Small Zener diode voltage regulator circuit with PCB, using transistor and few parts. it is simple and cheap fixed dc source circuit. As one of best choice.
Modify power supply source in old circuit PCB. I then build to is 9v regulated power supply circuit , that can give current 1 to 2Amp. Look 3 circuit below
If you would like to build Simple 3.3V Power Supply circuit for digital micro controller or esp8266. This circuit may be one way for you. It can power current at 1A maximum. We normally use common electronics parts, so easy and cheap.