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Char problem on TI99... some help for Repair ?


ti99iuc

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I had two TI-99s that I hadn't used for a long while that had the same problem when I started them up. Corrupt characters and then eventually the computer would hang. Luckily I had two F18As waiting to be installed so I thought I'd at least use one to practice the installation and lo and behold installing the F18A corrected the issue and I had a working TI. Seems it was the VDP chip in my case.

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It's more likely I think that one or more of the video RAM chips is faulty. It's displaying a picture OK, but the data is corrupted. If you remove the board you'll see a row of 8 off 4116 (or equivalent) RAM ICs. With everything connected back up again, you *might* find that one or more are running noticeably hotter or colder than the others - that might be a clue as to which needs replacing. Otherwise replace one, test, replace another if still faulty ... Replacements are fairly cheap and common on Ebay. To replace, I find it easiest to cut all the pins on a chip and throw it away, unsolder the remains of the pins and clean up the holes, then fit a socket.

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A guy from our austrian group has seen these video errors a few times (he fixes a lot of 8/16 bit systems), one of the 8 video memory chips could be faulty.

Open it and try to see if one of the memory chips is really hot compared to the others, this one needs replacement then:

http://www.8bit-home..._reparatur.html

http://www.8bit-homecomputermuseum.at/repair/ti_994a_2/ti994a_2_reparatur.html

 

Since the VDP is socketed you can further try removing the VDP chip and putting it back into the socket.

 

Of course you could try a F18A in there which bypasses the 16K VDP Ram all together because it is using it's own 16K as VDP.

Edited by kl99
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I once thought about adding a feature like "Simulate cartridge port wear-out" which should add random bits to the address and data bus lines. But I also remember the thrill in those days long ago, entering an Extended Basic program, hoping that the cursor that just disappeared on the screen will return, anxiously thinking about another quick save, although this may just trigger yet another crash. I was so glad to have the Geneve and no cartridge problems anymore. Maybe for this reason I never carried on with that thought.

 

But for your current problem: This seems to be a good indication that the RAM circuit for bit 0x04 is faulty. If you look at the schematics you see that the Video RAM is organized as 8 chips with 16K x 1 bit, i.e. each bit of a byte is stored in another chip.

 

Michael

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I have the same problem on a couple COLECO ADAMs.

When I load a game I get black vertical stripes where the VDP can't wrote to or read from memory.

That makes it a little more obvious than the garbled characters, which I also get.

Definitely RAM.

Edited by JamesD
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  • 2 weeks later...

Hi, i am the guy Klaus mentioned above (www.8bit-homecomputermuseum.at)...

 

The easy way to find most of the Videoram faults is to "piggypack" a working Ramchip ontop of the Ram's (one after the other) and see if the display changes...

Sometimes it is not easy to feel heat differences...

 

Ciao TOM:-)

 

Thanks for sharing an easy trouble-shooting tip!

 

-Dano

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