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lgallair

Troubleshooting TI 99/4A QI video RAM

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A TI 99/4A QI computer recently came into my possession but sadly it has video RAM issues.  According to this website, the issue is bit 5 is stuck at 0 because that screen shot is exactly the same screen I get.  My problem is I do not know which chip to replace.  The diagram shows the chip locations for a non-QI model.  Does anyone know which 4116 chip represents the bit 5 circuit on a QI board layout?

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11 minutes ago, lgallair said:

A TI 99/4A QI computer recently came into my possession but sadly it has video RAM issues.  According to this website, the issue is bit 5 is stuck at 0 because that screen shot is exactly the same screen I get.  My problem is I do not know which chip to replace.  The diagram shows the chip locations for a non-QI model.  Does anyone know which 4116 chip represents the bit 5 circuit on a QI board layout?

From the same page (below the examples) shows which bit is which chip. to test the theory try piggybacking a good chip on top of the old one 

 

Vram.jpghttps://www.ninerpedia.org/wiki/File:Vram.jpg

The image shows the bit numbers on the circuits. Note that Texas Instruments uses a big-endian bit order, i.e. bit 0 is the most significant bit (27), and bit 7 is the least significant bit (20). Accordingly, if your failure symptom is Mask 11011111, this means bit 2 is stuck at 0.

 

 

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Yeah I noticed but shouldn't it be the same bits just in a vertical line? Or do you think ti rekajiggered the ram locations too

Sent from my LM-V600 using Tapatalk

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I suppose you could meter the datalines from brain to RAM for continuity on bit 2. But that might require chip pinouts and/or schematic.

Edited by Ed in SoDak
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54 minutes ago, arcadeshopper said:

Yeah I noticed but shouldn't it be the same bits just in a vertical line? Or do you think ti rekajiggered the ram locations too
 

The 9918 has been rotated 180 degrees on the QI board, so anything might have happened ...

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Yes, changed! Now (more correctly) in respective order.:grin:

 

D0=U9 ram IC located closest to the video port.

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D7=U16 ram IC located closest to keyboard connector.

 

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Hokay, for what it's worth! Here's a clip of the 99/4A schematic and board layout, with RD line 2 called out. It goes from pin 30 of U100 (TMS9918) to pin 14 of U106 (4116 RAM). Seems to agree with the pic blaming the 5th 4116 from the left as the faulty chip.

 

So, just trace out pin 30 on the QI's 9918 to whichever 4116 it leads to. Stick one DMM probe to 9918 pin 30 and check each 4116's pin 14 in turn. Only one chip should have continuity. I leave it to you to figure out which pins those are. Aw, heck, I looked that info up too. :) I don't have a QI schem or layout, which would have made it all much easier!

TI99-4A RAM.jpg

TI99_4A RAM layout.jpg

TI99-4A 9918 pinout.jpg

TI99-4A 4116 pinout.jpg

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1 hour ago, HOME AUTOMATION said:

Yes, changed! Now (more correctly) in respective order.:grin:

 

D0=U9 ram IC located closest to the video port.

.

.

.

.

.

.

D7=U16 ram IC located closest to keyboard connector.

 

U11 for the win! All we need now is the board pic called out like on Ninerpedia.

Edit: Or is that too obvious? :dunce:

Edited by Ed in SoDak

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40 minutes ago, Ed in SoDak said:

U11 for the win! All we need now is the board pic called out like on Ninerpedia.

Edit: Or is that too obvious?

Well, U asked for it! But my camera Scks8). Don't have a board to photograph either.

 

DIAGRAMREDLINED.thumb.JPG.1c915839ddf05419476f89a1daa46d73.JPG 

PLACEMENTREDLINED.thumb.JPG.92aa1c8b1559ce868b61e315d110ab98.JPG

:grin:

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2 hours ago, Ed in SoDak said:

Thanks Homey!

Hmm, Your TI-99/4A parts placement diagram differs from the one in my book somewhat. Particularly around the VDP XTAL.

 

Here's another copy of the book I'm using...

 

https://archive.org/details/TI994AConsoleAndPeripheralExpansionSystemTechnicalData/page/n17/mode/2up


I didn't know we were HOMEIES!:lust: ...much Love.;)

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If someone could do a good photograph of a QI board, we can add that to Ninerpedia with the appropriate chip numbering.

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3 hours ago, mizapf said:

If someone could do a good photograph of a QI board, we can add that to Ninerpedia with the appropriate chip numbering.

I have a few more days till my Vaca is over, if no one had posted one by then, I have a Q1 already out of the case (non working), I can take a photo or two then. Monday or Tuesday.

Edited by RickyDean
spelling

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2 hours ago, HOME AUTOMATION said:

This post contains a decent pic...

 

 

..not sure about it's terms though!

Looks as good as any I'd take, should be sufficient.

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Thanks for all the help guys.  I recently upgraded the RAM in my Joust cocktail with 4146 so I have a bunch of 4116 RAM's to try.  Was going to piggy back onto the offending RAM chip when all of a sudden, the power board went "poof" before I could even try.  After the little tiny puff of smoke dissipated, I could not identify what exactly blew.  I'm guessing an electrolytic cap.  There is a small burn mark on the white wire which goes to the two-pin power connector.  So I'm off to Digi-Key for some caps and am going to re-cap all 10 or 11 caps on the power board.

 

Now I have at least two problems with the TI-99.  Sheesh!

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