RE: Help...dead 8K PET

From: Chandra Bajpai (
Date: 2001-02-22 04:20:41

William, I'm beginning to suspect the EPROM as a problem.  I'd like to rule
the 6502 first.  I have a Radio Shack hand held logic problem - will that
I assume I check on the 6502:
- RESET Line
- READY (What does this do?)

I potentially could get a oscilloscope...will that be any better? Logic

I assume there is nothing I check on the 2708 EPROM.  Are 2708 still
I wonder if the 2708 is a drop in replacement for the 6540.
I don't quite understand what you mean by replacing more than one 2K ROM.
Are you saying that the ROMS in H7 (F800-FFF) and H4 (F000-F7FF) should be
at the same time...BTW-I assume this machined worked with this ROM in place.

-Chandra you don't get confused...I'm fortunate enought to own 2 8K Chiclet
Unfortunately neither of them work 100%.  This particular machine is
dead and the other has a weird lockup problem.

-----Original Message-----
[]On Behalf Of William Levak
Sent: Wednesday, February 21, 2001 9:42 PM
Subject: Re: Help...dead 8K PET

On Wed, 21 Feb 2001, Martijn van Buul wrote:

> Chandra Bajpai wrote:
> >
> > Can you help me with this problem?
> >
> > What I have: 2001 PET 8K...Board has 2114 RAMS + 6540 ROMs.  The ROM
> > controls F800-FFFF had gone bad and someone replaced it with a 2708
> > a little daughter card that fits on top of the ROM slot.  I was told the
> > computer worked great.  When I turn it on all I get is random
> Which means that the videoRAM isn't initialized yet. The video logic is
> probably working, though.
> > So far I tried to reseat all the chips with no luck.  Only other clue is
> > RED LED on the motherboard does not light up.

The red LED is for a built-in diagnostic.  There is a connector that goes
on the user port and one that goes on the keyboard connector.  With these
connectors in place, the computer is turned on and it goes through a
diagnostic with the output displayed on the screen.  When the diagnostic
finishes correctly, the red LED turns on.  The connectors just jumper
contacts on the user port and keyboard connector.  The connectors are easy
to make, but I do not have information at hand.  I will look at my
connectors and give you the results in the next few days.

> > I don't know how to check if the 6502 is actually executing
code...doesn't it
> > usually start at FFFF?

You need a logic probe.  This is an inexpensive device that allows you to
check the logic state of a single line at a time.  You should check the
reset on the CPU and interface chips and the select line on the ROMs.

> It fetches a start address from FFFE and FFFF indeed. The EPROM
> is a possible cause indeed. However, there might be a more 'low-level'

Replacing one 2K ROM is not very reliable.  Both ROMs in a 4K block are
addressed identically.  It is the ROM that selects whether it is the upper
or lower 2K.  When replacing ROMs, it is best to replace both ROMs in a 4K
block with one 4K EPROM which can be located in either socket for that 4K

> problem. I hope you have a multimeter of some sort. Check the 5v supply
> on a few chips. I don't really know the 2001N, but later CBMs had seperate
> supplies for the logic and for the video circuitry. If the logic one
> is shot, you'd get results like this. Happened to me one time; a 4032
> perfectly before I took it home. When I arrived home, it did about exactly
> the same as your PET seems to be doing.

The fact that you get random characters means that the video board and the
digital display logic are working.  If either of those were bad, the
screen would be blank.  It sounds like either the processor or the ROMs at
$F000-FFFF are bad.

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