NES lacking audio

[K_Devel]

I got an NES here that has no audio, either RF or RCA, I think something MAY have shorted. Originally it had no video either, but I installed a 40-pin socket for both the CPU and PPU and the new PPU produced video, pretty much ensuring that there was some sort of power problem, also it was getting pretty hot. Video fixed.

As I understand it the CPU handles the audio. So I have a donor board for a new CPU, I put that in, still no audio. Makes me think the original board's CPU isn't blown in that capacity.

I was hoping I could get some help, or tips perhaps where I should be looking to solve the audio problem. I have, what I believe to be the audio pin from the 5 pin (pin 2, chip side up) connector of the outside board hooked directly to R2, ensuring there isn't some flim flam with the traces between the power box and the NES board. Is there another chip that I should inspect or perhaps some pins and leads I should check out. I'm not COMPLETELY familiar with the NES system hardware but I've been fixing up quite a few "broken" system and this one is ALMOST there. Just the dang audio left.

[lidnariq]

Touch an external amplifier (such as the TV's audio line in) to the CPU pins 1 and 2. You should be able to hear things there, if it works at all.

[K_Devel]

No audio with either CPU I exchanged, and I know one of them is DEFINITELY not broken. However putting the multimeter to pin 1 and pin 2 I get some voltage out of pin 2 but not pin 1, any thoughts?

[lidnariq]

Because of how the 2A03 generates audio, the DC voltage you see on pin 2 is probably due to the DPCM channel. But the lack of a DC voltage there isn't indicative of anything.

Were you testing on CPUs in known-functioning boards? The voltages coming out of the CPU are probably on the order of millivolts, not 1Vpp as expected by a TV's line in.

[K_Devel]

Nix that, I did get audio out of pin 1 and 2, I derped and didn't use the video input on my TV lolol.. so Im like, oh, wait, bone head. Any ideas on how I should proceed from here?

[lidnariq]

Trace down the circuit until the audio disappears. It'll get real quiet in places.
These two schematics should help:
http://nesdev.com/NESAudio.gif
http://console5.com/wiki/File:NES-001-S ... M,-CIC.png

[K_Devel]

It'll get real quiet in places.

So the quietness isn't an indicator of a problem, just wait until it's gone.

[lidnariq]

Yeah, the signal does some weird things where it's mixed. It should get loud again after.

[K_Devel]

Alright, I've got it down to what I believe is R6, the 47K resistor, I'm able to get audio from the side leading into it, but none out of the other side. I know you said quiet, but I hear nothing. I can get the jumping sounds of SMB, the music all wrapped into there when I touched the audio wire to it. After it, zilch... as far as I know, I cannot hear it. I have hearing issues on top of it, but not like horribly deaf. Is it normal for the audio to sound almost dead at R6 (47K)?

[K_Devel]

Im getting audio out of pin 11 on the 74HCU04P ic

[lidnariq]

More likely the 74'04 inverter is what's dead, or at least that specific gate. Resistors tend not to die ... or at least, if they do, they're usually obviously burned.

The other five inverters are used for the CIC (1-2, 3-4, and 13-12 for CIC CLK, and 9-8 for the power LED) and for the PPU (5-6, producing PPU /A13). If the CIC's already dead/removed you should be able to repurpose one of the other inverters for the audio path.

[K_Devel]

Even if I'm getting audio out of the 11th pin from it?

[lidnariq]

Pin 11's an input. 10's the output.

[K_Devel]

My logic probe just emits a lo squealing tone on pins 11 and 10 on the 7404, but yeah, no output for audio on 10. Just 11, which yeah, I looked up the pinout then was like, oops.

[K_Devel]

I replaced the 7404 with the 7404 on the NES donor board I've been using as basically parts. Still no sound. To say the least, I put a 14 pin socket in so if that chip is fine, I'll just toss it back in. However, I'm not getting audio out of pin 10 here either. Slim to none chance that both of these are suffering the same problem as I am not aware of the donor board having audio issues.. I *do* have some other 7404 chips, I can just toss any 7404 in there right? I got about 2 I accumulated from god knows where.