Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!rpi!ego
From: ego@itsgw.rpi.edu (Erik G Olson)
Newsgroups: comp.sys.ti
Subject: Re: Sick TI99/4A - Weird video
Message-ID: <{{R^~S@rpi.edu>
Date: 30 Dec 90 00:07:49 GMT
References: <90363.005816GERLACHJ@QUCDN.QueensU.CA>
Organization: Rensselaer Polytechnic Institute, Troy NY
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Nntp-Posting-Host: jec421.its.rpi.edu

It's easy to see that all of the incorrect characters and colors
exhibited by your "sick ti" involve dropping the last bit of each byte.
Odd valued characters become even valued: I becomes H, S becomes R, etc.

This would suggest that one of your ram chips is bad. Each ram chip
is 16k by 1 bit, so that a byte (8 bits) is spread out over all 8
chips. If the lower-end chip went bad, you'd lose the capability to
store that lowest-order bit. 

The same thing would happen to the color tables. Cyan (value 07) would
become 06 (dark red.) 

The cure for this is to replace the unsocketed ram chip that is causing 
you trouble. This is far from a trivial job-- you have to take off
all the solder, pull out the chip, get a new one, etc. You would
probably be better off buying another console at a garage sale.

If you were to replace the faulty TMS4116-15 ram chip, you would need 
to purchase a 4116 (or several) which go for dirt cheap these days, say, 
around 49 cents. Just about any brand will do, but it has to have "4116"
in the name (there are a FEW equivalents, but I don't recall what
they are.) There are different speeds of 4116's, too, like the 4116-20.
You need to get a -15. (I think that's the fastest one available.)

If you do replace a ram chip, you're better off putting a plastic socket
in the formerly unsocketed place, just in case.

Oh yeah-- which of the eight chips would be the culprit? I don't
have the complete schematics, but I did some poking around to identify
the chip on the right end of the row of 4116's as the one 
holding the least significant bits, the ones that seem to be the cause
of your trouble. That's the one at the very edge of the motherboard.

Good luck!

-- 
=======================+=========================================
Erik G Olson              "There was virtue in the world before 
                            there was orthodoxy in it."
ego@pawl.rpi.edu            --The Independent Whig