Finally built a proper Nintendo Famicom Test rig for testing CPU, PPU, SRAM, 74HC139, 74HC373 and games. Have been needing this for a long while. Using @backofficeshow‘s FC Power VAMP to provide stable power and safe composite video output without leading to PPU damage.

VAMP:
https://backofficeshow.com/shop/famicom-psu-av-replacement-board