I need a post or link to a primer on tolerance information of going +/- 1 volt or more with same/different amperage and how dangerous that deviation is to 6V, 9V and 12V devices.

So I'd like to know the affects of using a power adapter that has
• Higher vs. lower voltage than the device asks for, +/- 1 volt vs. +/- 2 volts
• Higher vs. lower amperage specs than the device asks for

Background: An old Sega Genesis console says it needs a DC 9V 1.2A adaptor.
I had a bunch of 9V adaptors but they were 9V 200mA to 500mA range.
I also had a 10V 1.2A adaptor.

I resolved my original issue of SEGA Genesis console emitting a loud buzz, something it is apparently known to do, by trying several adaptors I had.
So once again the console says it needs a 9V 1.2A adaptor.

Buzzing went away with a 9V 500mA adaptor.
It was still there with a 10V 1.2A adaptor.
It was still there with a 9V 300mA adaptor.


What are the tolerances in general on this topic?

 

the buzzing is probably due to the filter caps going bad., you shouldnt go over the voltage ratings, that is bad on the load. the current rating should be the same or more., and make sure they are regulated, unregulated wall warts were common for quite a while. the sega genasis used the adapter output as input for the internal regulator, just like the trs80, which used a large adapter. a lot of adapeters now are actually switching supplies, and work better than the older ones.

 

What are the tolerances in general on this topic?

Unpredictable. There is no hard and fast rule because every load is different. Something that is designed to be supplied by a regulated voltage might be destroyed by supplying it too high a voltage. Another device might have voltage regulation built in and not care much what the supply is.

About the only certainty is that your adapter should be rated for a current that is at or above the load requirement, otherwise you risk an overload of the adaptor.