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

From: Ethan Dicks <>
Date: Wed, 13 Jul 2016 16:36:26 -0400
Message-ID: <>
On Wed, Jul 13, 2016 at 2:50 PM, William Levak <> wrote:
> 320351 is from a 2001N or early 4000 with 9 inch screen.  Check the voltage
> regulators.  This machine uses 4116 dynamic RAM.  These early RAM were not
> reliable.  Can you swap them around and get a different result?

Also check the 7905 regulator for Vin and -5V on Vreg.

The screen RAM is a pair of 2114s, but if you see the random character
splash at power-on, the screen RAM is likely good (2114s are also
notoriously unreliable).

I haven't had to debug a DRAM failure, so I'm not even sure at what
point the ROM code will fail if zero page or the stack are broken.
I'd probably start looking through a ROM disassembly starting at the
reset vector and seeing where the firmware starts reading back and
depending on correct values in low memory, including the first RTS
after a JSR.

If this were my PET, I'd pull the 6502 and stick in my Fluke 9010A
w/6502 pod and start poking around to ensure ROM and RAM contents were
reasonable, but that's not a common tool that everyone has.  Without
such a tool or some other hardware-level debugging gear (logic probe,
etc), once (bad or lack of) DC power was eliminated as a cause, I'd
consider writing some custom firmware to go in the $F000 socket that
could see about using screen RAM as temp storage and have the code
sniff RAM and ROM and display the results on the screen.


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Received on 2016-07-13 21:00:03

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