Had a 7800 come into the ITC lab to get refurbished, and upgraded with a few things. Among the updates was to get a UAV plus mount board setup as I've been doing for over a year now.

This is where it is important that you always...ALWAYS test the current status of electronics before you dive in and start doing stuff. A rule I didn't follow so what I discovered after putting all this work in, isn't something I could verify was an issue before the work started. I had been told that it was working 100% without issue as it was the daily driver 7800 for the owner. 

So what was the issue I discovered? Well, after installing the UAV setup and the 10pin mini din AV connector to test the system. I saw something odd when I ran Ballblazer for burn in testing. What I saw was odd additional background graphic information on the far left and far right sides of the screen that I'd never seen before? More specific...this is what Ballblazer looked like:

So if you look on the left and right hand side of the image, you will see what looks like the blue in the sky being extended out along with some burgundy like brown/red below that. Now oddly enough the 5200 version of the game through a UAV actually looks kinda like this, but on the 7800 this isn't normal. Here is a closeup detail of the effect and you can see it is more like a checkerboard pattern within those extra graphics being shown.

Making matters more confusing was that NOT everything had this effect going on. Ballblazer seemed to show it the worst but any game using a colored backgrounds would have this kind of effect going on. But anything using a black ground was fine as this color hue test screen shows from the 7800 utility program:

I did figure out pretty quickly that the issue had to be something with the Maria IC because this same effect wasn't happening on 2600 games and was only present on 7800 titles. So a few days of delay (Waiting on a new heater for my desoldering gun), I removed the original Maria IC, installed a pair of new 24pin sockets (As I don't have 48pin on hand). And installed a different Maria pulled from a parts donor 7800. And what did ball blazer look like after that?

It worked! So yes after nearly 150 Atari 7800s that I've worked on, this was a first. My guess is that this 7800 always had this issue but as it was mainly used on a CRT and was only present on 7800 games, it likely wasn't noticed or the owner simply thought it was normal. In any event the original Maria was working in that the actual game play area on the screen looked normal otherwise. But the blanking to mask the extra graphics junk on the sides, wasn't working properly on this Maria and only by swapping it out with another was the issue fully resolved.

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