Having problems with my new TI-99

[tokyorock]

I collect old video game consoles, and recently came across a TI-99/4A (without cords), so I purchased all of the necessary items plugged it in. When the boot screen first displayed, everything looked good, but it quickly (in about 5 seconds) degraded to a sort of sloppy mess. The sound quality (beeps and boops) is still good, but all of the text on the TI Basic prompt screen is unreadable.

 

I took pictures of the screen, the system itself, and the video modulator. The delay and gradual degradation leads me to believe it could be a bad capacitor on the main board, particularly one having to do with video display, but I figured I should ask here to see if any of you can diagnose the problem.

Edited November 4, 2014 by tokyorock

[gregallenwarner]

My guess is that one of the VRAM chips has gone bad. The standard fix for this these days is to install an F18A module. The side effect of this would be that you now get VGA output to boot!

[Iwantgames:)]

My guess is that one of the VRAM chips has gone bad. The standard fix for this these days is to install an F18A module. The side effect of this would be that you now get VGA output to boot!

Too bad the F18A site is still down I really wanna buy one myself

[tokyorock]

My guess is that one of the VRAM chips has gone bad. The standard fix for this these days is to install an F18A module. The side effect of this would be that you now get VGA output to boot!

 

It seems like it would be cheaper to just buy a new unit than trying to find and purchase an F18A.

Edited November 4, 2014 by tokyorock

[Iwantgames:)]

The F18A is about $80

[dphirschler]

It looks like not all yours are bad. If I can get my vram chips desoldered (they are quite difficult to remove), I will sell you a set. One of them is bad, but I believe the other 7 are good. I had the same issue with my TI and the F18A made those vram chips unnecessary.

 

 

Darryl

[Tursi]

That's the most common failure. If you can get a 4116 DRAM chip, as noted above, it's likely only one has gone bad (there are . The F18A is easier since it just drops into the VDP socket.

 

Don't turf the console, anyway, it's easily fixable.

[RobertLM78]

Definitely bad VDP RAM. I had a similar thing happen to one of my machines. An F18A will fix the problem

[Stuart]

This seems to be a common problem. A while ago Michael (I think it was) done a temporary tweak in MESS to replicate a failed VRAM and show how it looked on screen. Might someone take up the challenge of adding this as a 'feature' to an emulator to help diagnose "if your startup screen looks like this, replace VRAM chip Uxx"? Or do temporary tweaks and take a series of reference screenshots? Not entirely sure how practical this suggestion is - I'm guessing that it's probably only one bit in *part* of a VRAM failing, rather than the entire contents of one VRAM failing.

[Wildstar]

The problem is implementing that in emulation falls into the accuracy issue. These components can fail in alot of ways. There is alot of say, components that could fail. Be it resistor, a capacitor, partial to full failure of a RAM chip or a video chip which consists of digital and analog components. A respository of high quality photo with description of the type of failure would sometimes be sufficiently helpful for diagnostic purposes.

 

How the chip's failure will look depends on many many variables.

[Stuart]

But the failures seen tend to be quite specific, and as reported in this thread - the display is absolutely normal except that some characters are garbled. And that's down to one of the VRAMs. They're fairly easy to replace, if one knows which one of the eight is faulty. I might have heard of a VDP failing once or twice, but have never heard of a problem with any other component in the video subsystem.

[+mizapf]

Changing

m_tms9928a->vram_write(space, 0, data);

to

m_tms9928a->vram_write(space, 0, data & 0xdf);

yields this:

 

[+Lee Stewart]

So...I guess that means bit 2 (TI backwards, with MSb = bit 0) is bad, which is U106 from the schematic and the 5th chip from the joystick port.

 

...lee

[Stuart]

So if Michael could just repeat that another 7 times for the other 7 bits ...

[+mizapf]

As for the suggestion to integrate such a feature into MESS: We can, admittedly, use that for tracing problems in real machines, but this is only one simple kind of fault. There is a myriad of other possible faults, and I'm afraid if we start to include them we'll bloat the emulator controls, so we might lose our goal of making it more user-friendly.

 

MESS has open sources and can be pulled from Github. I don't believe I'm the only one who is able to set up a build environment.

 

... thinking of it, I understand your suggestion now, so here you go:

 

 

[gregallenwarner]

Now can we get every permutation of combinations of chips from 2 to 7 out?

[+mizapf]

Even worse, these were only the stuck-at-zero symptoms. We should also consider stuck-at-ones.

[Wildstar]

We could perhaps have a special edition of MESS that can be used strictly for such diagnostic purposes while ordinary versions of MESS won't necessarily have this. I suppose that could be a concept so we can call that version of MESS.... say.... "DR.MESS Diagnostic Emulation System".

 

Tool that will emulate failure in components to aid in diagnosing problems.

[matthew180]

It seems like it would be cheaper to just buy a new unit than trying to find and purchase an F18A.

The F18A is not hard to find, it has been for sale on my website (codehackcreate.com) since mid-2012 and the checkout is pretty easy. Yes, consoles are cheap, VGA output is not (depending on your interpretation of expensive). Collecting and messing with old hardware / computers is anything but cheap.

 

... They're fairly easy to replace, if one knows which one of the eight is faulty. ...

In my experience they are hard to remove, especially without damaging the motherboard. Oh course I was trying to preserve the chips, so if you are willing to cut the legs off then it is a little easier. You still have to be careful not to pull up a pad or a trace, or do other such damage to the board. Not as easy as popping the VDP from the socket though. :-)

Edited November 6, 2014 by matthew180