Left Joystick Port troubleshooting

[robcatron]

Hello, all

 

I just finished my composite A/V mod on my 7800 that I have for several years. I had been unable to play the 7800 because the RF port had gotten broken away from the motherboard. I figured I wasn't going to use it anyway, so I just did the A/V mod (which was simple) but when I started it up, I noticed immediate left joystick port issues. Once any game is inserted, you can immediately tell the system is in a left joystick down position state. River Raid is a good test. All movements works, but the ship is stuck in slow flying deceleration mode). Most other games, like Ms. Pac-man for instance, you character just goes straight down and gets stuck there (depending on the title).

 

Doesn't matter if there is a stick inserted or not - the system just automatically forces the left controller in the down position.

 

After a search of these forums, I found that resoldering the pins to the port on the motherboard may fix it, they looked good, but I resoldered them anyway. That didn't help. There is a bit of corrosion (looks like rust) at the solder points and on both ports pins.

 

What are the next troubleshooting steps? Could there be another piece on the MB that may be unsoldered that would cause that situation?

 

Thank you

[+Stephen Moss]

From the circuit diagram available here on AA the direction control for the controllers is wired to U9 (6532).

 

First, with the power off I would check the continuity between pin 13 of U9 and pin 2(?) of the controller port to ensure there is a connection between them, if possible try to make the connection on the IC and socket legs directly as opposed to the PCB solder pads. If you are not sure which pin is pin 2 it should be the second one in from one end on the top row (alternatively follow the PCB track round to identify where it is connected) .

 

Second, check the continuity between pin 13 (Down) and pin 1 (0V/GND) of U9.

 

Finally, with the power on measure the voltage at pins 8 - 15 of U9 with no controller connected to either controller port.

 

Let me know what values you get.

Edited February 13, 2017 by Stephen Moss