13 Nov
2007
13 Nov
'07
7:56 p.m.
Date: Tue, 13 Nov 2007 11:17:41 -0600 (CST)
From: "Jeff Walther" <trag at io.com>
Subject: Re: How not to fix a classic mac (or: fried logic boards)
To: cctech at classiccmp.org
Message-ID: <12318.209.163.133.242.1194974261.squirrel at webmail.io.com>
Content-Type: text/plain;charset=iso-8859-1
> Date: Mon, 12 Nov 2007 22:45:09 -0800
> From: Josh Dersch <derschjo at msu.edu>
> First point of business, I discharged the CRT.
> To the main chassis. This, as I have now discovered, is not what you
> are supposed to do to discharge the CRT unless you want to destroy the
> logic board.
That particular failure is documented in Larry
Pina's "Macintosh Repair
and Upgrade Secrets" and probably in "The Dead Mac Scrolls" as well.
I'd
look it up for you, but I don't have my books with me here.
Okay, I'm home, I have my books. It says on page 98 that
discharging the CRT without a big honking resistor may blow a 74LS38N
(U2) on the analog board and the LAG chip on the logic board. The
former sounds like it might be fairly standard. The latter sounds
like it may be one of the custom programmed PALs or GALs or whatever
that Tony was writing about.
I wouldn't be surprised if folks had already figured out all the
internal logic for the various Mac 128/512/Plus chips though.
Finding it might be a bit of a challenge.
OTOH, the LAG chip may be fine and it could be U2 on the analog board
that has the problem.
Jeff Walther