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PostPosted: Fri Nov 16, 2018 9:53 am 
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Hi, I finally could put my hands at a japanese Gimmick! cartridge.
It's a nice game but I wasn't getting the aditional audio out of my clone, a Phantom System.
I traced it's audio path and thats what I've got:
Attachment:
File comment: Audio path Phantom System
Phantom System audio path.png
Phantom System audio path.png [ 6.36 KiB | Viewed 1215 times ]

As seem on the schematics, it had a connection to the cartridge pin 54.
I tried to route the external audio to this pin inside the adapter and got no external sound, even with a resistor.
I ended cutting the trace of the pin 54 and routed to the negative pin of the electrolytic capacitor, with a 100k resistor inside the adapter.
Sounded fine to me. Now I think that small video would help to show how it is sounding... I'll do it as soon as possible.

My doubts are:
1- Is this mod correct? I mean, won't It fry something on the long run?
2- How different is this mod from the original NES mod recommended in many places? Sorry, I didn't find the schematics for that...
3- Would an simpler adapter mod, without the need to mod the console be possible? How?

Thanks in advance!


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PostPosted: Fri Nov 16, 2018 12:15 pm 
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If it is possible to add something between the cartridge and the console, and pass the audio through that, maybe.

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PostPosted: Fri Nov 16, 2018 12:29 pm 
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Quote:
pin 54.

45 rather?


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PostPosted: Fri Nov 16, 2018 4:34 pm 
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Is this a 72-pin connector (NES) or a 60-pin one (Famicom)?

Fisher wrote:
1- Is this mod correct? I mean, won't It fry something on the long run?
In that schematic, pin #54 there is directly connected to the output of the inverter-used-as-amplifier. I agree that it makes me uncomfortable; I'd really rather have some explicit mixing impedance there. That location is what's pin 3 on the official NES expansion port, where it's audio out.
Quote:
2- How different is this mod from the original NES mod recommended in many places? Sorry, I didn't find the schematics for that...
The standard NES external audio mod connects two pins on the expansion jack (EXPx to AUDIO IN). This is equivalent (per the official Sharp TV schematics) to a 22kΩ+≈47kΩ resistor mixing in to the junction of R1 and R2
Quote:
3- Would an simpler adapter mod, without the need to mod the console be possible? How?
Something like a Game Genie could be modified connect the cart audio output EXP pin to an RCA jack.


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PostPosted: Fri Nov 16, 2018 6:20 pm 
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Sorry, I didn't told exactly wich NES clone it is...
It's a Phantom System, a nice 72 pin Brazilian clone.
The same one that I was helped to fix on this thread.
I even could get its full schematics, that I checked against the board and confirmed it's OK.
I even posted pictures of its naked board on that thread, just like the ones here and here.
And yes, part of this mod is done inside a 60=>72 pin adapter.
Would be nice if I could only mod the adapter and don't touch inside the console's guts.


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PostPosted: Fri Nov 16, 2018 6:59 pm 
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Oh, sure. Connect AUDIO OUT from the 60-pin side of the adapter to an external RCA jack. You'll have expansion audio on one jack and the 2A03 audio on another, so there'll be a weird stereo separation, but that's certainly the simplest option.


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PostPosted: Fri Nov 16, 2018 7:02 pm 
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Combining the signals externally should be easy though, right?


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PostPosted: Fri Nov 16, 2018 7:05 pm 
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They can be mixed externally unless the console's output is through RF.


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PostPosted: Sat Nov 17, 2018 5:34 am 
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Looks like the easiest way is external.
Maybe the "best" (or less worse) approach sould use an male RCA that plugs on the console and goes to the cartridge's audio in and plug the cartridge's audio output in another cable with a female RCA that becomes the new audio out, letting the cartridge be responsible for the audio mix.
I think that is similar to what happens inside the original famicom.
Unfortunatelly I will lose the extra audio through RF, that I still use with some very old TVs I have access.
lidnariq wrote:
This is equivalent (per the official Sharp TV schematics) to a 22kΩ+≈47kΩ resistor mixing in to the junction of R1 and R2

So by cutting the pin's 54 track and plugging it to the R1 and R2 junction (or the negative pin of the electrolytic capacitor as I stated before) I've done almost the correct thing.
The only thing I got wrong was the resistor value, (100K) but that was just a wild guess.
Maybe a 62k or a 68k resistor would give me best results.

If I duplicate the circuit formed by C1, C2, R5 and the inverter gate inside the adapter and plug it on the unmodified pin 54 would I get good sound results or just distortion?
Anyway, thanks for the information. You guys are always great!


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PostPosted: Sat Nov 17, 2018 8:00 am 
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Oh, here's what I was thinking of (except that the cable would be installed in the 60-to-72-pin converter): http://callanbrown.com/index.php/castle ... icom-audio


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PostPosted: Sat Nov 17, 2018 11:18 am 
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Fisher wrote:
The only thing I got wrong was the resistor value, (100K) but that was just a wild guess.
Maybe a 62k or a 68k resistor would give me best results.
Many people use a potentiometer to allow for fine tuning after the fact. 47kΩ is just a "typical" value I've seen used, but I know I've seen people complain about it producing a mix that's too loud or too quiet.

Quote:
If I duplicate the circuit formed by C1, C2, R5 and the inverter gate inside the adapter and plug it on the unmodified pin 54 would I get good sound results or just distortion?
Pin 54 is a low impedance output from the inverter; it's not really a great place to mix in audio. For one, as you've correctly identified here, you need another low impedance driver to even stand a chance of injecting audio. For another, it'll cause both the original driver and your new one to dissipate more heat. Finally, this design is a 100x amplifier; you'll need to drastically attenuate the input signal for it to not distort.

But ultimately, it probably will work.

Here's a picture from Falstad's circuit simulator, and the netlist I used for it:


Attachments:
File comment: http://falstad.com/circuit/
two-inverters-as-amplifiers-mixed.txt [1 KiB]
Downloaded 21 times
two-inverters-as-amplifiers-mixed.png
two-inverters-as-amplifiers-mixed.png [ 3.32 KiB | Viewed 1048 times ]
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PostPosted: Sat Nov 17, 2018 2:51 pm 
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tokumaru wrote:
except that the cable would be installed in the 60-to-72-pin converter

That was more or less what I meant, I just didn't say the cartridge audio pins should be taken inside the adapter. :oops:
I think an slightly modified AV cable can be used for this.
The only issue I see is to find a nice spot on the adapter to put a passing hole that wouldn't disturb the cartridge connection on an original NES and in most clones.

I could do a little video trying to show the result of the 100k resistor mod here.
Please, share your toughts about the mod.
If someone needs another track of the sound recorded, just tell me and I'll try to do.
I'm just not good at this game, plus the capture gear adds some delay, so expect a really lame player. :mrgreen:
Many thanks!


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