Ultra Pong not switching to 2 player mode

[drurywoodson]

I have an Ultra Pong that is mostly working, but it is stuck in the 4 player mode and will not switch to the two player mode no matter how many paddles are not plugged in.  Also, the Right B paddle will not move the cursor.

 

Here is what I have tried so far:

1)  All 4 of the paddles will work in any of the Left A, Left B, or Right A jacks, so I know it is not a problem with the paddles.  They all range from about 0 to 800 kOhms.

2)  I have replaced all four of the paddle jacks.  The jack in use switch is good in all 4 jacks.  Each of the 4 jacks can read the paddle potentiometer.

3)  The paddle sense line at the back of the jacks is VCC if any of the 4 jacks are unplugged, and is disconnected from VCC if all 4 paddles are plugged in.

4)  The signal from the right b jack is present at pin 13 of the pong chip, and I can correctly read the resistance value from there.

5)  I changed out the chip with one from a water damaged console, and I get exactly the same problem with the other chip.

6)  The 68nF cap hanging off the Right B line tests as 71 nF, which is within spec.

 

 

The circuit is really simple, and I am out of ideas.  Does anyone have any suggestions?

[drurywoodson]

I have done more research, on it, and it is really weird.  With everything turned off, the paddle resistance reads properly in the circuit.  When I apply power, the paddle line is stuck at a high voltage.  I would guess it is a problem in the pong chip,  but I have two of them and they both do the same thing.  Has anybody else seen this failure mode?

 

 

[OmegaMax]

Check the electrolytic capacitors. Electrolytic capacitors can age and fail over time. 

[drurywoodson]

This product doesn't have any electrolytics.  I am using a modern external power supply.  The 68nF capacitor on that particular line measures at 70 nF.