Re: Help...dead 8K PET

From: Larry Anderson (
Date: 2001-02-22 05:01:19

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 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..

But the diagnostic clips may provide one - there are a couple special
connectors to plug into the parallel port (connects some upper row
diagnostic pins to lower row parallel) and the keyboard connector (loops
pins to other pins) for some tests...  Will see if I can find my
diagnostic book...  I got all the stuff (diagnostic cassette,
connectors, booklet, shematics) but have never tried it (I'm not much of
a hardware person)...

> > 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.

How hot is awkwardly hot? on a couple of my PETs they get mighty hot...

> 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..

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