Arrrrgh! (was Re: Static PET repair)

From: Ethan Dicks <>
Date: Fri, 2 Apr 2010 01:29:10 -0400
Message-ID: <>
I wrote:
> The board I got from you (Mike) produces $FF garbage in video RAM (not the
> expected random characters), and has the additional fault of the video
> timing pulses being "wrong" (the image is short and squeezed so much
> left-right that it wraps).  Other 9" PET boards in this 9" chassis
> display the right thing, so I'm looking for a "bad" 74107 or 7474 in
> the timing chain - i.e., some clock isn't getting divided down enough
> stages.

I found the problem... it took me a long time because there *wasn't* a
fault in the video or the timing circuit... someone (I'm guessing it
was the guy who tried "converting" the board) replaced the 8MHz
crystal with a 16MHz crystal!

So... with the right crystal in the board, video is the right
proportion, video memory tests fine (visually and with the Fluke
9010A), ROM reads fine (Fluke), but RAM fails (Fluke).  I'm currently
suspecting the '244 at H8.  Video RAM and ROM use D0-D8, but main RAM
uses BD0-BD8, buffered through H8 and H9.


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Received on 2010-04-02 06:00:04

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