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

From: Gerrit Heitsch <>
Date: Thu, 14 Jul 2016 08:55:37 +0200
Message-ID: <>
On 07/13/2016 10:36 PM, Ethan Dicks wrote:
> 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 your PET has 32 KB, meaning 2 RAM banks, you can switch those by 
desoldering one side of 2 resistors and crossing them. If only one RAM 
is bad, that should make it work again and give you a working system 
which you can use to find the broken RAM. I don't have a PET or the 
schematics though, so I can't tell you which resistors. Likely the ones 
the /CAS signals are routed through.


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Received on 2016-07-14 07:00:08

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