Re: PET 2001 fix Part 3 - RAM/ROM board etc.

----- Original Message ----- 
From: "Ethan Dicks" <ethan.dicks@gmail.com>
To: <cbm-hackers@musoftware.de>
Sent: Thursday, May 12, 2011 1:54 PM
Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc.


> On Thu, May 12, 2011 at 1:21 PM, MikeS <dm561@torfree.net> wrote:
>> Have you tried replacing the video RAM?
>
> Besides the fact that I did test (and mark) all the 6550s, bad video
> RAM is not likely to cause pixel-level defects in the video... that
> tends to be a fault around the chargen and attached shift register or
> with the timing chain of flip-flops and such.
>
> -ethan
-------

AFAIK the old 2001 had the video RAM connected directly to the CG ROM 
(unlike the newer ones which had a latch in between), so if one or more bits 
of the video RAM were unstable (or you had an intermittent socket) which 
caused the data to change part way through a character line, wouldn't you 
then get corrupted characters as the CG switched back and forth?

I just thought it'd be an easy and worthwhile thing to try, especially since
someone on the VCF PET forum just fixed a 'flickering snow' problem by
replacing one of the video RAMs.

m





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