Jim Gerlach <GERLACHJ@QUCDN.QueensU.CA> (12/29/90)
Years ago - when TI's were going for a song - I bought a second machine as a backup. I used it to replace my existing TI, which was about a year old, and packed the other TI away. Today I unpacked that machine and plugged it in. The video (hooked up to color TV with RF modulator) was verrrry strange. All character were in the right places but some were "off" by one character. e.g. "TDX@R HNRTRTLDNTR" instead of "TEXAS INSTRUMENTS" The background color was brown instead of blue and the screen was flickering. I proceeded to open up the machine to see if anything obvious was wrong. I wiggled all the connections, re-seated the socketed IC's and generally mucked about - no joy. Any and all suggestions would be welcomed. If it isn't obvious, I know nothing about these computers. I would like to get this "spare" computer running so I could give it to a friend who has a couple of kids interested in computers. Any chance of picking up a replacement "motherboard" for example? Thanx in advance - Jim Jim Gerlach - Cancer Research Laboratories James.H.Gerlach@QueensU.CA Queen's University GERLACHJ@QUCDN.QueensU.CA (NetNorth/BITNET) Room 358, Botterell Hall tel (613) 545-6446 Kingston, Ontario fax (613) 544-9708 CANADA K7L 3N6
ego@itsgw.rpi.edu (Erik G Olson) (12/30/90)
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!
--
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Erik G Olson "There was virtue in the world before
there was orthodoxy in it."
ego@pawl.rpi.edu --The Independent Whig