Lunar Lander glitch - please help!

Vectorman

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Guys,

Moved my Lunar Lander across the street, and now it's doing this. Before I go dig out the schematics and try to trace down the problem, I thought I'd throw a picture up here to see if anyone knows where to look off the top of their head. There are so many of you guys who are smarter than me and have experience on these Atari vectors. It's obviously a glitch in the x-circuitry. Any ideas?

Thanks for any guidance you can offer...!

John
 

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Looks like the X DAC, or possibly one of the position counters feeding it. Hard to say exactly without more info, but that's a first-order assessment.

I would socket and swap out the X DAC first. You can also take a probe and look at the data inputs to the DAC (which are the position counter outputs), and see if any are stuck, which would imply a counter problem. But most of the time for issues like this, it tends to be the DAC. Socketing a DAC is always valuable (for the next time it goes bad), so even if that ends up not being it, it's still useful.

How long had the game been powered off for, since the last time it was powered on? These chips tend to blow when a game sits for months or longer. For whatever reason, these old TTL chips don't like sitting unpowered. That's why the best thing you can do for your games is power them on once a week, or put them on a timer to turn on for an hour or two.

If you end up not being able to figure it out, you can always send the game board in. PM if needed.
 
Moved game = connection issue.
Might not even be a failed component. 🤔
Simple stuff first.


This is specifically why I asked how long the game had been sitting unpowered for, prior to moving.

Moving a game that hasn't been turned on in months, then turning it on = same as not moving the game.

Yes, all voltages and connections should be checked first. But I'm assuming a knowledgeable 20-year member like Vectorman (with one of the largest collections in VAPS) knows this already.
 
Just because someone on KLOV has the word "VECTOR" in their user name doesn't necessarily imply they know anything. Same is true for collection size and time spent in the hobby. LOL - I know from personal experience.

Too many times an old game has malfunctioned due to connection issues and only determined AFTER the board or monitor had been needlessly shotgunned. But yes, more details of what happened and the history of the game is warranted.

(Waiting for the inevitable - have you checked AC ripple comment). LOL X2
 
Ok, so then to make VC happy, make sure you check the 5V on the game board first. :)

If it were especially low, I suppose that could cause a counter to malfunction. But it would have to be down around 4.5V, with just that one chip operating right on the edge. And when things are that low, the board usually just won't boot. Which is why I'm less inclined to think it's going to just be a bad connection.

That banding effect is definitely either a counter or the DAC dropping out, we know that from many other threads here. Whether it's a bad chip or just low voltage, can easily be checked.
 
Maybe a DAC or counter is socketted and has dislodged itself. That goes in the "bad connection" category. 😁
 
I agree, DAC or Counter are the likely culprits.
 
And you are in McLean, Illinois. If you want, you can use a logic probe and check what your Counters are doing. I'll check on mine and give you notes which you can compare it to.

Otherwise, you can send it to someone like andrewB who repairs such boards, or others.
 
SUCCESS!

Guys, thanks so much for all the input and suggestions. I thought the suggestion of checking the counters made the most sense based on what it was doing on the screen. So I got out my logic probe and the schematics and quickly determined that DACX7 was stuck high. Traced it back - counter was fine, but the buffer / multiplexer chip between the 191 and the DAC was failing on that circuit.

It's a 74LS399 which I didn't have in stock. I also don't have a lot of Lunar lander parts boards laying around and it looks like only the first couple revisions of Asteroids boards used that chip. Fortunately, AI was able to tell me that the Sega Choplifter PCB uses an ls399, and I do have about 10 non-working Choplifter PCBs, so I desoldered one and popped it in the new socket on my Lunar Lander PCB and it's fixed!

BTW, I had just plugged it in yesterday before moving it and it was working perfectly. I have no idea how dollying a game about 300 yards makes a chip fail, but I chalk it up to 45 year old circuitry and God's plan for me to learn a bit more about troubleshooting vector problems.

Yes, I admit that although Vector is in my username, I am indeed NOT an expert and only a very mediocre tech. But I do have a EE degree and 35 years of at least tinkering around attempting to fix classic games including lots of vector games. But I am very limited in my abilities, hence my need for help from you guys who are far more expert than I.

Thanks so much for responding to this plea for help, and you are all welcome to my new Arcadia Unlimited arcade for free anytime (as soon as I get it open sometime in the next month or two). It will have ~275 video games to play in phase one, double that in the next phase (which will be in about a year or two). And LOTS of vector games.
 

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