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). m ----- Original Message ----- From: "Ethan Dicks" <email@example.com> To: <firstname.lastname@example.org> 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 <email@example.com> 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 > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2011-05-12 20:00:23
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