i bought this 12v 2.5amp adapter for modem is it good? someone with any idea of circuits?
It's difficult to say.
From the number of discrete components, it is clearly not a simple linear PSU. For that type of PSU I would expect to see a transformer, a bridge rectifier, two capacitors and a choke. It doesn't look like a switching mode PSU, so I'll make a guess that it is probably some form of regulated linear PSU.
As long as your modem draws less than 2.5 A at 12 V DC then I would expect that PSU to be satisfactory.