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From: ingoldsb@ctycal.UUCP (Terry Ingoldsby)
Newsgroups: comp.sys.m6809
Subject: Re: dead coco
Summary: Check CPU, SAM
Keywords: ampersands and such
Message-ID: <507@ctycal.UUCP>
Date: 8 Nov 90 19:33:04 GMT
References: <1990Nov5.215152.28456@cbnewse.att.com>
Distribution: na
Organization: The City of Calgary, Ab
Lines: 29

In article <1990Nov5.215152.28456@cbnewse.att.com>, gmark@cbnewse.att.com (gilbert.m.stewart) writes:
> My coco seems to have died.  At powerup, ampersands and such appear on
> the screen, whose black border and green background look normal.  I
> guess it's the ROM.  Anybody know where the cheapest place to get
> a coco I ROM nowadays?  Since the background looks okay, can I safely
> say it's the extended BASIC ROM rather than the "stock" original ROM?

First of all, which CoCo are you referring to?  I am guessing that you
mean Coco I or CoCo II.  In any case, it is unlikely that the ROM would
go (unless you have been tinkering around inside and managed to short
something).  Much more likely is the CPU or (if it is a CoCo I/II) the
SAM chip.  Your VDG is quite capable of producing video output without
CPU, ROM, RAM or SAM in the system.  It will come up in some default
mode, and as long as some power and timing conditions are met will
display the data it sees on the data bus while the address lines are
clocked through certain values.  My guess is (I don't have schematic
or specs handy) is that some value is locked on the data bus (perhaps
NULL?  I think that gives inverted @ ??) and that your VDG is just
clocking them out to the screen.

What usually kills CoCos is unplugging a ROM cartridge with the
power on.  That can toast your CPU or SAM chip.  Hope this helps.



-- 
  Terry Ingoldsby                ctycal!ingoldsb%cpsc.ucalgary.ca
  Land Information Services                 or
  The City of Calgary       ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb