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

From: Baltissen, GJPAA (Ruud) <>
Date: Thu, 19 May 2011 15:34:01 +0200
Message-ID: <>
Hallo Philip,

> Are there any test I can do with a scope or logic probe
> that could help me narrow down the bad ic?

To be honest, in this case I have no idea. You already prooved that the
video-RAM/CPU connection is OK. Which means that the trouble should be
found in the circuit generating the image and especially the counter
part. There are two weird things happening:
- only the last three bytes of a row are copied to the next line
- the counter "forgets" to increase after the first column

<some hours later>

A horizontal line is made of a border, the visual part and a border
again. During the visual a counter reads out 40 memory places. The data
is sent to a ROM which outputs the bits that have to be displayed. The
only counters directly related with the video RAM are the two 74177s.
and that's why I suspected them.
But because there are borders, the 177s don't count continiously. So
there must be a circuit that starts and stops the counting. And this
circuit must count to 40 (20? 10?) in some way. What maybe looks like it
is the circuit mid-left around parts of B5, C5, C6 and C7. 

Two things go through my mind:
- The 74100 latches the address to be read by the video circuit. What if
the circuit stops the latching too early (but the rest goes on)? At the
next line the circuit starts with the too early latched address.
- What if the circuit starts one bit too late with clocking the 74107/D5
and 74177 counters?

My problem: I keep on encountering confusing things, like the
combination at the top of the schematic of the vertical synv and the
video on signal. I only can hope that the above ideas trigger someone
elses mind to find a possible reason for the problem.

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Received on 2011-05-19 14:00:02

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