HP-41 - port not working

[wojtek]

I would start with checking connections to this port, especially on the flexible tapes and checking the screws holding the body of the 41c together.
Then you could check with an oscilloscope if the port gives any signs of life. As the other three ports are OK you could compare their electrical activity with dead port.
Technically it can be not so simple because you would have to insert a module to a port what will block the access to the connectors inside the port