda, as I pointed before, the case is already connected to the negative rectified voltage, so connecting it to the Earth will simply reference the Negative to Earth which is not good at all.

You could probably ask: why not then simply disconnect the case from the Negative? the answer is simple - it is practically military construction and everything is veted/pressed intop the chassis, it is not a simple jumper. It is all across. There is a Negative Common bar running in the middle and it is pressed in the chassis at both ends.

I think the obvious slution is what you and I thought of - the isolation transformer, but the case will have to stay floating.