Gravitar help! (please)

DogP

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Hey,

I finally got a Gravitar, which I've been trying to find for a long time. Anyway, it had a dead flyback on the Amplifone, so after fixing all the monitor related stuff, it had problem playing.

Basically, it only boots sometimes, and when it does, the text and other static objects seem to be wrong and/or in wrong places. The cool Gravitar attract screen works fine, but the title screen w/ text has either random text on it, or sometimes duplicates, and while the game plays just fine, there's some static characters randomly placed over the screen (and my ship's thrust is drawn in the center of the screen).

At first the game seemed to work completely fine only with an A/R II board from my Tempest... so I rebuilt the original w/ a Bob Roberts kit, and it still didn't work, then I tried putting the Tempest A/R back in, and it's the same problem, so it was probably just coincidence or slight adjustment difference why it worked then. But I've checked the voltages and made sure +5 was properly adjusted but (at least now) there's a consistent problem.

Self test never tells me anything good (at least to my knowledge)... some have blank screens, others draw just fine... others draw messed up (I think).

I'm not very familiar with the Atari hardware, so if anyone could give me a pointer to where I should start to attack the problem, I'd appreciate it. Attached are some pics:

Thanks,
DogP
 

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A few more pics:

Picture captions in order from left to right (above):
Normal Gameplay
First Self Test Screen
Next test screen to show anything
Next test screen to show anything
Next test screen to show anything

Pics below:
Next test screen to show anything
Title screen that comes up sometimes
Title screen that comes up other times
Gravitar animated screen

And I've tried the obvious things like reseating the chips, checking voltages, and I looked over the board pretty closely for damaged traces and stuff... but everything looks very untouched (even the dryrotted foam behind the PCB was still attached to the wall until I knocked it off ;-) ).

Thanks,
DogP
 

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Last edited:
When I go into test mode I don't get any beeps, but I get loud static sound (until I press a button, which makes the tones corresponding to the buttons). I also noticed when I just checked it tonight, everything was running really slow... like 1/4 speed (along with extra graphics on the screen).

Does that make sense?

Thanks,
DogP
 
Fascinating bug.

You've got a working game, but when the vector state machine tries to draw a character, it ddraws it ttwice (after the first character), and intersperses some z00ee00rr00e00s.

So - "E0 10110110 00000" is actually "E", "111", as is "Press Start 1 for Test", and so on...

I'm going to assume a problem with a counter, latch, or address decoder somewhere in the vector RAM/ROM area.

Not sure what the "S/F G/H/I" refers to, but "1/2/3/4/5/6/7/8" screen seems to indicate that the self-test routine thinks all the ROMs are bad. That's highly unlikely (were this the case, the game would most likely crash), but would be consistent with this hypothesis.

So on to sheets 5B, 6A, and 6B, and 7A of the schematics. If you have a signature analyzer, that's probably a good place to start probing. If not, start looking for pins that are stuck high or stuck low.
 
Thanks for the reply... I'll look at those schematic pages and see if I can figure anything out. About the 12345678 screen, that's the start screen, same as the one next to it... it just seems to randomly pick one of them on boot (the G and I are from _G_ame Over _I_nsert Coin).

Unfortunately, I don't have a signature analyzer, but I do have oscilloscopes, logic analyzers, logic probes, etc, so hopefully I can get some time with it and figure this thing out. It's maddening though... I've been on the lookout for a Gravitar for years, and now I have one and playing it is just out of reach. That seems to always happen though :p . Fixing games is sometimes more fun anyway :) .

Thanks,
DogP
 
Argh... I shoulda followed my instinct. I fixed this tonight, and it ended up being the AVG chip. All along I kept saying that other strange intermittent problems I've had have been AVG chips, but I didn't think these would be AVG related problems. And I was too lazy to pop open my Space Duel to borrow the AVG off it since I couldn't get to it easily, but finally tonight after checking a bunch of signals with a logic probe, and nothing seeming out of the ordinary, I decided to just shut myself up and try the AVG. AND IT WORKED!

Is there anything wrong with the Arcadeshop AVG replacements? I've got some other things to order from AS, so I'll probably order a couple of their replacements as backups and for testing in the future.

Thanks again for all the help!
DogP
 
Show us some Pics Doug! I think AS AVGs are fine also, buddy of mine had put their product in his Space Duel and they worked fine.
 
Heh... since you asked... here's a few pics. Overall the cab is pretty nice, except some water damage at the bottom... and the monitor still needs a few minor adjustments. But the price was right, and I can play it now :).

DogP
 

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Argh... I shoulda followed my instinct. I fixed this tonight, and it ended up being the AVG chip. All along I kept saying that other strange intermittent problems I've had have been AVG chips, but I didn't think these would be AVG related problems. And I was too lazy to pop open my Space Duel to borrow the AVG off it since I couldn't get to it easily, but finally tonight after checking a bunch of signals with a logic probe, and nothing seeming out of the ordinary, I decided to just shut myself up and try the AVG. AND IT WORKED!

Is there anything wrong with the Arcadeshop AVG replacements? I've got some other things to order from AS, so I'll probably order a couple of their replacements as backups and for testing in the future.

Thanks again for all the help!
DogP

Well, I'm sorry I skipped past this thread when it first came up (the little popup box when i highlighted it didn't seem to be something I could help with), but I would've told you to swap in another AVG if you had one as I'd seen similar issues that another AVG fixed...
 
Thanks... yeah, the only Gravitars I came across before this were very reasonably priced ones in CA (several fully working $400 ones), but once you add shipping, it's not worth it :( .

DogP
 
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