Player 1 port: no down input

[Kaiju]

Hey guys. Odd issue, fired up my 7800 and noticed none of my controllers (tested several) register the down input on the player 1 port. 
 

tested player 2 (wizards of wor) and that works fine. I assume it’s an issue with the left 9pin? Is it something one would be able to troubleshoot, fix or replace on their own? 

[+DrVenkman]

Could be an issue with the connector itself, but that’s easy to check. Look to see if you have any bent or broken pins on the connector. If not, the problem is elsewhere. If I recall correctly, there are filter capacitors and Zener diodes on each direction signal, and then the circuit goes directly to input pins on the RIOT chip. If you are experienced with electronics repair, it should be easy to ID the appropriate diode and capacitor and determine if they are failed. If so, replace. If not, the RIOT will need to be replaced. Unfortunately, while those chips are plentiful, most 7800’s have soldered chips. Replacing them takes some time and some degree of skill and experience. 

 

EDIT: here is the schematic.

 

[Kaiju]

Thanks for your time. 
 

Guess I’ll break out my meter and watch some videos ?
 

Question, if the riot or cap is the issue wouldn’t it effect player 2 as well? Or is that separate 

Edited March 25, 2021 by Kaiju

[Stephen Moss]

17 hours ago, Kaiju said:

Question, if the riot or cap is the issue wouldn’t it effect player 2 as well? Or is that separate 

If the entire ROIT chip was down, i.e. no supply voltage then it would affect both players, as it only affects on player the problem is likely to be localised to that one input pin, it could be a dry solder joint on either the controller port pin or RIOT pin or possibly a track break where the controller port pin solder pad joins the track.

Alternatively, it could be an internal fault with the RIOT such as a failure of the bond wire that connects the external pin to the silicon or with the silicon itself. 

 

As you say it is not registering Down rather then permanently registering it, I would think it unlikely to be the result of a fault of any zenner or cap on the input line.

If the voltage on the relevant pin of the RIOT is 5V without the Joystick in the down position and does not change to 0V with the Joystick in the down position then a bad solder joint/track brake would be the most likely issue.

However, if the voltage on the RIOT pin does change from 5V to 0V when the joystick is in the down position then the issue is more likely to be an internal fault with the ROIT.

[juansolo]

I had a similar problem with a 65XE where two directions didn't work on the joysticks but I could find no other issues. Turned out to be a dodgy PIA, swapped that and it was away. If everything else is working and the signal is getting to it, as @Stephen Moss says, it's likely the IC.

Edited March 27, 2021 by juansolo