Re: option ROM into a 3032

From: Ethan Dicks <>
Date: Wed, 27 Sep 2017 14:41:46 -0400
Message-ID: <>
On Wed, Sep 27, 2017 at 12:13 PM, Francesco Messineo
<> wrote:
> Hi Ethan,
> On Wed, Sep 27, 2017 at 4:50 PM, Ethan Dicks <> wrote:
>> On Wed, Sep 27, 2017 at 10:24 AM, Francesco Messineo
>> <> wrote:
>>> Hi all,
>>> I'm repairing a CBM 3032....

> well, the changes are really small, actually part of the same
> character changes (modifying the
> character itself), not the character.

Ah... the pixels in a character cell are changing?  That's really odd.
The path is the timing circuit reads the pair of 2114 RAMs that are
gated over to the CHARGEN ROM then the ROM's data outputs are hooked
to a shift register for the specific dot that's then sent up the
6-wire cable (video, h sync, v-sync and 3 grounds).

If the character cells aren't stable, could be the shift register,
could be anything between the shift register and the ROM, or could be
the inputs to the ROM are changing during a character time - several
items have the potential to interfere but I can't think of an obvious
first thing to check.

If you can at some point determine if the machine is running BASIC but
not displaying RAM properly, there are probably some test values you
could POKE into screen RAM to help figure out what the video circuit
is doing.  A logic probe or an oscilloscope might be needed here.
I've fixed a couple of video problems and they did take me a while.

> the behaviour is the same even with no other ROMs but the petester.bin
> image in UD9, it
> just moves parts of some characters but it doesn't of course respond
> to keypresses.

If you have a petester ROM installed, then BASIC and the KERNEL ROMs
aren't a factor, but address and data buffers could be, or address
select logic.


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Received on 2017-09-27 19:00:03

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