111203-2236 EST
Put a short length dead short across the voltage input terminals. Two banana plugs with a #16 or #14 wire between them (mostly this size wire for mechanical strength) or a dual banana plug with #20 shorting it. Select millivolts AC and see what the reading is and report to us. Switch to DC and it should read zero. The RMS AC to DC conversion has a substantial noise level.
Probably a poor idea to measure current over about 1 A using the internal shunt. Rather use and external resistor (or 4 terminal shunt) and a voltage range. Reason the internal fuse in current mode is probably $15 or so.
If you have available a function generator, then apply different waveforms, use both AC and DC measurements and calculate what to expect and see how the meter reads.
Apply a constant amplitude sine wave as the input and gradual increase the frequency from 5 kHz to maybe 500 kHz and see what is the response.
Lots more you can do.
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