I have an NES with picture, no sound.

I am wondering if anyone has had to troubleshoot the audio circuit on an NES?
I have tried the R/F cable and a set of A/V cables on this system, which both work on another NES I have so it is not a cable issue.

There are two pins, pin 1 and pin 2 (AUX0 and AUX1) on the U6 CPU, which, though a resistor network, provide the audio input to the circuit.

Does anyone know if the 72 pin connector can cause this issue? I am not sure how the cartridge gets the audio and video generated, internal to the system. Any help in diagnosing this circuit problem is greatly appreciated.

Well, since this posting, I swapped 72 pin connectors, both recently new, between this system and a working system. THe problem stays with the system, did not follow the connector. Looks like I will have to get an oscilloscope out and compare the bad system with a good one, both with Super Mario Brothers playing. Once I figure this out I will post my findings.

I repaired many of these Nintendos with sounds problems and it's very easy if you can solder. First you have to remove the RF board from the main mother board then remove the top metal cover from the RF board on the board there's two glass diodes orange with a green stripe and have designators marking them D1 and D2 but that depends on the model of the RF board on other boards they can be D5 and D6. Desolder those two diodes and replace them with part number 1N5819. I also recommend replacing all capacitors while having the RF board removed because bad capacitors will cause sound, video, and power issues. Ive never seen capacitors go bad on the mother board but very often on the RF board especially the 100uf 16v in the sound circuitry. I'll post links to photos of a rebuild I did were I had to replace the diodes. Everything in the photos marked with yellow dots are components I replaced.

http://i1080.photobucket.com/albums/j331/dimav09/Mobile%20Uploads/20150402_122454.jpg

http://i1080.photobucket.com/albums/j331/dimav09/Mobile%20Uploads/20150402_090345.jpg

http://i1080.photobucket.com/albums/j331/dimav09/Mobile%20Uploads/20150402_090358.jpg