Re: Fixing a Apple IIc

[dempson@actrix.gen.nz (David Empson)]

Tom Kelly <tomk@pro-nsdapple.cts.com> wrote:

>  dempson@actrix.gen.nz wrote:
>  The upgraded ROM has a full self test, which should report which RAM
>  chips are faulty.  It is then a matter of replacing the chips in
>  question.  The self test displays a RAM fault as a series of eight 0 and
>  1 digits, with 1s corresponding to the faulty chips.  The leftmost digit
>  is bit 7 (RAM chip MRD7), the rightmost bit 0 (RAM chip MRD0).
>  
>  If an asterisk (star) is also displayed next to the numbers, then the
>  fault is in auxiliary RAM, not main RAM.  The auxiliary RAM is labelled
>  ARD7 (bit 7) through ARD0 (bit 0).
>   =====================================================================
>  When I ran the self test I got: RAM 0 1 0 0 0 0 0 0.
>  I replaced MRD6
>  
>  Next I got: *RAM ZP 0 1 0 0 0 0 0 0.
>  I replaced ARD6.
>  It seems ODD that the same bit would fail in both main and aux memory.

Yes.  They might have been damaged by something affecting data bus bit
6.  If the fault was still indicated when the RAM chips had been
replaced, then I would look for a motherboard fault such as a shorted or
broken track.

>  What does the ZP mean?

The fault was detected in the zero page area (first 256 bytes) of the
affected bank.

>  Now it seems to pass RAM test, but stops and displays GLU on the screen.
>  I presume that means that the chip on logic board labeled GLU is bad.

The GLU is a PAL ("programmable array logic").  It generates at least
two decoded addresses (chip selects for the IWM disk controller and the
6551 serial chips), and a few other odds and ends.  (Some of its
operation is unclear, because there are several signals which might be
inputs or outputs.)

The self test indicating a fault with the GLU might not necessarily mean
it is faulty - it could be something else which is causing the test to
think that the GLU is faulty (e.g. one of its inputs might be invalid).

The test only points to the GLU or the area around it as a likely
candidate for the fault.  It would be necessary to use a logic probe or
analyser to identify the exact fault before the GLU could be
conclusively pointed to.  This is the sort of thing that would normally
be done by an Apple technician.

>  With the proper programmer, can it be duplicated?

Doubtful.  Most PALs have a "security fuse".  If this is blown, then it
is not possible to read the internal logic connections.  It would be
necessary to determine its logic functions by identifying all the inputs
and outputs, and working out how they were supposed to behave.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand