The Syncron AU7a was one of the earliest transistorised condenser microphones, and was also sold as the Fairchild F-22. It ran on four mercury batteries, which had a tendency to leak after a few decades and are now obsolete. Xaudia offers a replacement circuit board which allows the microphone to run on standard 48V phantom power.
The Xaudia upgrade includes swapping the entire circuit board and fitting a new, low noise PCB which runs on standard 48V phantom power, and switching the DIN connector to an XLR socket.
The cost for this conversion is a fixed £119 including parts. You can read further details here at the Xaudia blog.