TI99/4A starts to distort sound and picture after 10 minutes

[jschaap26]

I was really excited to have acquired a cosmetically excellent TI-99/4A. I was even more happy when it powered up and worked, seemingly, without issue. However, now I've notice that after about 15 minutes or so of game play video distortion and / or graphical errors will start to happen. Shortly thereafter, the sound and / or game play will begin to distort or act crazy. Eventually it will freeze the machine.

 

If I leave the system off for while and come back later it things will be good for another 15 minutes or so but eventually the behavior always returns. It does this on an original Parsec cartridge as well as a FinalGROM99 cartridge/3D printed case I have. It gets pretty warm on the face of the console just below where the cartridge is inserted. But perhaps this is normal...

 

After buying the TI, a joystick adapter, speech synthesizer and the FinalGROMM cart it's so disappointing to realize that the TI has a major issue. I really did not want to have to crack it open... sigh. Any suggestions anyone has about how to diagnose and fix this issue are greatly appreciated.

[mindlord]

Sounds like as it heats up something is getting loose. If star by taking out apart, reseating any socketed chips, checking the voltages on the power supply.

Sent from my SM-G960U1 using Tapatalk

[Omega-TI]

My first GUESS would be the 9918 overheating.  It could be that the heatsink compound is dried out and useless after all these years.  Opening up the clamshell, removing the "white stuff" and replacing it with new << thermal paste >> may fix the problem.   

 

Be very careful when unplugging the keyboard from the motherboard!  The ribbon cable may be very fragile.

[Nick99]

Is the problem the same without the speech synth?

[Omega-TI]

35 minutes ago, Nick99 said:

Is the problem the same without the speech synth?

Good question! I overlooked BOTH sound and speech.  Take a stopwatch or clock and see the length of time it takes from a cold boot to distort with and without the speech.  If it's faster with the speech., the power supply may be weak/failing and values change enough when warm, reducing the supply current to the motherboard.

[fabrice montupet]

I suppose that the audio chip installed in your computer is a TMS-9919 (or 447 KHz equivalent).
As the sound is distorted at the same time the graphics have problems and GROM cartridges end up to crash,  the problem seems to be created by an unstable VDP GROMCLK signal due to a faultly TMS-9918A or an excessive heat. You should replace the thermal paste for a better dissipation and, if the problem persists, replace the TMS-9918A.

 

[jschaap26]

2 hours ago, Nick99 said:

Is the problem the same without the speech synth?

Yes, the behavior is the same when the speech synthesizer is not connected.

[Ed in SoDak]

As a stopgap, try a small computer fan laying on the slots behind the cartridge port. I power a little 12v fan with a 3v wallwart. Slow and quiet but it does keep the "coffee warmer" atop the power supply cooler.

-Ed

Edited December 8, 2019 by Ed in SoDak

[jschaap26]

So after watching some disassembly videos I was able to very carefully disassemble my 99/4a tonight. I was surprised to find a MASSIVE amount of thermal compound on the 9918A. This can't be normal... can it?? I am not sure if this is a sign that someone was being very sloppy the day it was assembled or if someone has been in it before and tried to resolve what they thought was a heat issue with a load of thermal paste. The paste seemed a little rubbery but not completely dried out.... So I can't tell if it was original or not. At any rate it took my 45 minutes of detailed cleaning to get it off the chip, heat sync and board. What a mess!!

 

The plan from here, I suppose, is to reapply thermal grease, reassemble and see how it behaves. Or, I suppose I could run it bare with a fan on the 9918A and see what happens? Thoughts anyone?

[Omega-TI]

Damn!  Someone really went to town with that stuff!      It's more like a blanket on the chip to keep it warm rather than to dissipate heat.  Yes, try it with a fan to see if there is a difference.  

[JB]

That looks "right" to me. Vigorous application at the assembly line, let the block mash out the excess. It is non-corrosive and not electrically conductive, so the excess is just gross, not a problem.

[+Lee Stewart]

Whatever you decide to do, be careful you do not damage those white/gray vertical bands that run across and along the circuit board. They actually carry multiple circuit traces around the board and can easily be broken by too much bending, which then acts the same as though you had cut foil traces with a razor blade. They may also control air flow for heat dissipation, but their principal role is circuit routing.

 

...lee

[Omega-TI]

7 minutes ago, JB said:

That looks "right" to me. Vigorous application at the assembly line, let the block mash out the excess. It is non-corrosive and not electrically conductive, so the excess is just gross, not a problem.

I've opened up quite a few TI's in years past and have never seen one quite that bad.

[JB]

29 minutes ago, --- Ω --- said:

I've opened up quite a few TI's in years past and have never seen one quite that bad.

That's why I put quote marks around "right". It isn't PROPER, but it isn't a problem or evidence of rework. It's just... a lot of excess goo.

 

(I was pretty surprised the first time I opened a 4a up and there was this big wad of rubbery snot hanging off the VDP. I THINK it was that bad, but can't swear to it. Best hypothesis we came up with is that they were near the end of the pot of goo and just decided to slather the whole glob on so they could get a new pot.)

[jschaap26]

Thanks everyone for your thoughts. So I ran it bare and played Donkey Kong for like 3 minutes and the issue came back. Then I put a fan (the only one I had handy so don't judge! Hahaha) on it and it ran like a champ without any issue. And had my best game of DK ever!

 

So does this mean:

 

A) The chip is good and I should just reapply some quality thermal grease and reassemble?

 

B) The chip starting to have issues and should be replaced before being reassembled?

 

Appreciate all the help.

[Omega-TI]

I used to use a can of cold shot as you could isolate individual components.  

[+FarmerPotato]

17 hours ago, Lee Stewart said:

Whatever you decide to do, be careful you do not damage those white/gray vertical bands that run across and along the circuit board. They actually carry multiple circuit traces around the board and can easily be broken by too much bending, which then acts the same as though you had cut foil traces with a razor blade. They may also control air flow for heat dissipation, but their principal role is circuit routing.

 

...lee

 

What's in those horizontal bus bars?

 

Is it voltages for the 4116s? Address or data lines from the 9918A?

 

Is there a PCB photo pinpointing how these lines get to the 4116s?

 

This is a practical question - I have tried to route a PCB with 6 x 4464s and my attempts are a mess.
 

 

 

[HOME AUTOMATION]

1 hour ago, jschaap26 said:

Thanks everyone for your thoughts. So I ran it bare and played Donkey Kong for like 3 minutes and the issue came back. Then I put a fan (the only one I had handy so don't judge! Hahaha) on it and it ran like a champ without any issue. And had my best game of DK ever!

 

So does this mean:

 

A) The chip is good and I should just reapply some quality thermal grease and reassemble?

 

B) The chip starting to have issues and should be replaced before being reassembled?

 

Appreciate all the help.

The fan is blowing on the power supply too. I've never had an issue running w/o the heat sync on VDP.

 

If the VDP is overheating, the issue could still be elsewhere...

[HOME AUTOMATION]

24 minutes ago, FarmerPotato said:

 

What's in those horizontal bus bars?

 

Is it voltages for the 4116s? Address or data lines from the 9918A?

 

Is there a PCB photo pinpointing how these lines get to the 4116s?

 

This is a practical question - I have tried to route a PCB with 6 x 4416s and my attempts are a mess.
 

 

 

I had wondered recently myself how to best troubleshoot these...

I have one board "down" and suspect these to be a possible cause.

[jschaap26]

So I did some more testing tonight. I got rid of the big room fan and fired up Donkey Kong (this game tends to make the issue appear quickly) from my FinalGROMM99. Sure enough, 3 or 4 minutes in the issues started. When it did, I hit the 9918A with a shot of compressed air (not cold air, just compressed for blowing out dust). After a 3 or 4 second burst things went back to normal. They would stay fine for about 30 seconds or so before the sound and / or graphics started to distort again. I would hit it with another spray and things would again immediately stabilize. The system never crashed as long as I did this which was about 7 or 8 times I think.

 

After that I pulled a heat sync fan from an old PC, wired it up to a power adapter and put that right over the 9918A (see picture) so that it was cooling that and nothing else (not the power supply components). I played Donkey Kong and other games for quite some time and the issues never came back.

 

At this point, I am again, suspecting the 9918A as the culprit and wondering if just putting thermal compound on it will resolve all the issues. But, I am very troubled by the fact that Dragonstomper is saying that the 9918A should have no problem running without cooling or the heat sync... The last thing I want is to put it all back together have the same issues.

 

Again, your thoughts are welcomed.

 

 

[HOME AUTOMATION]

Dragonstomper! I didn't notice, I'd been promoted.

 

Don't be too troubled... Sometimes, things like this are hit or miss.

If the fan alone works, I would think that a good heat sink should as well! Se la vie. 

 

Does the socketed I.C., ending in 4624N, near the cartridge port, get hot?

It would likely be the most intolerant of over-voltage.

Edited December 11, 2019 by HOME AUTOMATION

[Omega-TI]

On 12/9/2019 at 9:12 PM, JB said:

That's why I put quote marks around "right". It isn't PROPER, but it isn't a problem or evidence of rework. It's just... a lot of excess goo.

 

(I was pretty surprised the first time I opened a 4a up and there was this big wad of rubbery snot hanging off the VDP. I THINK it was that bad, but can't swear to it. Best hypothesis we came up with is that they were near the end of the pot of goo and just decided to slather the whole glob on so they could get a new pot.)

 

I stand corrected.  I HAVE seen one almost that bad before.  When a TI I had was opened up for a Tursi 32K upgrade, it's viewable in the gallery below...

 

 

[wierd_w]

That would be this one, yes?

 

Reminds me of when I cleaned off waaaay too much thermal compound off a co-worker's home build, when they were trying to figure out an overheating issue.  I discretely informed them that a rice sized dollop was "more than adequate", and hooked them up with some arctic silver.

 

Speaking of which, it looks like the default choice in the late 70s was the white junk. Blech.  Put a nice chipset heatsink on there with some silver compound. Will probably get the overheating issue sorted.  Another thing to check would be reseating the IC. It could be working harder than it should be, due to poor signal fidelity, and generating heat.

 

 

 

[Omega-TI]

8 minutes ago, wierd_w said:

That would be this one, yes?

Yes, but the best way to eliminate the overheating problem (IMHO) is...

(ducking & running)

[Tursi]

13 hours ago, jschaap26 said:

At this point, I am again, suspecting the 9918A as the culprit and wondering if just putting thermal compound on it will resolve all the issues. But, I am very troubled by the fact that Dragonstomper is saying that the 9918A should have no problem running without cooling or the heat sync... The last thing I want is to put it all back together have the same issues.

99/4A consoles, while built ruggedly for the minimum spec, do seem to vary a lot to the maximum spec. I've had quite a few VDPs that can't run without the VDP heat sink in place for more than a few minutes, and I've definitely run my plank with some VDPs without a heat sink for at least a few minutes. But I think it's safe to say that the heat sink is required in your case, and I wouldn't deliberately run long term without it.