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Atari 7800 - Self activating fire buttons?!



I received an A1 series 7800 main board in the mail recently where the owner stated it was working properly but was missing a pause button for some reason. Well, when I receive a 7800 system for services, one of the things I do is to pull up a test utility without any controllers plugged in. The reason for this is to ensure that the system controller ports are working properly.

But you ask... "How does not having any controllers plugged in, help to know if the ports are working properly?" Excellent question!

You see the TIA controls the trigger functions of the joysticks and depending on the mode the console is loaded up in, the components in use differ a little. But here was the issue with this one that might help explain things a little better...

The issue was after powering it up without a controller plugged, it the menu selector of the utility kept selecting the first option, exiting, entering...exiting over and over. This behavior was as if the fire button was being held down. But without a controller plugged in?! Quickly using the select and reset buttons on the console I was able to get into the controller test screen and this screen was telling me that not ONLY was the first fire button being pressed, but both fire buttons were showing as being pressed down. It was showing the same behavior on controller port 2 as well. 

Then I plug in an actual controller and.... it stops auto selecting options and now shows the fire buttons aren't being pressed. Pressing the fire buttons would activate normally. In other words with a controller plugged in, the console seems to work exactly as it should. But that doesn't mean there wasn't a problem here because technically without a controller plugged in, it shouldn't read anything at all. Plugging a controller into port 2, would also remove the condition on port 2 and make everything appear to be fine. So what is going on?

It turns out that the 7800 is pretty sensitive to the readings it gets from the controller ports and any deviation beyond a certain point will confuse the console. To correct the issue on this 7800 I had to replace at least 2 components and also replaced a 3rd for good measure. The main culprit was unfortunately the TIA chip. Yeap... I replaced the TIA because in most trigger issues the TIA is my first item to blame and usually that is correct, but it didn't solve everything in this case. I did confirm the TIA was faulty using my chip testing as it was showing an immediate fault with the Inputs and audio portions of the chip. Replacing the TIA solved half the issue. Basically controller port 1 was working properly now, but I was still getting errant trigger readings from port 2? This ended up being due to a marginal resistor located at R35. There is a pair of them that control each of the port readings in 2600 mode that each measure 220Ω. The one at R35 was only measuring 217Ω and while that should have been good enough, it was causing odd triggering issues. When I removed R35 from circuit, then the 7800 port 2 buttons were now working properly but of course, no fire button action in 2600 mode testing. So I replaced the resistor with a used one from another 7800 donor board. This replacement measures 221Ω and is much closer to the 220 reading than the original. Installed it, and sure enough everything is working properly again. There is also a 3906 transistor in this mix and it was also replaced out as they are cheap to have on hand and I've a few. But the two main issues were the TIA and this slightly out of spec drifted resistor at R35. Here is a pic of the console where I've outlined the components mentioned.



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