Buying used amplifiers, how to test them right?

[Crosstalk]

Many of us buy lot of audio gear from the used market. There are lot of times, when I got amps which had irritating issues. What equipment should we have at home at the bare minimum, and what tests we need to perform on the amps before connecting to our beloved speakers?

When connected to the same exact speaker(one speaker, swapped from channel A to B) on many amps, I have noticed
the left and right channels do not sound exactly the same in terms of tonality for most of the them anymore.

How do I test if one channel is delivering less power than the other? this may lead to clipping and burn out of tweeter when pushed.
How do I test if there is some DC voltage coming from the speaker outputs and how much is the safe level?

If transformer hums, is it a sign of aging? If not, how do we fix them?

 

[Sokel]

Others may be more technical,but the bare minimum is a DMM where you can check for DC at the speakers outputs.*
If there's no,turn the DMM to AC,play a 50Hz tone at low volume and check if both channels are the same.

*Edit:it's not the worst idea to also check the rest of the chain prior so to know there's non there also as if it's there is the amp will just amplify it and it won't be it's fault.

 

[Crosstalk]

Others may be more technical,but the bare minimum is a DMM where you can check for DC at the speakers outputs.
If there's no,turn the DMM to AC,play a 50Hz tone at low volume and check if both channels are the same.

how much DC voltage will be safe? DMM = Digital multimeter?

 

[Sokel]

how much DC voltage will be safe? DMM = Digital multimeter?

A few mV,tops 10 or so in my book,others say that as much as 50mV are ok too.
Yes,digital multimeter.A MUST have,despite new or used gear,everything must be tested before entering the chain.

 

[Sokel]

...and since we're talking about used,older amps may take their time to settle,so letting them on for half hour or so and then remeasuring may produce different results.
That's normal as long as the readings get better.