NES motherboard with a strange problem.

Here I have a NES that I cleaned because as usual the connector would not work. Routine procedure for me.

I test the cleaned machine and the game boots as usual, however there are two problems.

1.There are some interferences on the screen.

2. The biggest problem; controllers wont work on this board. In fact I managed to get the game to start at some point and it is like every button on the controller is being mashed at Gatling gun speed, A, B, D-pad, Start Select, it is like everything is on permanent turbo.

I did the basics of testing with different controllers and different Controller ports. the problem happens with both controller inputs. I checked continuity on the motherboard and everything seems good. Ground and +5v at the controller ports is good. I studied the NES controller input schematic and probed around with my scope.
I compared with a healthy board and may have found some anomaly




Clock line seems good. Look similar from the good and the bad board
Latch line seems good too but somehow it looks as though it may not peek high enough on the bad board, so maybe it is not detected by the controller. ( I am not the best at using a scope so I wonder if I am missing something. )
From what I understand, the Latch signals the controller to start polling data from the buttons.

The Data line is pulled high when there is no controller connected. This happens on both the good and the bad motherboard.

However, on a healthy board when you connect the controller you can see the data being sent with the scope as you press de keys. On the bad board it is pulled low permanently, just a flat line.

The data line connects to 74HC368.
The latch line seems to connect directly to the CPU. If the latch signal is indeed bad, it would means the console CPU is damaged.

I could try to swap it with the CPU of another board to verify this but somehow I wish I could do a diagnosis with measurements only before starting experimental surgery.

 

Update: Ok I did some calibration and more thoughtful readings with my scope and I can confirm the Latch signal coming out of the CPU is abnormally weak.

It is pretty weak by TTL standard even on the good console where it peaks a bit over 3 volts.
On the bad board however it is more like 1.25V definitely should be too low to trigger logic circuitry. This is measured directly on the CPU pin so cannot be explained by by bad traces or cold solder joints.

Either the CPU has a fault or something is leeching the signal. I an tempted to lift that pin.

Edit: Just lifted the CPU pin connected to controller latch. Even completely isolated it still peaks super low. What if I amplify it? hmmm.