How not to fix a classic mac (or: fried logic boards)

Josh Dersch 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 :).

- Josh
>
>> 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
>
>



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