NES lacking audio

[lidnariq]

Not all 7404s will work. You probably need at least 74HC, maybe even HCT or HCU. Older LS, S, F, or letterless ones definitely won't work.

[K_Devel]

man, I have two 7404N's. Nothing newer. So you were saying that if I disabled lock out I could then use, or attempt to use the inverter of that function instead. I'm completely boggled right now as to why two chips aren't working, I cannot get a definitive "yup it's broken" other than yup, it's not putting out audio to go on to what I see it as, FC1 and then C4 which brings it right to the 5 pin to the outside board. So close.

Edit: I meant to ask, what two pins were the lockout using? I'm having difficulty locating where it goes atm.

[l_oliveira]

Apparently Nintendo only used Toshiba "H" series on the Famicom. Because they're not using ports from the 74368s on the NES for the audio amp (because they had to add a 7404 for the CIC) they ended using a 74HCU04. I think if you find a 74HCU04 you can use parts from a wider range of makers.

Unfortunately it's as lindariq said. Most logic ports won't operate on analog levels under the circumstances "induced" by the NES circuitry.

[lidnariq]

Edit: I meant to ask, what two pins were the lockout using?

The lockout chip actually uses four of the inverters in the 74HCU04 (one drives the LED on the case).

The inverter from pins 1-2 generates the clock source the CIC, and is the one that I stole for my "CIC removal and replacement" instructions here. The PCB already connects 74HCU04 pin 2 to pins 3 and 13 (to produce a buffered clock source for the CIC and for the CIC in the cartridge, so you'll either need to lift pins or cut traces to get them out to reuse as an audio amplifier.

[K_Devel]

I've got some new HCU 7404's coming, same state as me too, should be here soon. Got something cheap and local. I'm hoping replacement will fix this, under the strange and unlikely case that both of my NES pulled 7404's are dysfunctional at pin 10 to 11.

http://www.digchip.com/datasheets/parts ... 4N-pdf.php

Should work, right? If not, I'm gonna need some direction, because I'm totally lost if a viable brand new chip replacement isn't gonna fly. I don't want to too heavily mod the CIC area, I'm fine with the pin 4 pull up to make games load more effectively, as that's really the important thing, playability. I'm trying to not distance myself too far from the original NES setup as it is, ignoring CIC pin 4 mod cuz that's just wise to do and then progressing to a stereo audio mod because that seems cool. I liked the output I've heard from people who've done it.

[lidnariq]

It is quite odd that both boards would have had just that one inverter die in the 74'04, and not had another one fail and so break the CIC, power LED, or video output. I suppose it's possible that you might have killed (caused to become a short) the 10nF and 1nF capacitors on the edge between the console and the RF daughterboard or the first BJT inside the RF daughterboard.

Does the logic tester indicate the voltage on pin 10 is always logic low or logic high?

It's worth noting that if you are going to do the dual-mono mod† there's no point in rebuilding the audio path here, because you won't be reusing ≈any of it.

I forget: do these machines work in every regard other than audio?

† caution: the dual-mono mod gets some people here to emit rants.

[K_Devel]

Well the stereo mod is not completely on the table with THIS system yet, I might, I have better boards to muck with. I'm not sure both inverters are dead, the board the 2nd inverter came from is a parts board and is no longer in a capacity that it would be able to test anything on. I could, use my GOOD board, and change out the inverters believed to be broken and see if I can produce audio with it. I'll have to wait till the morning though, I just now thought to do this. I can add the 14 pin socket and make it all exchangeable on my good board I'm using for a possible handheld mod. I am interested in making the board in question work by default before I decide where I'm going with it.

I just feel so close to finishing this one up, I'll test capacitors you mentioned but like I said prior, I get audio out of pin 11 on the 7404, just not pin 10. I'm not sure right now if the capacitors you mentioned are before or after the chip. I'll get some more information about the now deemed "broken" 7404's pushed into a known working board and go from there. If they produce audio, then I suppose they're not broken and further diagnosis needs to occur outside of the 7404.

[lidnariq]

The other parts I was contemplating could have been dead would be the 0.01µF capacitor labelled C4 on the mainboard, the SMT BJT inside the RF modulator (labelled LC), or the brown SMT capacitor next to that BJT.

My NES is a NES-CPU-07 with a Mitsumi RF (J82TC? E210034?) modulator, things might vary slightly for you.

[K_Devel]

This is an NES-CPU-05 CPU w/ a NES-CPU-08 PPU, which worked to fix the video. As far as I can tell, the audio, because it's not reaching outside of pin 10 of the 7404 (presumably) that it's not reaching FC1 or C4. The trace from pin 10 on the 7404 goes directly to FC1 then C4. I would *think* that if the next steps had any faults to diagnose at this point, pin 10 would be functionally putting out audio first and I won't know the state of FC1 or C4 until pin 10 is outputting audio, On my WORKING system, pin 10 does in fact produce louder audio than pin 11, so, great, we know once I switch this chip, pending there is no OTHER board issues, that this should work to then further test the connection from FC1 and C4 and onwards into the outside board. Getting to it now, gotta clean up my room first, such a mess from pulling things apart lately.

[K_Devel]

WORKING chip is NOT working on the other board, verified. Something is up with the transfer between pin 11 and 10. Just gotta figure out what exactly. If they were somehow connected together by some accident on my behalf, then I assume both pins would play same audio level due to it just not going through properly, so there has to be something else. The board is not burned either, its in pretty good shape around that area.

[K_Devel]

I also verified the 7404 I was working with on the original board with audio issues works, I put the 7404 into the functioning board I swapped chips with and added a ic socket from this morning. Still not sure why there is an issue between these two pins on the board level now.

Edit: I have also replaced C20 from the parts board I'm pulling from. I get audio from the entry part of C20 but not the exit which is suppose to come up from pin 10 from how I'm seeing things which just further reiterates there is a problem between entry point A and exit point B from the IC but the IC itself is completely functional as it DOES work in another board just fine.

[lidnariq]

So, the inverter there, in combination with the 47kΩ resistor, makes an amplifier. Pin 11's the input, and should be quieter than what you see at pin 10. Since that's not the case, and swapping out the inverter didn't fix things ... it implies there's something wrong attached to the output.

Looking back at this reverse-engineered audio path: http://nesdev.com/NESAudio.gif there aren't very many places that could be broken. If one of the three components I mentioned has shorted out, that would explain what's going on ... but beyond that, I'm kind of at a loss.
You could try temporarily removing the 39µH inductor (the green blob), and see if the inverter now works as intended once that's gone.
You could also use the multimeter to measure the average voltage at pin 10, or once the machine is off measure resistance from pin 10 to +5V and ground.

[K_Devel]

You could also use the multimeter to measure the average voltage at pin 10, or once the machine is off measure resistance from pin 10 to +5V and ground.

voltage at pin 10 is about 0.16 as is pin 11
pin 10 to 5v ground resistance is 4.2

[lidnariq]

pin 10 to 5v ground resistance is 4.2

What? Is that the resistance from pin 10 to +5V (pin 14) or to ground (pin 7) ? And what units? kΩ? Ω?

[K_Devel]

pin 10 to 5v ground resistance is 4.2

What? Is that the resistance from pin 10 to +5V (pin 14) or to ground (pin 7) ? And what units? kΩ? Ω?

Ω It's 4.2 from pin 10 to ground pin 7