🚀 go-pugleaf

After a recent, severe thunderstorm I found that my modem no longer
worked and that the receive data line of the modem port of my fat Mac
was insensitive to RS-232 signals.  (It would work with the higher levels
of a local, direct-current adapter.)  This was the second such failure of
a Mac modem port I have observed, so perhaps there are others who might
be interested in the solution.

The main suspect at first was the differential driver, an AMD 26LS32,
that connects the SCC (an 8530) to the modem port.  When the 26LS32 was
removed for replacement, a resistance check of the receive data line RFI
filter showed double the expected amount, so it was also replaced.  With
the replacement of both parts, the modem port again functions with RS-232
signals, but its still not clear to me which part corrected the problem.
I would recommend starting with the RFI filter.

Replacement of components on the four layer processor PC board is not a
job for the faint hearted or inexperienced.  If you have never done it, I
recommend you find someone who has, because it takes time, patience, skill
and a few special tools.  I wouldn't have succeeded without the aid of some
local engineering friends, who knew how to do the job and where to get the
parts.  The diagram of the serial ports from Inside Macintosh (page III-26)
was also helpful.

Vic Abell, asc.purdue.edu, ...!pur-ee!pucc-j!abe