Why the ac adapter give a wrong voltage?

Hey mate, this will be due to the capacitors in the power adapter. Over time they dry out and their ESR (equivalent series resistance ) increases. Its usually the output filtering or oscillation capacitors. This leaves you with three options. 1) see if you can buy a adapter (might be expensive and cause issuse down the line), 2) carefully crack the adapter open and replace all of the capacitors 3) make your own new adapter. If you want to do 2 or 3 a d want help or advice, feel free to message and I'll help you as best i can.

Zakk