I've never worked on a Atari 2600 before. The first time I did, I wasn't getting anything on my old box TV. Not being able to do much at the time, I decided to check the voltage. The output voltage is 9v DC, but I was getting 15v DC. Yesterday I got my hands on another Atari 2600, so I decided to check the voltage again... it's also 15v DC. Is this the actual voltage, or do both power supplies have a short (or something)?
The design of the Atari 2600, and many other devices that use "9-volt DC" wall bricks, doesn't really care about receiving exactly 9 volts, subject to a few constraints:
The voltage must be far enough about 5 volts to allow a cheap regulator circuit to convert it into 5 volts.
There is an upper limit which will cause essentially immediate damage--typically somewhere between 15-35 volts.
Heat dissipation in the regulator will be proportional to product of load current times the amount by which the input exceeds 5 volts. A 13-volt supply will produce twice as much heat in the regulator as a 9-volt supply, but under conditions of light loading that won't be an issue.
The Atari 2600 draws a moderate amount of current itself, but cartridges draw additional current from the same supply, so total current draw may depend upon what cartridge is being used. On the other hand, a typical unregulated "9 volt" supply will only produce voltages significantly above 9 volts under conditions of light loading when heat dissipation isn't likely to pose a problem.