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View Full Version : Need help fixing a Ti-99/4A



LancePrice
May 22nd, 2017, 06:33 AM
So I found this boxed Ti-99/4a computer in an attic. Was in fantastic condition, probably had the goodies it came with originally.
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I plugged it into a television and turned it on and, well, it almost works. It's responsive, however the graphics are pretty jumbled, text is unreadable gibberish.
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I assumed this was due to a rom chip storing the necessary data going bad or something, I opened it up and looked for any visible damage. One chip in particular was absolutely covered in a white squishy goop, presumably an adhesive that had gone off. I removed the chip and cleaned off the adhesive with a soft bristle toothbrush and a microfiber lens cleaning cloth.
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(The chip I cleaned highlighted in red.)
No idea if this is the chip causing the problem in the first place, either way cleaning it did not fix the jumbled graphics.
Anyone know what could be causing this? If I'm right about it being a bad rom chip, does anyone know which one it is? Was it the one I cleaned? Where might I find a replacement?

barryp
May 23rd, 2017, 05:00 PM
The "white squishy goop" was on the VDP Video Display Processor, TMS9918A and is there to facilitate heat transfer to the heat sink and is needed. Could you go beyond the title screen? Just press any key.

SomeGuy
May 23rd, 2017, 06:36 PM
It is probably a bad RAM chip or two (of the eight on the left). Someone might be able to suggest which one(s) based on the patterns. The way the TI-99/4a works, all character patterns are copied in the the video RAM at startup. So it looks like two columns are missing from each character, bars down the screen, and some of the wrong letters appear.

At least there is not too much guesswork there. If any of the other components die you often only get a black screen, and no idea which is bad.

You will want to put some fresh thermal grease on the VDP before you reassemble everything, but it won't hurt to run it with the heatsink/shield off.

Ksarul
May 26th, 2017, 06:28 AM
That is the classic symptom for a bad RAM chip. You have two options at this point. First would be to replace the RAM (assuming you have good soldering skills, as it is really easy to destroy an otherwise working board). The other option is a bit more expensive, but it also lets you connect your TI to a VGA monitor. It is called an F18A, and replaces the VDP chip that you removed the white heat sink compound from. You can get them here (http://codehackcreate.com/store#!/F18A-V1-7-Video-Board/p/14022176/category=0) (they are listed as out of stock, but if you contact Matthew, he will usually have some that aren't on the website yet). The VDP RAM is no longer needed with an F18A in place.

gertk
May 27th, 2017, 09:55 AM
I would suggest to replace all ram chips by sockets first, then you can decide if you want to put in 4116 type DRAM chips or modify the powerlines and use single supply 4164 (or even 41256) chips. For the latter two you need to cut the -5 and the +12 Volt supply lines.

I did a replacement with 41256 chips and it works well. I removed the original power rails so the old ram chips could be removed easier.

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Note this will not increase the total amount of memory..