Asteroids Video / Vector Fault

[ataridude74]

I've got a video / vector fault with my Asteroids PCB.

It's not the monitor, PSU etc as it all works fine with an Asteroids Deluxe in the same cab.

The game plays fine, sounds work, coins up etc. When it first comes on the problem is worse, it takes a minute or so to stabilise.

The vectors are all screwed up with horizontal tearing on the Asteroids in particular, strangely the player and particularly the alien ships aren't affected as much, but the test pattern is, as is the text.

Photos below, final one is the test pattern. Sorry about the reflections, not easy to photograph a vector monitor.

 

[1bradders]

So the problem stabilises after a few minutes/"the game warms up"? No visual issues after that?

 

[Nes4life]

Interesting issue. Check the LS374 at D10.
It's an x-axis issue for sure; and appearing as an issue on one of the most-significant x-axis data lines. After that I'd check x-axis DAC and then go to x-axis data counters. When checking the DAC, be careful not to accidentally connect the in data lines to the out data lines; otherwise you'll put a higher voltage on the in-lines and likely fry an IC.

If you're unable to see an obvious error using a logic probe or oscilloscope (e.g. a line isn't permanently being held high or low), then you can desolder an IC, fit a socket, desolder it's Y-axis counterpart, fit another socket, and swap them. If the issue changes then you've found your IC. I typically do that with the DACs anyway; but really hope the DACs are fine for you! Also, I do most of my testing with the test pattern on.

 

[ataridude74]

It's taken a while... but I have tested every IC in the X position circuit with the scope and they are all showing good 5v peak-to-peak logic signals. Whether those signals are correct I can't tell, but nothing that should have a logic signal is stuck high or low. I've also swapped the DACs, handily in sockets already, no change.

Weirdly one time firing up the issue was worse with video concentrated into 2 or 3 narrow bands along the X-axis. I flipped it into test and back again and it reverted to the previous fault as per the photos above, so I couldn't capture it.

I'm still thinking it's something in the X position circuit, unfortunately my only option now is to shotgun it. 4 ICs are already socketed (3 x LS157s test good in the Minipro), so only leaves 5 more ICs to socket and swap. Possibly 6 if I include the LS174 @ F10 although that's shared with the Y position circuit so that'd be a last act of desperation.