Asteroids Help

Monstermug

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I got a Asteroids pcb that has a slight glitch. It was working one minute, switched off. Turned on again and this happened.

Firstly, large saucers are showing up as small saucers. There are also no medium size meteors either. Only Large and Small. When a large meteor is blown up it turns into a small meteor and when the small meteor is blown up it then split into 3 other small meteor. Points are still awarded the same as if working.

Also, which seems to be related, some of the text shown is not centred. (The high score table, High score congratulation message, press start, insert coin). The main score and Atari logo inc is centred though. The texts looks smaller than what they should be and shifted left of the screen.

Things I have done so far:

Replaced all 3 program roms and 1 vector rom. No change.
Replaced aging Rom sockets. No change.
Checked capacitors.
I have all correct sounds, and everything else function correctly.
No errors in test mode.
No visual damage on board. No missing components. No dry solder joints. (I went through this pcb with a fine tooth comb) (PCB looks brand new and untouched)

If you could point me to which part of the schematics this might be related too? I was thinking a slight glitch in the addressing circuit? However probing the entire circuit against skats, revealed nothing interesting. (I could have missed something out though)

If you have read this far then thank you!
 
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Sorry, I don't have much of a clue but wouldn't mind seeing a video of the symptoms, It might help others as well.

You said your replaced all 3 program roms. Was this just a shotgun or was the code corrupted? To me, if the small saucers and different sizes look normal but are just appearing at incorrect times, I would think it is something on the programming.

Maybe articulate the dip switches a few times and then reset to their defaults.
 
Thank you for your response.

Here are some screens of the problem. As you can see the high score table fonts looks smaller than what they should be. Also not centred.

20161224_125614.jpg


High score congratulations shifted too far to the left! Also smaller text as well.

20161224_125959.jpg


I replaced the Rom socket because it was watchdogging and in self test I had a 5th Low tone indicating a ram error. But as I was touching some of the rom chips, it passed the test. The rom sockets looks to be of low quality so I replaced them with newer low profile sockets. I re-burnt the roms because I just wanted to store the original roms somewhere safe :D

I have been digging around the net for clues and there were no mention to this problem anywhere on the net. I did stumble upon a tech sheet which listed a few common problems. One of them was down to some oversized letters.

20161224_130644.jpg


As my problem had this issue, but rather a reversed effect, I figured this would at least be a good place to start.

I probed pins on M7 and all was fine except pins 14 was stuck high and 15 was stuck low. All the other pins were flipping as normal.

20161224_130437.jpg


Checked the data sheet for 74LS175.

20161224_130620.jpg


The logic diagram shows that input on pins 13 should be driving outputs on pins 14 and 15. I probe pin 13 and it was indeed flipping. This should mean that outputs on pins 14 and 15 should not be stuck in either states.

20161224_130557.jpg


Got a 74LS175 and some sockets on order and hopefully this should fix the problem.

Not sure if this will fix everything but I guess it would be a good place to start.
 
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nice work. Hope it fixes your issue. I hate not have spare parts and having to wait. keep us updated on progress.
 
Today was looking for something else and found a 174N which I had purchased when I repaired a Gravitar High Score save problem last month.

Pulled out faulty 174N.

20161226_161135.jpg


Socketed in a new 174N.

And got back medium asteroids and large saucers..

20161226_183700.jpg


And also the large text and in the right place too.

20161226_183745.jpg


20161226_184015_001.jpg
 
awesome job / fix. I guess 74LS174 and 74LS175 are similar? I know if I looked at parts # and datasheet it would be obvious.
 
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