Re: Help...dead 8K PET

From: Martijn van Buul (
Date: 2001-02-21 12:10:57

Chandra Bajpai wrote:
> Can you help me with this problem?
> What I have: 2001 PET 8K...Board has 2114 RAMS + 6540 ROMs.  The ROM that
> controls F800-FFFF had gone bad and someone replaced it with a 2708 EPROM on
> 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 characters.

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 the
> RED LED on the motherboard does not light up. 

Hmm. I just checked the schematics, and said LED isn't supposed to be lit. 
I have no clue what it's doing there, but unless I misinterpret things, it
will light if row 14 of the keyboard is selected. AFAIK, there *is* no
row 14..

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

It fetches a start address from FFFE and FFFF indeed. The EPROM replacement
is a possible cause indeed. However, there might be a more 'low-level'
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 worked
perfectly before I took it home. When I arrived home, it did about exactly
the same as your PET seems to be doing. 

Check if there are chips which get awkwardly hot after powerup.

I've Cc'ed this mail to the cbm-hackers mailinglist; there is a couple
people subscribed to that list who know a great deal more about this
machine than I do.

> potentially I could get a scope or radioshack logic problem to check other
> things.

Well, if the supply is within tolerance, you could check the processor and
see if it's actually doing something..

    Martijn van Buul - -
	 Geek code: G--  - Visit OuterSpace: 3333
   Kees J. Bot: The sum of CPU power and user brain power is a constant.
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