NES Stereo Mod 1 pot

Hi, I just saw a video on Youtube from GameTechUS, I see that he uses a method different to everyone else, he uses a 1kOhm Pot and solders a pair of wires to pins 1 and 2 of the CPU, each of those wires are soldered to the negative side of a capacitor, (one for each wire) the positive end of one capacitor is soldered to the center pin of the pot and the other capacitor to an outer pin of the pot. He then taps a wire on each positive of the capacitor to the center pin of the RCA jack. Basically he's shorting both pins to a degree by mixing them directly without the mono channel, like this:

http://s9.postimg.org/cpvrksnfj/nes_mixer.jpg

Now, wanting to do this mod myself, and it seem much simpler and one pot only, my question is, is there any risk by shorting/mixing the CPU pins with the pot directly?

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Originally Posted by t0nito

Hi, I just saw a video on Youtube from GameTechUS, I see that he uses a method different to everyone else, he uses a 1kOhm Pot and solders a pair of wires to pins 1 and 2 of the CPU, each of those wires are soldered to the negative side of a capacitor, (one for each wire) the positive end of one capacitor is soldered to the center pin of the pot and the other capacitor to an outer pin of the pot. He then taps a wire on each positive of the capacitor to the center pin of the RCA jack. Basically he's shorting both pins to a degree by mixing them directly without the mono channel, like this:

http://s9.postimg.org/cpvrksnfj/nes_mixer.jpg

Now, wanting to do this mod myself, and it seem much simpler and one pot only, my question is, is there any risk by shorting/mixing the CPU pins with the pot directly?

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Jason Rauch has actually stated on his website that the potentiometers he usually uses tend not to work - which is why he's question using them at all. With that being said - he says in his TopLoader walkthrough that "You short the signals together with the wiper".

What he means is; he's combining the two signal by turning the pot all the way to the right (or left, depending on which side it's wired to). Even when a potentiometer is "shorting" two signals, there's still a great deal of resistance between the two. If it just so happens that I'm wrong (although I've watched his video several times before I got the hang of doing TopLoader mods) - connecting the two CPU pins is how the console produces mono audio to begin with. You probably wouldn't want to do it without a 1kΩ resistor between the two, but it's totally safe otherwise.

I actually sent a note to Jason yesterday, asking if he'd tried different methods, using potentiometers to mix Audio 0, and Audio 1. He sent me a message back letting me know that he no longer uses the method.

Thanks! Well I tried and it seem I f*cked up my NES, I don't know what happened, I did the mod and tested it and I was only getting one channel, I thought maybe the capacitor was bad as they are salvaged from other boards and stuff. I swapped the capacitor and when I turned it on it didn't do anything, I think I shot the CPU, I don't even know how, nothing was shorting anything, it simply dead, I did all the soldering with the unit unplugged of course but still.

The voltages are present 9V and 5V but the LED won't turn on, the CPU gets hot, I see a small flash on the screen when I switch it on otherwise its black and dead.

I think it may be the CPU, but there is no way I can be sure. I see these chips on ebay. Mine being an NTSC version is a RP2A03G the cheapest ones I find on ebay are RP2A03E from China, does it make any difference being E or G?

http://www.ebay.com/itm/1PCS-RP2A03E...8AAOSw6dNWTogo

The only RP2A03G's I find are from the USA, but shipping costs to Portugal are ridiculous for such a small item.

Your CPU likely isn't broken (unless you burned the shit out of the pin.) Run continuity checks on all the surrounding traces.

Thanks for replying, I think the problem is greater than that, I still don't know if it's a short on the CPU or somewhere else, I inspected the board thoroughly to see if a blob of solder fell somewhere, it didn't. I use a 9 VDC supply, measuring the voltages I get 8.3 V when off, and around 7 V when on that's a full volt drop, I don't know if it's normal or not, I measured the VCC pin on the CPU and it's bang on 5 V.

When switching the unit on and off the LED flashes very briefly sometimes and if I time it right I'm able to get a blue screen but the led remains off. If I drop the power supply to 7.5V I'm able to turn it on all the time with a blue solid screen and the LED is dimly lit. It won't read carts in this state only a solid blue screen. The screen doesn't flash because I disabled the lock out chip years ago to play PAL games on it.

I have a video demonstrating this:

https://youtu.be/pCZKhHRF9K0

What do you think about this?

EDIT: Well I guess the CPU is fact shorted as leaving the NES on for less than a minute the CPU gets scalding hot, hot enough to burn, anyway I ordered a CPU from China, and see how it goes when it arrives. If it fixes it no more mods for me! I have no idea why or what went wrong.

I don't believe that's a CPU problem. It sounds to me like something's wrong with the power circuit inside the RF modulator (you may have fried the 2200uF cap). You can check to see if the CPU's been damaged by putting the negative lead of your multimeter on any ground area - and checking each pin for continuity between the CPU and ground. One pin SHOULD have continuity between ground, but only one. If you get more than one pin with continuity to ground than something shorted inside the CPU and it indeed does need to be replaced.

Ok, so I checked the power circuit, all seems OK.

Continuity on the chip, the audio pins were 100 Ohm each to ground, pin 40 VCC 263 Ohm, but then all the +5 V rail gives that resistance to ground. I did test voltages and while pin 1 measured 300 something mV pin 2 was measuring 0.8 V which seems quite high for an audio channel, I disconnected the 100 Ohm resistor to figure out if this voltage was coming out from the chip or in from the board, with the resistor disconnected this voltage raised to around 2V, I don't think this is normal.

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Originally Posted by t0nito

Ok, so I checked the power circuit, all seems OK.

Continuity on the chip, the audio pins were 100 Ohm each to ground, pin 40 VCC 263 Ohm, but then all the +5 V rail gives that resistance to ground. I did test voltages and while pin 1 measured 300 something mV pin 2 was measuring 0.8 V which seems quite high for an audio channel, I disconnected the 100 Ohm resistor to figure out if this voltage was coming out from the chip or in from the board, with the resistor disconnected this voltage raised to around 2V, I don't think this is normal.

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...what?

So you took apart the RF modulator and checked the cap, the bridge rectifier, the 7805, etc? Also...

Did you run continuity checks from every CPU pin to ground? (You know? The function that beeps at you if you hold each multimeter probe together?) If you did - and you only got ONE PIN with continuity, then more than likely - it's a good CPU. Voltages coming out of the pins have NOTHING to do with continuity.

Also, if I'm not mistaken - there's no external amplifier for audio out of the NES. The CPU itself drives the audio signals. That's why you need to terminate voltage using a pair of caps when you grab AUD0, and AUD1 directly from the CPU. 2Volts+ sounds about right for both of the audio pins.

Also... wait a sec? You turned the system ON when checking for continuity? All you're gonna get is beeps with that! Not to mention that if one of your multimeter probes slides and jumps two pins - then you WILL have a broken CPU!

Quote:

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Originally Posted by t0nito

Thanks for replying, I think the problem is greater than that, I still don't know if it's a short on the CPU or somewhere else, I inspected the board thoroughly to see if a blob of solder fell somewhere, it didn't. I use a 9 VDC supply, measuring the voltages I get 8.3 V when off, and around 7 V when on that's a full volt drop, I don't know if it's normal or not, I measured the VCC pin on the CPU and it's bang on 5 V

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Wait a sec I missed this part.

You're getting current when the system IS OFF?! You shouldn't be getting ANYTHING when the system is off! Let alone 8 VOLTS! If you're getting 8 Volts on anything your entire NES is toast.

No, I get 8V on the input pin to the power switch only, when I turn on the switch, that voltage drops, the rectifier is working as I have 5VDC on the 5v rail.

That's your problem ;)

The forward voltage drop off caused by the diodes, is not turning the 7805 on. Therefore, the system itself is not turning on correctly because there's not enough voltage. Also how many amps if the A/C adapter you're using? It should be at least 1 Ampere (or 1000mA). You need to bump the A/C voltage from 9V's in to around 12Volts in.

Again I could be wrong. So to confirm, can you check the input rail of the 7805 and see how many volts in you're getting?

It's the same adapter I've always been using, perhaps you misread, the rectifier is outputting 5V, the input is that 7 something volts when turned on, I did try a 12v supply, to change is behavior, but as you can see on the video it works better (if you can call that) when the voltage is lower.

What do you reckon about the 2 volts out of the audio pin on the CPU (pin 2)?

Quote:

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Originally Posted by t0nito

It's the same adapter I've always been using, perhaps you misread, the rectifier is outputting 5V, the input is that 7 something volts when turned on, I did try a 12v supply, to change is behavior, but as you can see on the video it works better (if you can call that) when the voltage is lower.

What do you reckon about the 2 volts out of the audio pin on the CPU (pin 2)?

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The bridge rectifier should be outputting at least 9 volts. I don't know what diodes are used in the NES bridge rectifier, however, let's say they're common 1N4007's. The voltage drop off will be anywhere from 1.1 to 1.4 volts. Double that since there's two diodes working in series at any one time, and the voltage drop off from a 12Volt A/C adapter becomes (around) 9 volts DC out.

If you're only getting 5-7 volts from the output of the bridge rectifier, it won't be feeding enough current to the 7805 in order to correctly power on the regulator. So try eliminating that as a problem by probing the input pin of the 7805. If it's 8 volts or less - the 7805 is likely not receiving enough power to turn on.

To quote what I said above about the CPU (in terms of audio)

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Originally Posted by SwampFox56

if I'm not mistaken - there's no external amplifier for audio out of the NES. The CPU itself drives the audio signals. That's why you need to terminate voltage using a pair of caps when you grab AUD0, and AUD1 directly from the CPU. 2Volts+ sounds about right for both of the audio pins.

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Quote:

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Originally Posted by SwampFox56

...what?

So you took apart the RF modulator and checked the cap, the bridge rectifier, the 7805, etc? Also...

Did you run continuity checks from every CPU pin to ground? (You know? The function that beeps at you if you hold each multimeter probe together?) If you did - and you only got ONE PIN with continuity, then more than likely - it's a good CPU. Voltages coming out of the pins have NOTHING to do with continuity.

Also, if I'm not mistaken - there's no external amplifier for audio out of the NES. The CPU itself drives the audio signals. That's why you need to terminate voltage using a pair of caps when you grab AUD0, and AUD1 directly from the CPU. 2Volts+ sounds about right for both of the audio pins.

Also... wait a sec? You turned the system ON when checking for continuity? All you're gonna get is beeps with that! Not to mention that if one of your multimeter probes slides and jumps two pins - then you WILL have a broken CPU!

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I'm sorry I missed this post entirely (how embarrassing).

Yes I took apart the rf mod, cap checks fine, I get around 8 volts out of the bridge rectifier, using a 9v supply.

I did check the continuity with the results posted above. Only pin 30 beeped as it's the test pin pulled to ground so it's normal.

Of course I did this with the power off, I'm not THAT much of a noob, that's why I don't understand what went wrong when doing such a simple mod :p. I've done a region mod to a Mega Drive add two crystals to work on both NTSC or PAL 60, and other small stuff.

The voltages I stated were after the continuity checks.

On the NES chip, one audio pin has 300 something mV, the other pin has 2V so there's a discrepancy on both pins, the NES has a preamp to raise the weak audio cpu signals to line level before reaching the RCA plug. That's why I find 2V on audio pin way too high.

Quote:

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Originally Posted by SwampFox56

If you're only getting 5-7 volts from the output of the bridge rectifier, it won't be feeding enough current to the 7805 in order to correctly power on the regulator. So try eliminating that as a problem by probing the input pin of the 7805. If it's 8 volts or less - the 7805 is likely not receiving enough power to turn on.

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Sorry my mistake, to clear things up, when I said rectifier I was referring to the 7805, I have 8v in the 7805 and 5v out. I also measured 5V between ground and the CPU VCC pin, so we can rule out a power supply problem.

Since the CPU is burning hot after seconds after turning on I believe it's most definitely shot.

This mod sucks anyway. The NES was never designed to output stereo sound. The vast majority of NES games sound horrible with this mod, a few others are only marginally terrible. The simple fact that a pot is required to be adjusted for every game is proof that this mod just doesn't work.