IMHO never trust header solder connections on a 40 year old PC Board.
As I have been repairing several old PCBs I pry off the old plastic 9 pin header leaving the pins in the PCB.
I then heat the pins to let them fall out, this protects the PC board traces from heat and mechanical forces when using a de-soldering tool.
What I have noticed is that a large number of these pins come out when I pry the plastic off indicating they are not 'really' still connected.
These pins solder connection looked fine before I pried on the header housing.
So, the bottom line is the 'solder' ages and de-bonds from the pin to the PC board trace even if they look OK from the bottom side.
This de-bonding is likely caused by temperatures cycles as well as mechanical forces from plugging and un-plugging the connector over many years.
That is why Todd Tuckey's shop re-solders ALL header pins on PC boards as a standard procedure on all serviced pins and arcades. (watch the YouTubes)