Re: Defective RAM on Apple IIgs

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

Bryan Parkoff <nospam@nospam.com> wrote:

>     I have experienced when I tried to copy files from DOS 3.3 to ProDOS
> using Copy II Plus 9.1.  I compared both files on source and target.  I have
> discovered that both are not identical.  I did notice garbage data from
> sector to sector.  I assure to be defective RAM built-in on motherboard of
> Apple IIgs (256KB).  I did diagnose using Ctrl-Open Apple-Closed 
> Apple-Reset, but it shows, "System Good".  It does not make sense.  It
> should have scanned bank $0 through $3 to detect corrupted data in RAM.
>     I have tested another Apple IIgs motherboard.  It is working fine when
> files are copied using Copy II Plus 9.1.

With those symptoms, I agree with your analysis. Something is corrupting
the data. If it isn't bad RAM then it would have to be something like a
faulty ROM resulting in a firmware bug which is overwriting memory that
Copy II+ is using, or some kind of data corruption through the disk
controller hardware (unlikely).

The built-in diagnostics might not have a good enough RAM test to detect
all types of fault.

The fault might not be the RAM itself. It could be a dry joint on either
the RAM or one of the other chips connected to the RAM. These would have
identical symptoms to some types of RAM fault.

> Maybe, I will need another software to test defective RAM. 

If you have AppleWorks GS, there is a reasonably good memory test
program included on its floppy disk. I haven't used it for ages so I
don't remember whether it is a ProDOS-8 or GS/OS application. I think it
runs in text mode, but that doesn't narrow down the OS.

It will test the RAM in the memory expansion card, but might have
trouble testing the built-in RAM, because part or most of it is used by
the operating system.

(If you don't have AppleWorks GS, perhaps someone else can send you a
copy of the memory test program, with slightly more detailed
instructions about using it.)

> If it determines errors in RAM. there should be a way to replace RAM chips
> on motherboard, but they are all soldered.  Any suggestions?

Desoldering RAM can be tricky because the IIgs is a multilayer circuit
board. It is best done by someone skillful to avoid damaging tracks.
Assuming the RAM is faulty, the best option would be to install a
sockets so the RAM itself doesn't need to be soldered back in again.

It may be easier to substitute another IIgs motherboard.

-- 
David Empson
dempson@actrix.gen.nz