How not to fix a classic mac (or: fried logic boards)
derschjo at msu.edu
Tue Feb 5 00:01:18 CST 2008
Just to follow up on a very old thread -- I finally got a replacement
LAG for my 512k Mac (thanks to a very helpful list member with a GAL
programmer) and after replacing it, the Mac is once again functioning.
So if you do accidentally do what I did (discharge the CRT to the main
chassis) and you end up with a very long, drawn out boot beep, the LAG
is quite possibly the culprit.
Thanks once again to all you helpful folks. You've been, well, helpful :).
>> That particular failure is documented in Larry Pina's "Macintosh Repair
>> and Upgrade Secrets" and probably in "The Dead Mac Scrolls" as well.
>> 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
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