New to me Gravitar can't start a game

I repair them.

And yes, if you have an original AVG chip on the board, it is almost certainly bad in this case.

Biltronix (above) sells the best replacement AVGs out there, IMO. There are other alternative ones out there that are a little cheaper, but don't have some of the features that Bill's has, like power and activity LEDs, which are nice.

You should definitely replace the one that's there, if it's original. There's a good chance that will solve some, and possibly all of your issues with the board. If it's still not working after a new AVG, then you can send it out for repair.
 
Sounds like a plan! Thanks a million guys! I'll update after I get some parts to play with.
 
These problems can't be fixed and you should sell me that Gravitar for pennies on the dollar. :)
 
Well crap. I was afraid of that. What's you address and I'll get it headed that way. [emoji38]
 
Got it cleaned up a little. This is its best side. Other side is almost as nice.


c5a89ebff4d11b09ca4eb690fb31f84f.jpg


a823d7b703a7525f0174cc0bbb626dce.jpg
 
Well crap. I was afraid of that. What's you address and I'll get it headed that way. [emoji38]

We're in the same state so how about you just delivering it to my house rather than shipping it?

I'll cover pennies on the dollar for your gas!
 
I've seen the noise in test mode, I think it was pokey related. Also had a bw board that would reset after the start button was pressed, which was caused by a bad pokey also. Probably unrelated, but heck given the failure rate of the pokeys, it's worth a shot. Switch the two and see if anything changes.
 
Swapped the 2 pokeys but got no change. Going to order a new avg and possible that buffer referenced early on. See where that gets me.
 
Logic probe and analog scope (and voltmeter) will make repair go much faster. :cool:

Send it out for repair if you get to the point where you're randomly desoldering ICs. Just sayin' ...

Good luck ... Gravitars must be repaired and ultimately played/enjoyed.
 
I was able to borrow a scope from a friend which could be a bad thing since I'm not sure what I am doing. I found a post talking about checking avg 0-13 for activity. Looking at the schematics I checked the corresponding pins on the avg for those and found that there was no activity on avg 6-13. I'm guessing that this means my avg is dead. Any merit to my thinking?
 
...no activity on avg 6-13. I'm guessing that this means my avg is dead. Any merit to my thinking?

Absolutely. The AVG chip takes data from vector generator RAM and ROM (plus some control signals) and outputs addresses on the AVG0-AVG13 lines. So if you have activity at the inputs but nothing on AVG6-AVG13 it's probably a bad AVG chip.
 
Last edited:
Thanks for the reply. I'm brand new at using a scope and reading schematics at the logic level. I'm not sure which legs would be inputs but had some activity on some of the pins other than the ones listed above. Trying hard to learn how to diagnose this stuff.
 
Atari schematics are pretty nice. One, they are legible. Two, they are divided into sections that are labeled with the function that group of chips performs. Three, the inputs to the chips are generally drawn going into the top and left sides of a chip, and the outputs out from the right and bottom. This is not always true, so you should look up the chip's datasheet to verify that (and to learn what the chip is supposed to do).

There's no easy to read datasheet for the custom Atari AVG chip, but if you want you can check out the "Stack and Program Counter" box on sheet 3A of the Tempest schematics to get an approximation of what's inside it. It's there you can see why Atari made the AVG: they needed an extra bit (DVY12/AVG13) and there was no bandwidth left. The 11 chips (would be 14 with the extra bit) and the space they took up on the board made it worthwhile to create the custom AVG.
 
Back
Top Bottom