On pin 10 of Serial B, there are 12.68 k ohms of resistance between it and the metal chassis, but that is not present on pin 10 of Serial A. Might that be a clue? Or is that the way Serial B was designed?
Hopefully all my poking and prodding isn't making anything worse.
If you've got the power unplugged and staying away from the video circuitry and power supply, poking around with an ohmmeter is unlikely to damage anything in a Lisa.
Pin 10 of the RS-232 ports isn't supposed to be connected to anything, so double-check that you've got the pin numbering scheme correct...
Looking at the rear of the Lisa, you see the receptacle side of the female DB-25 connector, so the pins are like this:
\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /
\ /
\ 25 24 23 22 21 20 19 18 17 16 15 14 /
A minimal RS-232 connection requires 3 wires/pins
Pin 2 - Transmit data from Lisa
Pin 3 - Receive data to Lisa
Pin 7 - Ground
Sometimes more wires are used for handshaking and modem control.
Assuming you have a 2/10 I/O Board plugged into a 2/10 Motherboard, you should be able to measure low resistance from the I/O Board to Serial B as follows:
Serial B
DB 25 - to - I/O Board
Pin 2 - to - U10E-15
Pin 3 - to - U11E-1
If you turn the power on, the voltage at Serial B pin 2 should go to -5V or so when nothing is being transmitted.