[Ham-Mac] Mac Classic II Serial Port Kaput

[KLØS]

I've been using a very old Mac Classic II at the workbench to keep track of
my HP GPS frequency standards.  The I/O is RS-232 and use a simple terminal
program to talk to the boxes.  System has worked fine with my Mac IIsi also.
Recently the terminal program stopped talking to the box and I thought that
I may have fried the serial port by inadvertently unplugging the cable with
the machine on.  The computer makes a funny sound after I send the
interrogation character sequence to the box which makes me think the box is
actually receiving the query and returning the status data; the sound almost
sounds like a hard disk in the act of writing.

I found a very reasonable replacement motherboard and after installing it
all was well once again.  After a few weeks I cranked up the machine today
to check the GPS receiver status and the old problem has now returned and
I'm afraid I may have now fried this serial port too---the only thing I can
think of is that I've left the always on RS-232 connection plugged into the
Mac and that the higher voltages of RS-232 vs. RS-422 have broken down some
component as I have not unplugged or "hot switched" the two boxes connected
to the new motherboard since I installed it.  I used "PortPeek" today to
look at the serial port and seemingly the terminal program is opening and
closing the port ok.

Anyone have any ideas or experience with faulty serial ports on the older
machines?

Thanks in advance!

Dino KLØS/4