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Bad Video on TI-99/4A?

TMA-1

TMA-1

Experienced Member
Joined
May 3, 2013
Messages
67
Location
Ontario Canada
I recently picked up a TI-99/4A on Kijiji. (Shhhhh,... don't tell my wife I bought ANOTHER computer.) Back in the day I was a poor student and therefore a ZX81 man, but I was always jealous of the full keyboard and features of the better but more expensive TI. Now that I finally have one, I find it may have gone bad in the interim.

Turning it on (for the first time in years?), I get the picture below. It looks similar to what I expected, but garbled somehow. Alas I do not yet have direct experience to compare it with, but it doesn't look right at all. With a cartridge in place it seems to play a game, but I can't follow what is happening and my keystrokes are random guesses at best. That would indicate most of the computer is still functioning, and perhaps just the video section is bad somehow. I've tried it on two televisions, a modern flat and a vintage black & white. Both show the same strong yet jumbled output.

Does anyone recognize the distortion shown? Does it point to something fixable? Thanks in advance.

IMAG0001.jpg
 
Does it do this consistently, or does it intermittently operate normally?

It looks like either an issue with the video RAM or the VDP chip.

The VDP chip is in a socket, so a good place to start is to disassemble it and re-seat the VDP chip, while checking to see if there is any corrosion in the socket. A bad connection somewhere wouldn't be surprising at all.

If you had a second TI console you could swap out the VDP chip to rule out if a faulty VDP chip is the problem or not.

If things intermittently go back to normal, it could also be a bad solder connection around the VDP or RAM.

It looks like it could be a faulty RAM chip. Replacement 16k DRAM chips are not too hard to find, but those are soldered on to the board, making them difficult to replace.
 
Hi SomeGuy,

The problem is not intermittent. I have never seen the display "properly". The entire unit was stored in a plastic box and extremely well cared-for. I took it apart. There is no corrosion or visible signs of trauma; actually it looks like it just popped out of the assembly line.

I re-seated the only chip which has a socket, (and also a heat sink), TMS9918ANL, which is apparently the video processor. No joy. I see them available on eBay however for about $13.

The RAM seems to be 4116's. Oh the mixed emotions--I know these finicky flaky chips from trying to repair ZX81 add-on RAM packs, with mixed success. The good news is I am capable to unsolder and replace these chips, and I have replacements available. Quite the pain in the posterior though.

I'm inclined to think it's the VDP, reason being, I can see a basic ">" prompt, or run games from cartridges. I just can't tell what's going on because of the display. But the computer itself seems happy to run programs, and I don't think that would happen if a RAM chip had gone bad.

Any other opinions before I gamble 13 bucks on a VDP?
 
Your VDP is fine. That display is the classic sign of a bad TMS4116. Notice that many letters are correct: S and O are two that I noticed right away. The computer will run programs fine because the 4116s are VDP RAM, not CPU RAM. It just gives you a totally munged (modified until no good) display. You can actually use MESS to test to see which one is bad, IIRC.
 
Thanks Ksarul. I'll warm up the soldering iron. First thing is to get the dang RF shielding off. Darn thing is armored for a jousting match,...

[...edit...]

Well I don't know what I messed up, but something has gone amiss. I have extracted all 8 video RAM as carefully as I could, and socketed the board. Lost a couple of traces in the process, and jumped the connections with wire. Then of course I populated the sockets with new RAM. No fly. The screen is worse now than it was before, being mostly pink with only a vague shadow of features which are totally unreadable.

I have printed off a schematic of the connections and verified every one at the legs of the chips with my multimeter. All good. Dunno where to go from here, except that there's a Kijiji ad locally for another TI console, cartridges, and documents, but sans cables. This fits perfectly with what I have, and I'm waiting for the poster to respond to my email.

IMAG0001.jpgIMAG0002.jpgIMAG0002.jpg
 
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Couple of things:

- You say that you've verified every IC leg; but have you checked RAM is receiving the correct voltages?.
- Are you 100% sure that you've correctly rebuilt all the broken traces?. Double check them against the schematics...
- Look carefully that there're no solder spills or unexpected solder joints.
- Are you sure the new RAM is good?
 
The RAM is receiving voltages that look a little "off", but unfortunately I didn't measure them before I started, only afterwards when it didn't work. I don't have the information in front of me, but IIRC the -5 was in fact about -1.7, and the +12 was about +14. (The +5 was pretty well on.) I know these chips are finicky, but I've seen one with no -5v at all perform perfectly except for 1 bad bit. (In that particular case, I didn't even look at the voltages because I was sure it was a stuck bit in one of the chips. I was shocked out of my socks when I discovered it was a supply problem!) So the bottom line is, it's a definite maybe.

I have several of these chips in two categories: brand new, and proven good. (The proven good ones were laboriously substituted into a socketed ZX81 RAM pack and verified.) All the combinations I've tried produce the same results, so I'm as sure as I can be of the RAM itself.

As to the circuitry, I would say I was 100% sure that there are all the correct linkages and no shorts, except for the stark reality that that assertion is demonstratively false. :-/

The only other thing I can think of is that an ill-timed power-up with one missing trace somehow destroyed the VDP. I'm picking up another console tonight, and if it is functioning well I will test this rather desperate theory with substitution, as well as quantify the RAM chip voltages present in the 2nd unit. If that doesn't change anything I will just have to "rinse and repeat" my probing to find the fault.

Thanks for the help.

[...edit...]

The bad news is the 2nd console doesn't work either. Well dayum. The good news is I found a short in the 1st board, and that one is working now!

I had thought to make the 2nd purchase my go-to machine, but the newly acquired TI sends no video at all that the television will display. It does send a dog-awful audio tone, summoning the wife who wants to know what I'm doing--"no never mind just cut it out." :) The video RAM voltages on this board are right on. So I swapped the power supply boards--this had no effect on the behavior of either machine or the voltages observed, so I assume that means both power supplies are fine. Tried switching the VDPs too; again no effect on either machine.

So I went to work again with the magnifying glass on the 1st board, and found a short! Bad TMA-1, 50 lashes with a hot wire,... Low and behold all is now well. Did I say "all"? The -5V chip voltage is still suspect, at -2.3, but apparently nominal enough to function. Disquieting though.

While the unit was still apart, and having socketed the RAM, it made a fine trial-and-error chip tester, so I went thru the original set of 4116's and binned the one that was zonked. (Yes, I saved them, only breaking one leg in the extraction. Luckily that turned out to be the bum one anyway.)

As for the 2nd console, it is rather filthy and far less well cared-for, so I'm not sure how much effort I will put into fixing it, unless someone has a good idea what to look for. I suppose I should start by hooking it up to my pedestrian but undiscriminating black & white to see what video, if any, is actually being sent. A geeks work is never done,....

05 Good Screen.jpg06 All Done.jpg
 
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Good to hear that you at least get a console working!. About the second console, if its display is pitch black and not analog noise can be a lot of things ,ranging from fried RAM to toasted CPU. Hard to tell; but if you have time to spent on it, you can always check the voltages in the board for a start.
 
Thanks jltursan. I'm just happy I now have something to play with.

The second console is pitch dark on the modern TV, which (like most now) suppresses any input it deems unfit for the human eye. (Snooty so-and-so,...) On my vintage old black & white I use for the Sinclair, there is an all-grey image being sent. I.e. it is not just static. And there's that steady audio tone, which is perhaps a two voice tone. I'd undertake to unsolder and test at least one of the video RAM chips again, were it not for the tone. Is there any chance that's an error code of some sort? [edit--no such luck; I see on the net this condition is referred to as "a black screamer", and can be caused by a number of failures. Great.]

The video RAM voltages are bang on. I didn't check the other chips, but you're right--that's an excellent place to start. And again, I do know that the problem is on the main board, not the power supply board, since swapping power supplies had no effect on either machine. When I take a break from saving the Earth from blocky beeping alien invaders, (who with their advanced intelligence line up conveniently to be shot), I will snoop around and see what I can find.
 
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I think I have the original troubleshooting guide for the console as used by the TI engineers back in the day. I do have their guides for troubleshooting most of the PEB cards and some of the sidecar peripherals. I should probably scan those in and put them up online with the other HTF documents I've put up on WHT over the years. One of those to-do items I haven't gotten around to yet. . .too many things to do and too little time to devote to them.
 
jltursan said:
That would be great, there's not too much info about repairing the old TI :-(
Click to expand...

Yeah, what he said. :) That would be most welcome if you could manage to do it Ksarul.

I've been searching for information and resources about this nifty little machine, but have not found as much out there as I was expecting. I know TI took a bit of a bath in the marketplace, but the computer itself is cool, and its fans loyal. I would hate to see any of them going to landfill at this point.

Uhm,.. the computers that is, not,.. you know what I meant.
 
I'm guessing you guys have all seen the doc below? I'll post a link just in case...
ftp://whtech.com/articles/limanews/CONSOLE.TXT

"Black screamer"..lol. I like that. Yes, the first one I got was one of those. Could have been anything. Being short on time and not being as masochistic as I used to be I solved the problem simply by getting a replacement board.

Tez
 
Epilogue:

I put the 2nd unit back on the bench and had a look.

Although that document is very good, tezza's accompanying comment that it "could have been anything" is, I think, even more spot on. On a whim I replaced the two MCM6810P memory chips, but it has made no difference. It's still a "black screamer". Diagnostics say it could be CPU, but I'm not going to go taking any more pot shots. I "accidentally" won an eBay auction on another machine for a little over $10 plus reasonable shipping, which is far cheaper in the long run than tearing my hair out over this motherboard. So I will have a spare power supply, keyboard, VDP, etc. and hope to have two working units. Enough is enough.

Thanks again guys for the help in finding and fixing my original unit. Maybe someone else with the same distorted screen will find this thread and see a resemblance in the photo. Meanwhile, I'm still struggling with sound waves and trying to acoustically load up a program "from the net". (Thread here.)

Cheers,
Ian
 
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