Atari 400 fire button won't work on P1 controller port

[acromite53]

Hi everyone, I finally bought my first game for my Atari 400 and to my disappointment, I discovered that the controller button won't activate when pressed. The d-pad / joystick works fine though. P2 works just fine with all buttons. I opened up the console to check the connections. I checked the continuity from all the controller port pins to the board and all go through fine. There are no broken traces or corrosion anywhere. It seems that pin 6 is used for the action button so I followed it to the big row of resistors but didn't see anything wrong.

If anyone has experience with these and would like to share info, that would be very helpful. Thank you!

[SoundGammon]

I believe the GTIA chip handles the fire buttons for all 4 controllers. If you have a spare, replace it and see if it cures the problem!

[+Nezgar]

Or even just reseat the GTIA if socketed.

Edited June 2, 2018 by Nezgar

[+DrVenkman]

I believe the GTIA chip handles the fire buttons for all 4 controllers. If you have a spare, replace it and see if it cures the problem!

 

 

Or even just reseat the GTIA if socketed.

 

Indeed. Hopefully re-seating the chip resolves the issue. GTIA's do go bad - I've had two fail on my various machines over the years.

[Rybags]

Since it's a 400 it's somewhat harder to test such things with the machine powered up. There's multiple possible failure points but luckily the overall circuit for the fire buttons is pretty simple. Pin 8 of GTIA is input for TRIG0 - you can test by grounding it though on a 400 not so easy to do when powered up.

 

Since you have to pull the CPU card to get to GTIA you may as well clean it's edge connector as well. If you apply a bit of contact cleaner to the card then insert/remove a few times into the motherboard it helps clean the semi hidden contacts in the slot as well.

 

Rather than running a game, a quick Basic program can show you fire button state:

10 ? STRIG(0) : GOTO 10

[acromite53]

Thanks everybody for the suggestions. I reseated all the chips on the CPU board (I didn't know which was the GTIA). I cleaned the connector of the card while I was at it. Put it all back together but still no response.

Then something strange happened. I opened the lid while the system was on to change a game. This turned off the system and then resumed once the cartridge was in and the lid was down. Now I'm getting a bar of wavy blue and red line that scroll up the screen. They disappear when the fire button or a diagonal button is pressed.

 

 

EDIT: looks like after turning it off and on a few more times, the line is gone

Edited June 2, 2018 by acromite53

[acromite53]

Since it's a 400 it's somewhat harder to test such things with the machine powered up. There's multiple possible failure points but luckily the overall circuit for the fire buttons is pretty simple. Pin 8 of GTIA is input for TRIG0 - you can test by grounding it though on a 400 not so easy to do when powered up.

 

Since you have to pull the CPU card to get to GTIA you may as well clean it's edge connector as well. If you apply a bit of contact cleaner to the card then insert/remove a few times into the motherboard it helps clean the semi hidden contacts in the slot as well.

 

Rather than running a game, a quick Basic program can show you fire button state:

10 ? STRIG(0) : GOTO 10

 

To ground that pin, could I run a wire from that pin to the metal shielding on the system? Then I could see if it is registering my button input?

[+Nezgar]

You should be able to register it by shorting pin 27 (ground) to pin 8 (Trig 0)

 

Pinout: https://user.xmission.com/~trevin/atari/gtia_pinout.html

[acromite53]

You should be able to register it by shorting pin 27 (ground) to pin 8 (Trig 0)

Pinout: https://user.xmission.com/~trevin/atari/gtia_pinout.html

I'm looking at the CPU card trying to find the GTIA. I'm not sure which one it is. There are 3 40 pin chips.

[Rybags]

CPU board, as pictured in this link https://commons.wikimedia.org/wiki/File:Atari-800-Expansion-Board-CPU.jpg

 

40 pin chips right to left are Antic , GTIA, 6502. Goto this page for serials and pinouts of the major chips https://www.atarimax.com/jindroush.atari.org/achip.html

[acromite53]

I had some time to test this fully disassembled. I tried grounding trig0 to pin 27 but couldn't get the fire button to register. Does this tell me that the chip is bad or that something else is going on? Thanks for all the help guys. I'm learning a lot.

[Rybags]

Make sure you have the right pins. Pin 8 is Trig 0 (port 1), 9 thru 11 are for ports 2 thru 4.

 

Try another as a test case, just change the program from earlier, e.g. 10 ? STRIG(0);STRIG(1);STRIG(2);STRIG(3) : GOTO 10

 

And remember pins run anticlockwise around the chip, start counting with the notch in the chip on your left, from the bottom 1-20 then top is 21-40.

You could select a GND source from somewhere else, e.g. exposed ground plane from another place - CPU board doesn't have a very big one and it looks like it might be laquered over.