Re: PET-2001 showing just '?' on screen

From: Ethan Dicks <>
Date: Tue, 19 Jul 2016 13:43:21 -0400
Message-ID: <>
On Tue, Jul 19, 2016 at 1:09 PM, Ville Laustela
<> wrote:
> ... I did switch the power on and off and I did see the whole screen filled with random characters.


> Then it goes blank (just as it should)


> but then it goes to the blank screen with the single reversed ? on screen. It’s always on the same spot

Can you tell where on the screen it is?  Is it where the first "*" or
"#" in the banner goes?

>  and also I noticed that the random characters were identical every time. If this helps any.

The random characters are the power-on state of the video RAM (2114
SRAM chips in most PETs, 6550s in the oldest ones).  Identical random
chars are common/usual.

Based on what I recall the reset code doing, it clears the screen
early in the sequence of events, sets up zero page and other
variables, then initializes BASIC and prints the banner.  I would
suspect you have a bad ROM or RAM to cause that sequence to fail
before it can get the banner out.  There are other causes, but those
are common ones.  Any of the techniques mentioned in this thread that
clip on or plug into the 6502 socket and do not depend on RAM and ROM
working (Fluke 9010/A, CBM diagnostic clip, etc) can help pinpoint
what's really broken.


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Received on 2016-07-19 18:02:08

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