Problem with broken TIA audio on my 7800

[Chicken George]

Hello everyone. I'm Chicken George and this is my first post !

 

I'm the loving possessor of an Atari PAL 7800, which gave me joy for almost 30 years.

I recently AV modded it with the "TFW8B" PCB and worked fine for some time.

 

Today I was trying to fix a minor issue with the audio causing little interference in the video (you know, when some sounds generate horizontal lines). I thought adding a small capacitor on the wire would do the trick.

Probably the capacitor was not really the problem, I must have short circuited something on the audio while working on it.

 

Long story short, now my audio is behaving in a reeeeeallly interesting way: It sounds like some pieces are missing completely, like the sound of the worm in centipede. Other sounds are broken and intermittent, and completely different from the way they should sound like (the alien ship in asteroids now does just "beep beep" instead of all that racket it's supposed to do). Also the overall audio experience suffers from peaks of low and high volume, just randomly.

 

It is hard to try and describe the sounds, I know, but what do you guys think? I tapped the sound directly from pin 13 of my PAL TIA chip, and it does not do any better. Is it possible I fried the TIA, either putting that capacitor or with an accidental short circuit?

 

Please help !!!

[+-^CrossBow^-]

I will admit that the TIA seems really fragile in the 7800. If you are tapping the audio straight form the TIA pins and getting those results then it very well could be the TIA at fault. I believe a different TIA is uses on PAL vs NTSC systems? I've not worked on any PAL consoles myself so I wouldn't know exactly off hand. If you had stated you were tapping the audio from the board somewhere, I would have thought that maybe a trace got damaged or something.

 

Do keep in mind that the exact same TIA is used in both 7800s and 2600s alike. So if you have a 2600 junker laying around with a TIA in it or possible have one with a socketed TIA you could pull to test...

 

Also..specifically what is the TFW8B AV mod?

[Chicken George]

the following is the mod I have:
https://www.thefuturewas8bit.com/atari-composite-video-mod.html

It looked very suspicious to me that there was nothing on the audio channel, not even an 1 microfarad capacitor. Well, I was wrong.

 

So if you have a 2600 junker laying around with a TIA in it or possible have one with a socketed TIA you could pull to test...

 

Nope, no 2600 around and unfortunately my 7800 does not have a socketed TIA, but will be great to get a TIA socket to solder on my board.

Anyways, is it remotely possible that the problem is BEFORE the TIA? like in the 5 or 6 audio registers that arrive in the TIA? If the answer is NO, I would not mind buying a new TIA (you can get them for around £10). If the answer is YES, any idea on how to troubleshoot?

Thanks for the reply!

[Chicken George]

Just a quick update: I changed the TIA on my 7800. Now it is socketed, so if it breaks again it will be much easier to change!

Anyways still have A LOT of interference from audio to video.

 

Any advice, anybody?

[+-^CrossBow^-]

So replacing the TIA at least brought the audio back to where you had it then? Interesting that the PAL TIA only has a single audio out channel as opposed to the NTSC where both pins 13 and 12 are audio out, and the description you gave almost sounded like one of the two TIA channels wasn't connected. But you having a PAL version I have to assume that something was fried internally that affected half of the TIA sound on the chip.

[Chicken George]

yup. It has only one audio. Weird.

 

[Osgeld]

It only has 1 pin but it's still two internally

[+-^CrossBow^-]

It only has 1 pin but it's still two internally

 

That is what I figured and was trying to state. So is it possible for only 1 part of that internally to get hosed up? Again, his description of the sound he was getting sounded a lot like only 1 of the 2 TIAs was operating.

[Osgeld]

Yes the other channel is probally damaged but not to the point of failure

 

Inside a chip is just like any other circuit (but smaller) two circuits doing basically the same thing made with the same parts will have different weak points

[Chicken George]

Inside a chip is just like any other circuit (but smaller) two circuits doing basically the same thing made with the same parts will have different weak points

 

Thank you for clarifying that. Any clue on what could cause the distortion of the audio aver the video? (remember I have a composite mod in the board.)

[Chicken George]

Ohhh, f*ck... I did not realize that now 2600 games have all colours wrong must be the spare TIA I used, or hopefully only some bad contact. It looks like the green is blue and the blue is red. Maybe I'm under a voodoo curse.

[Trebor]

Ohhh, f*ck... I did not realize that now 2600 games have all colours wrong must be the spare TIA I used, or hopefully only some bad contact. It looks like the green is blue and the blue is red. Maybe I'm under a voodoo curse.

 

There are two flat adjustment knobs in the middle of the motherboard that impact system colors [increases and decreases the degree delta between hue ranges].

Top one is for the 7800/MARIA graphics mode.

The one below it is for the 2600/TIA graphics mode.

Try adjusting the 2600/TIA one. Be forewarned though, the knobs are highly sensitive. A slight turn can make a substantial difference in the degrees of hue shift. If you do not have a 2600 color test cart/ROM, typically Frogger and Pitfall! are adequate reference games.

[InactiveX]

 

That is what I figured and was trying to state. So is it possible for only 1 part of that internally to get hosed up? Again, his description of the sound he was getting sounded a lot like only 1 of the 2 TIAs was operating.

 

I can confirm it is possible. Dig Dug's duophonic music sounded way off when it happened to me (UK PAL).

Edited October 16, 2018 by InactiveX

[Chicken George]

Hey Guys, those knobs are glued down on my 7800, I'm afraid I can ruin them. What would be the best option to remove that blob that appears to be hot glue?

[Osgeld]

dunno I usually just use a pick and it pops right off

[+-^CrossBow^-]

I actually just turn them anyway and it cracks off or cracks enough to allow the knobs to turn. Once I like how the colors look, I use nail polish to tack it back down again.

 

BTW, I don't think the knobs should require adjustment just from swapping out the TIA? Sounds more like you either put in an NTSC TIA or the TIA you installed might have issues of its own. One condition of a bad TIA is severe shifts in the color and even some color cycling during games while playing.

 

Have you checked the voltage coming off the 7805 regulator and the input voltage into it? I've seen colors be off when the input voltage isn't quite where it needs to be for the 7805 to provide properly current though I've mostly only witnessed that on 2600s.

[Chicken George]

I don't think the knobs should require adjustment just from swapping out the TIA? Sounds more like you either put in an NTSC TIA or the TIA you installed might have issues of its own.

 

Well the TIA i installed has exactly the same code, which is for the PAL TIA. But it could be broken, of course. Remember anyways that PAL has many substandards (PAL B, PAL G, PAL I) so it does make sense the chip is ok but the chroma needs adjusting.

(My 7800 was bought in italy, while the TIA I put in is most probably UK)

Anyways, I'll give it a try, I don't think I can break it more than this

Edited October 16, 2018 by Chicken George

[Osgeld]

if I replace like an older generation TIA with a newer one the colors will go wonky, even if the chips look identical I usually tweak them a bit anyway

[Chicken George]

Hey guys, thanks everybody.

I twisted the knob and now the colours are good. Anyways, it's a fragile piece of shit, and looses contact while you twist it, giving you only black and white images at times. Anyways now seems to be perfect.