Discussion:
My C-128 is throwing random chars
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PaulM
2020-06-28 19:34:03 UTC
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Recently my C-128 has started acting very oddly, it throws random characters as if I were typing them but I'm not. Sometimes it acts like I press keys even when my fingers aren't on the keyboard, and sometimes it will add random keys among the actual keys I am typing.

OK, this is old hardware, and things happen... but it was flawless until now. What would be a fix for this -- does this sound like something "recapping" would address? I am pretty clueless when it comes to hardware issues.

Is there a diagnostic image (in D64 or D71 format for an SD2IEC) out there that might help?

Thanks.
Felix Palmen
2020-06-28 19:59:15 UTC
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Post by PaulM
Recently my C-128 has started acting very oddly, it throws random
characters as if I were typing them but I'm not. Sometimes it acts
like I press keys even when my fingers aren't on the keyboard, and
sometimes it will add random keys among the actual keys I am typing.
I'm a software developer with only "basic" hardware knowledge, and,
although I own a C128, I know the C64 better, so, take the following in
this context…

The C128 has most of its keyboard matrix wired to a CIA (MOS 6526) I/O
chip called CIA #1. This chip also has all its data lines directly wired
to the control ports #1 and #2 (for joystick and/or mouse), therefore it
is directly exposed to any electrostatics on these ports. It's known to
break quite easily because of that. From what you describe, a broken CIA
#1 is kind of likely. The board has two identical chips, CIA #2 controls
the serial bus for peripherals like floppy drive, printer, etc -- IIRC
on the C128 only in C64 compatibility mode. So, a common way to be sure
is to exchange the two chips. If the result is keyboard/joystick/mouse
working perfectly while peripherals start to have problems, you know
your CIA chip is broken.

The problem is that many boards don't have these MOS 6526 chips
socketed, and unsoldering them without destroying them isn't a simple
task…
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Dipl.-Inform. Felix Palmen <***@palmen-it.de> ,.//..........
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