Oscilloscope with capture and FFT?

[BrokeLinuxPhile]

The hum adjustment is probably a humdinger, or a variant of it.  A pot or two resistors, 1 leg on each phase, joined together at ground.  Often used when there is no center tap to fix the ground potential for each phase on a AC filament transformer secondary.

 

A scope recommendation is a tough call without knowing how often you would use it.  If you are a hobbyist and a DIY type, they are great to have and are used often.  Other than that, it will probably collect dust.  Have to consider also things like building a proper dummy load, buying a quality function/waveform generator, cost of GOOD probes, cabling, proper isolation of scope so results have proper context, probably a bunch of other stuff I'm forgetting.  Can be a black hole of money.

 

Barring purchase, look at rental or lease of a network analyzer or high precision scope.  There are lots of places out there that do this, and you can use lab grade equipment to analyze your system without dropping $10-20K.  The A/D converters are just way superior on high end units, it makes sub $1K scopes look like toys in comparison.  They are much better isolated from the line as well.

[BrokeLinuxPhile]

A high quality rms voltmeter is a good idea, just read the hum measurement directly by putting it into AC mode.  Get a big heatsink, build a dummy load out of quality power resistors, hook up voltmeter leads across the load.  Should be that simple.  If you plan things out well, you probably won't even need to solder it up, and could use terminal strips and crimped connectors.  Put some thermal compound on bottom of the power resistors, then screw down to heatsink.  This way is better since hooking speakers up is a reactive load and can give odd measurements.

[BrokeLinuxPhile]

Something like this would work great:

 

https://www.amazon.com/Precision-2831E-Bench-Digital-Multimeter/dp/B072MKZ4DR/ref=pd_day0_hl_328_8?_encoding=UTF8&pd_rd_i=B004O8TUZ0&pd_rd_r=86372f3d-65da-11e9-b80c-09dbc8d3b34b&pd_rd_w=2ERyb&pd_rd_wg=8jkwM&pf_rd_p=ad07871c-e646-4161-82c7-5ed0d4c85b07&pf_rd_r=6G81YY96KP11SFGE06YA&refRID=6G81YY96KP11SFGE06YA&th=1

 

It's up to you if you want to pay extra for NIST calibration.  Considering what you are measuring and what you've probably spent on your system the extra $40-$50 is probably worth it for cert.