Asteroids vector generator debugging

ELutz

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Let me preface.....I suck at vector motherboard repair. The vector generator section kicks my ass. Anybody have any debugging tips they want to share. I've got an Asteroids I've been putting off....it's watchdogging. I pulled the DAMGO signal at L6.....resetting stopped, game played blind. So, my problem lies within the vector generator. I have diagonal lines running across the screen. In trest mode, I get repeating beeps...that never stop. Grounding the watchdog disable causes a constant tone (same in test mode or not). The vector ROM is good. What next?...vector RAM? Thanks!


Edward
 
RTFM... ;)

The beep tones on those old Atari boards can help you ID bad RAM chips.
 
Vector reset problems are very hard to solve....due to the looping nature of the logic. There is no magic troubleshooting technique to my knowledge. Sorry
 
Check the two LS157 inputs closer to the edge connector - on top of the regular control panel inputs, one of them deals with the HALT signal and causes watchdogging if its bad. If you have a Fluke you can test this easily (check memory map for the addy and bit).
 
I recommend, a logic probe, schematics, some sorta alcohol based beverage, and more hair than you can pull out in a night.
 
When I was by earlier I gave it a stern glance. Did that scare it into working?
 
If anyone's keeping track of this.....problem fixed. Bad bi-polar PROM at location C8 (well, that fixed the vector generator issues....image was also about 4-5 inches from the bottom of the screen and wouldn't adjust out anymore. This was a bad TLO82 at location A12).

Edward
 
I need better light in my garage

I neer got a good look at my board that was doing the same thing (watchdogging). There are missing chips at K4 and I4.

I'm embarrassed to even post this because I should have seen it. I have 4 boards that have various issues, so I got frustrated, but looking helps!

Also, looking at the schematic when you have a specific problem helps too! Sometimes I (we?) get a little frustrated though when too much shit piles up!

You guys are great. Thanks for all the help!
 
I've had 2 of those proms be bad before and cause all sorts of problems. I now put a probe on it, and if it's socketed pull it to check.

I've seen alot of failed proms lately, especially on Black Widows, and Gravitars.
 
nice work !!! what made you suspect the Prom ?

Well, I'd like to say I knew what I was doing......but:)....
Actually, after probing like crazy with a logic probe, chasing bad signals around in circles.....I pulled out the logic comparator. Everything I suspected looked good, and the PROM was in this mess. The PROM on this board was soldered in, so I didn't swap it immediately.

Edward
 
Will a logic comparator work on a prom? If you had the right one to compare it to...it might.

I'm not for sure. I didn't use the comparator on the PROM. I used it on the group of chips I was running around in circles with, while using the logic probe. The logic probe sent me in circles (no surprise).....so I pulled out the logic comparator....all the TTL chips checked out good with the logic comparator......the PROM was also in this "circle", though, I didn't try the logic comparator on it. I decided to desolder it and swap in a good one. At this point in my trials and probing....it seemed like a likely suspect. Sorry for the confusion.....though....you all have me wondering....I think I'm gonna dig the bad PROM out of the garbage, throw it back on the board, and check it with my logic comparator. See if the comparator will work with a PROM.

Edward
 
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