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

From: André Fachat <>
Date: Thu, 19 May 2011 20:55:02 +0200
Message-ID: <>
> So the summary is: look for a spike in the DIS.ON signal. Maybe even try
> to suppress it (temporarly) with a small capacitor to ground to see if the
> problem goes away.

Of course there may be different causes. 

A spike on DIS.ON could even haven been fed back from the video circuit through the E9 NAND which drives the TV VIDEO DRIVE signal.

Another possibility would be that there is one clock cycle missing in the counter for the addresses. This signal is produces by C8 in the master video timing, almost directly derived from the 8MHz master clock. The clock signal is fed back to C8 via D8 NAND with the 8MHz signal to produce a 125ns pulse only (and not a 500/500ns signal). It seems unlikely that something goes wrong there as no horizontal line signal is involved and the issues seems to be at the start of the line only.


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Received on 2011-05-19 19:00:21

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