Subject: Re: Er, does "correctable frying" of Apple IIs exist?
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From: Rubywand <rubywand@swbell.net>
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Date: Tue, 01 Aug 2000 14:09:22 -0500
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Neil Thayer writes ...
> 
> Perhaps eight years ago, at what will show itself to be the dumbstoopid age
> of 11, I had my dad obtain (for free!) a mint condition '78 Apple II from a
> friend of his.
 ....
> So one day I'm fooling around with the hardware inside,
> right, and I remove and reinsert the disk controller card while the unit's
> on.  (Yes, I know, the most idiotic thing possible, but I was 11.)  The
> unit hasn't worked correctly since.  Sometimes when it boots up it won't
> automatically access the floppy drive (and yes, it did before, I had the
> ROM upgrade); sometimes the keyboard characters don't correspond to the
> keys.  Is there anything obvious that might have been fried and could be
> corrected?
 ....

     In the case of the Disk Controller card, there does not seem to be any
easy way to mess up the computer by pulling or plugging the card with power
ON. The 'Controller card seems to just pass through +12V and -12V to the disk
drive. To mess up anything in the computer, it looks like the card would need
to be pulled or plugged at an angle or when the contacts are grossly
misaligned so as to cross-circuit Slot contacts.

     For example, if Slot pin 50 (+12V) should get cross-circuited to Slot pin
49 (Data 0), the chips that could be affected would be ...

Motherboard

o- the six ROM IC's

Does not seem especially likely since your computer is not dead and ROM IC's
are reasonably rugged.


o- the three Data 0 16k RAM chips (IC's C-3, D-3, E-3 on my II+ diagram)

Hard to say. The fact that your Apple II does anything at all makes RAM damage
seem a bit less likely.


o- the 8T28 IC which buffers Data 0 (and four other lines) to/from the 6502
microprocessor (IC H-11 on my II+ diagram).

Does not seem likely. The 8T28 is fairly rugged and, if the Data 0 line were
affected, it is unlikely that anything would work.


o- the 74LS257 dual 4-line multiplexer IC which selects between Data 0-3 data
coming from RAM, Char generator ROM, ... and Data 0-3 coming from the Keyboard
(IC B-6 on my II+ diagram).

This is a pretty good suspect. The 'LS257 is not especially rugged; and, a
damaged output (the side affected) for Data 0 might function to intermittantly
give you false 1's for Keyboard inputs (e.g. a "2" would be a "3"; an "R"
would be an "S") as well as occasional bad RAM reads.



Disk Controller Card

o- the PSAD3 seems to be some kind of ROM

Don't know.


o- the 74LS323 (C-3 on the 'Controller Card diagram in the DOS Manual)

Certainly possible; but, this IC would not affect Keyboard I/O.



Other Cards

o- Any other Slot cards which were plugged in and connect to the Data lines
may have been affected. The Data 0 RAM IC on a Language Card is an example.


Disk Drive

o- It seems unlikely that the disk drive circuit board would be damaged.


     So, replacing the 74LS257 at B-6 on the motherboard looks like a decent
bet for a fix. (A 74S257 should be okay as a replacement, too).  Probably, it
would be a good idea to replace both of the 74LS257's (at B-6 and B-7).

     Replacing the 74LS323 on the Disk Controller Card seems like a good idea.
(Or, since you are going to be shopping for IC's anyway and the chip does not
cost much, you might as well get one.)

     You could get enough RAM IC's to replace the 3-4 RAM chips which may have
been damage. But, if the place you get your other IC's doesn't have the 16k x
1 DRAMs you need, try replacing just the ones mentioned above.



Rubywand