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When you say "very underexposed" how much are we talking about? Were you able to get proper exposures using the same lights and same meter with your Nikon?
You may not be doing anything wrong; what one company calls "ISO 100" may not be the same as what another company calls it, even though ISO is supposed to be a standard.
You need to do some testing to find the "true ISO" of your camera, and add or subtract the difference for precise metering.
With my Sekonic L-758 I can save calibrations for different cameras in the meter's memory; I don't know if the Minolta can do that or not. If not, you can just make note of the difference and set it on your camera: for example, you may need to tell the meter you're using ISO 100, but dial ISO 160 into the camera. (That's just an example; only actual testing will give you the correct numbers.)