Re: pet 2001 repair

Den Sat, 6 Jan 2018 18:05:54 +0100 skrev Francesco Messineo
<francesco.messineo@gmail.com>:
> Edit: screen picture is not attached, here's a link:
> 
> https://photos.app.goo.gl/zy6vBiuoYziDXa6J3
> 
> 
> 
> Hi all,
> I'm not progressing much on this 2001. I'm now able to test the 6550s
> out of the PET and I've found two surely bad (also confirmed using
> them as video RAMs).
> Most of the times it just boots directly to an illegal quantity error,
> but sometimes displays the basic prompt without the bytes free amount
> as you see in this picture.
> I've suspected a problem reading the video ram (writing just works,
> otherwise it wouldn't clear the screen and display correct
> characters), but running the system with a nop generator showed no
> obvious problem on the bus transceivers or address decoders and
> anything else I could test.
> Screen scrolling looks a bit too "jumpy" compared to what I'm used to
> see on 3032, but I really don't see any problem in the video ram to
> data bus glue circuits.

Does it scroll like two lines at a time?

Not sure how a 40-column PET does, but if it is anything like a VIC-20
or C64 then it has logical and physical lines. If it believes that the
physical lines are linked together then it scrolls more than one
physical line at a time.

Illegal quantity, no shown amount of free memory and strange scroll
would lead me to check the RAM. Not sure which adresses the BASIC
floating point accumulator is at or where a PET stores the
logical/physical line links, but as your machine atleast starts and
tries to run basic it probably has working ram for zero page and the
stack (i.e. adress 0-511). I'd check the ram for adress 512-1023.

If all your ram is socketed I'd just try to move around the RAM chips
and see what changes.

(I haven't read anything about the old 65xx RAM chips as my oldest PET
uses 2114. Not sure what size they really are, if they are 1k or more
each then this probably doesen't apply).



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