I have an AIO HP 24-xa0057C (bought at Costco). The screen usually flickers with horizontal patt

[LeisureSuitLarr]

I bought another identical power adapter and it did the same thing.

They both read 19.5v consistently on my voltmeter.

And yes, it is connected to a good higher-end surge protector and always has been.

[itsmyname]

@LeisureSuitLarr -- They both read 19.5v consistently on my voltmeter.

Good.

Hopefully, your meter can also read Amperage.

Compare to a flashlight battery: the meter will show 1.5 V, if there is any chemical reactions happening, but the meter should display over 5 Amperes on a brand-new battery, or less than 1.0 Amperes on a "well-used" battery.  And, of course, the flashlight will not be as bright when the battery is producing less than 1.0 Amperes.