My cat chewed our cable modem's electrical input wire in two and dragged the plug off out of sight. I took the power adapter and modem to Radio Shack where I bought the proper power plug for the modem. I have a volt-ohm meter and can solder the plug on the cord, but I don't know the polarity of the plug. Is the center conductor of the plug positive or negative relative to the barrel of the plug?
Can you find the polarity on the adapter. There is usually a half little circle with a dot symbol and an line pointing to the half circle and the dot shows the polarity.
Then there are AC/AC power adapter and you don't have to worry about polarity.
Most power adapter I encounter have positive inside and negative outside.
My cable modem takes a 10 VDC, 1 Amp power adapter (modem's label even specifies that), but I can't find anywhere that tells me the polarity on the plug. The modem's circuit board doesn't specify polarity. The manufacturer doesn't maintain a technical questions desk and the cable company using the modem demands more information about me than I am willing to devulge.
I'll investigate your suggestion that, by convention, DC adapters all have positive inside and negative outside. I considered the possibility that DC adapters comform to some standard as you suggest, but haven't tried to confirm or deny the possibility.
Certainly, even if such a convention does exist, the modem's manufacturer isn't duty-bound to follow. The manufacturer could specify a DC adapter that goes against such a convention. I don't know why that might be the case, but such a decision by the manufacturer is possible.
Thanks for your suggestion. I'm going to check it out.