Some of the Atari made PSU's are ok, but the pyramid style one's can be a nightmare, as many an Atari owner has found out. So yes, very sound advice Cy, every owner who knows how to use a multimeter should have a quick check on a semiregular period. If it's giving out the wrong voltage, ESPECIALLY if it's higher by more than half a volt really should change that PSU. .5V isn't going to damage the Atari, but it implies the voltage is drifting, and that caps inside are going bad, either way, not worth a chance as you say if you have spares.
There's so many cheap 5V DC (centre pin positive) PSU's out there for not a lot of money, or you can do the USB variation and power it from one of those cables. Try and avoid buying unknown, often Chinese made PSU's, they are EXTREMELY cheaply made, with as few components as possible. Often they are already hot giving out enough voltage (or lots more) and are usually NOT the amp rating they say you can draw..(Easy sign of that, they get VERY hot and don't smell too good. Just recently my phone fast charger cooked itself, it was the genuine product that came with the phone, thus showing that even the 'good' stuff is prone...So look after your 40 plus year old gear..
PS, love the keeping stuff excuse for the other half...
Paul..
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Not old. Vintage. :)