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

From: MikeS <>
Date: Thu, 12 May 2011 15:28:21 -0400
Message-ID: <F3F4BC589AED4D1FA4329CAC098CA7FF@vl420mt>
You're right that it would probably be random, but it looks like it may be
power related since it clears up as the voltage drops, so maybe there's a
recurring spike that affects a weak bit as certain addresses or states come
up, whatever. Wouldn't be the first time that something worked that
shouldn't have and I thought it'd be a really easy way to rule out the RAM.

My first approach to a video problem is usually to pull the CG and look for
a clean blank screen, and then replace it, pull the video RAMs and look for
a clean checkerboard; often that points me in the right direction.

Looks like the problem is mostly in the fifth character column; wonder if
that's where the horizontal line was originally? That'd tend to point the 
finger at the '93 counter (A1) or the '165 SR (B2).


----- Original Message ----- 
From: "Ethan Dicks" <>
To: <>
Sent: Thursday, May 12, 2011 2:50 PM
Subject: Re: PET 2001 fix Part 3 - RAM/ROM board etc.

> On Thu, May 12, 2011 at 2:35 PM, MikeS <> wrote:
>> 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?
> You could, but in order for that to be stable, it would need to be
> synchronized with the pixel clock.  If the symptom was random
> pixels/no visible character shapes, I'd be willing to suspect the
> SRAMs.
>> 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.
> I can see how snow could be an SRAM problem because it's not tied to
> the same pixels changing in the same characters in the same way frame
> after frame - that is one way I'd expect random addressing of the CG
> ROM to manifest itself.
> -ethan
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Received on 2011-05-12 20:00:23

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