I picked up an Astron VS-35M power supply. When I plug it in and turn it on does there need to be a load on it in order to adjust the amps and the voltage? When I turn the knobs, the meters don't move at all. I can hear it humming but nothing else. I've never had a power supply before and the one I picked up didn't have a manual with it, and I know virtually nothing about these things. Thanks.