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Video Problems - Repair Questions.

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Posted (edited)

Hello....working on my TI99 and found this thread. Initially had "fixed" it using the MESS images to change a video RAM which at first seemed to work, however...when I turn it on, it boots fine/beep fine and goes to a normal appearing startup screen but then within a few seconds the letters, then boarders and graphics start to garble (color and character changes) not matching any of the MESS images. If I'm quick, I can get to basic and it can see my FinalGrom99.

 

Questions :

 

So to be clear..despite this article (https://www.classic-computers.org.nz/blog/2014-02-22-ti99_4a-reconstituting.htmTI-99/4A number 2), the rust square colored component is NOT capacitor 101 (it IS a inductor/choke) and  C101 is yellow resistor looking component with Brown, Red, Black, Gold, Black stripes  labeled in to above post?

 

Should I replace C101 with the issue I'm describing above? Can I use a 12 pf disc capacitor (one of the little brown pill type capacitors, about 4 mm across) to do this?

 

By the time I found the above article, (which does describe my symptoms)  , I had already started socketing and replacing all of the video RAM....so dead in the water waiting for pieces for now.

 

Thank you for any advice. 

Edited by WicoKid

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From what you describe, I'm thinking ...if its heat related, a chip has actually separated from its carrier. Maybe the 9918A or perhaps a voltage regulator on the P/S board. Yet, it could still be DRAM.

 

Somehow... I doubt c101 is at issue.:twisted:

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I would suspect an overheating chip here. Did you replace the heat sink compound on the VDP when you put the console back together the first time? That chip runs a bit hot and really needs the heat sink or it will give symptoms similar to your problem.

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Posted (edited)

I cleaned and replaced the compound.  If a I can salvage a sn74ls156n logic chip, I'll know if its the DRAM or not later today. I will admit I clipped of the logic chip thinking it was DRAM too. 

 

Really hope its not the 9918A. I had just gotten the 32k ram expansion built and played about three games before the original DRAM stuff showed up....then got a game to bot before the other garbled stuff started. Now on a mission and will consider F18 DIY if it failed.

Edited by WicoKid

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26 minutes ago, WicoKid said:

I cleaned and replaced the compound.  If a I can salvage a sn74ls156n logic chip, I'll know if its the DRAM or not later today. I will admit I clipped of the logic chip thinking it was DRAM too. 

 

Really hope its not the 9918A. I had just gotten the 32k ram expansion built and played about three games before the original DRAM stuff showed up....then got a game to bot before the other garbled stuff started. Now on a mission and will consider F18 DIY if it failed.

The 9918A is probably cheaper, and is certainly easier, to replace than a DRAM. If you're in the US, some kindly soul might be able to post you a used, tested one. Or I could send you one if you happen to be in the UK.

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Posted (edited)
18 minutes ago, Stuart said:

The 9918A is probably cheaper, and is certainly easier, to replace than a DRAM. If you're in the US, some kindly soul might be able to post you a used, tested one. Or I could send you one if you happen to be in the UK.

Just buy a F18A. More are being made this summer

Edited by GDMike

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10 minutes ago, GDMike said:

Just buy a F18A. More are being made this summer

This is actually pretty cool to read.  Though, it makes my original spare far less valuable :)

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Well....replaced the logic chip...it still beeps; sometimes boots to blue screen with bars at top and bottom and then fails, but seems to be getting worse with checkerboard. I've replaced/socketed 4/8 DRAM chips and even piggybacked others, really no changes (scan line  on the picture). Considering socketing the rest and trying 4164's with mod or boards from Arcade shopper. Have to put it all away for a while. though.

 

As far as buying a F18A, is that in reference to Hans on the F18 DYI thread or is someone in the US making them as well?

 

Did great for about 20 minutes with the 32K expansion and Flashgrom99, Frogger, Parsec, a taste of Mrs. Pacman then the crash.

TI99Screen.jpg

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Ksarul suggested a heat issue, when you ran it for 20 minutes, was it all bundled up with new heat sink compound on the tms9918?

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No, the old heat sink, I hadn't opened it at that point.

 

The computer hadn't had any problems for about 6 months with smaller games and had just finished up the 32k expansion and was trying it out when it crashed the first time. THEN, I opened it for the first time and quickly located the bad DRAM(my screen matched a MESS image pretty exactly), then the next time I tried it (aside from brief on/offs to make sure it was booting) I had reapplied heat sink and compound and closed case, it made it to Mrs. PacMan start up then garbled again; its been downhill since.

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Alright...got an untested TI99 from eBay....it works....just turned it on to startup screen at this point

 

1) What should I do maintenance wise before I start using it?

2) Do I risk removing the 9918A from it and trying it in the nonworking TI99?

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1 hour ago, WicoKid said:

Alright...got an untested TI99 from eBay....it works....just turned it on to startup screen at this point

 

1) What should I do maintenance wise before I start using it?

2) Do I risk removing the 9918A from it and trying it in the nonworking TI99?

Why?  I guess you are handy with the iron, but why run the risk of having two non-working machines?  If you need a 9918A to test, I can send you one.

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If you have it out of the case, wait until it garbles, and do a press check on each chip to see if one seems abnormally hot. That garbled screen could be caused by a number of different issues, usually VRAM, but it could be something else. I think it was a utube's Adirene's Digital Basement, it was a three part series of trying to fix a TI console, he went through, most likely, to least likely, unfortunately it ended up being a least likely. 😃

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On 7/5/2021 at 11:34 AM, Ksarul said:

I would suspect an overheating chip here. Did you replace the heat sink compound on the VDP when you put the console back together the first time? That chip runs a bit hot and really needs the heat sink or it will give symptoms similar to your problem.

Must be chip-dependent.  @OLD CS1 and I ran one without the heat shield and such (pretty much almost naked) at VCF-SE and had zero issues.  Maybe it needs the heat sink but not shield?

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Posted (edited)

Thank you for offering to send 9918A to test. Very kind. Had picked up 2 non working but socketed atari 800 xl and was able to sort out which chips were bad pretty quickly, so that was the theory when I ordered this untested TI99.

 

I figured putting what I thought was the bad 9918A into the working machine was the lowest risk, and it booted to normal startup screen. So pretty sure I have 2 good 9918A's.

 

The "bad" board is now garbled from the boot...no more warming up to worse, it is just worse as in the above picture, but normal startup beep and texture change when press 1. Next will finish socketing the video RAM and start checking traces on the bad board.  I have  about 10 4116 DRAM chips, however, no way to test them, so, once socketed  it will be traces vs  dram chips, unless it just fortunately works with the new socketed chips. 

 

Edited by WicoKid

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44 minutes ago, acadiel said:

Must be chip-dependent.  @OLD CS1 and I ran one without the heat shield and such (pretty much almost naked) at VCF-SE and had zero issues.  Maybe it needs the heat sink but not shield?

Oh, yeah.  Two boards, a standard and a QI, no heat sink or shield, running demos and games all day.

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Posted (edited)

Well I did find a broken trace that went under one of the DRAM sockets. Placed a jumper wire (U102 pin 14 to 9918A pin 25) but now back to a normal startup that quickly fails. When starting up on a CRT I get about 2-3 seconds of an absolutely normal startup screen then characters fail.  

If I turn it on and off immediately, then its just garbled....if I let it sit for 20 minutes and turn it on, I'm back to 2-3 seconds of a normal startup and fail.

 

Anyway, it is a work in progress. Will go the least traumatic way, and although they look OK, will replace electrolytic caps first.

 

Made a short video and uploads but will not play.

 

 

Edited by WicoKid

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It looks as though this is going to be at least partly power supply problem, maybe fried components.  I think this was failing as I tested it....the power brick is outputting 10.2 AC and 22.4 AC; to the board -6.06 Volts, 14.54 volts and 6.24 volts. I only have 1 external brick so  holding all power until a MEAN WELL USA Inc. / PT-45A arrives. 

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On 7/14/2021 at 6:13 PM, acadiel said:

Must be chip-dependent.  @OLD CS1 and I ran one without the heat shield and such (pretty much almost naked) at VCF-SE and had zero issues.  Maybe it needs the heat sink but not shield?

Late reply, but it totally is. I've had some that flaked out within 15 minutes or so, and one that completely cooked itself over 24 hours (that one had a manufacturing fault - a wire across the VDP under the heatsink. It would run for an hour or so without issue, but when I installed it in my BBS it died ;) )

 

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Battery in Voltmeter was low...wrong voltages; voltages are pretty much right on, except the 5 volt line reading 5.26 unloaded. So I'm back to looking for overheating chip....

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