Is it worth it to mod an NES top loader?

[Phetishboy]

Well, if you don't want to be confused, don't read below...

In order to prevent people from making piles of crappy games for the console (like what happened to the Atari 2600), Nintendo used a lockout system. Basically, a lockout chip sits in the console, and controls the reset pin of the processor. The lockout chip sits there, resetting the processor once per second, looking for a key chip on it's input pins. These input pins are connected directly to some of the pins on the Nintendo's 72 pin cartridge connector - and are the couple pins on the very edge, closest to the ridges that run down the front of the cartridge.

So, inside every cartridge is another lockout chip, configured in "key" mode. When the lockout chip in the Nintendo can communicate with the key chip in the cartridge, it stops resetting the processor and the game plays.

The problem is, this is very sensitive to dirt on the cartridge, and it's made worse by the fact that the pins on the very edge of the connector tend not to make as good a contact - and the edge pins are hardest to clean on the cartridge. So, if anything happens to this communication, the NES sits there resetting once a second - the flashing gray screen/power light. Sometimes you can even manage to get a game that loads the title screen perfectly, but keeps blinking. The program is trying to run, but the lockout chip is resetting the processor.

So, if you disable the lockout chip, it won't continuously reset the processor, making one less thing to go wrong. All that needs to be done is to clip pin 4 on that lockout chip. This is usually tied to 5v in the Nintendo, but contains enough internal pulldown so clipping the pin ties it to logic 0. This works because the same exact lockout chip is used in both the console and the cartridge. The console's chip is configured to be a lock (pin 4 high) and the cartridge chip is configured as a key (pin 4 low). By clipping the pin, and making the console's chip behave as a key, it won't reset the processor.

Third party games got around the lockout chip in a weird way, "zapping" it with a weird negative voltage, causing the chip to lock up and stop resetting the CPU. This is why some third party games have that switch on the back, and why sometimes they blink three or four times before they load. Later versions of the NES included a diode to prevent these from working as well. The only third party to really truly crack the lockout chip was Tengen (Atari), reverse engineering it and making a clone.

-Ian

I wish you were my dad.

 

[Tighe]

I wish you were my dad.

Yah Ian is awesome!

 

[MaximRecoil]

No I did not. Not even sure what a lock out chip is. I'm just a tired old man who wants to enjoy the occasional NES cart to relive the year 1986. Please don't try to confuse me with your technical jargon. You know too much.

RetroHacker explained it really well, and it is really easy to do. The only annoying part about doing it is dealing with forty-eleven Japanese "Phillips" screws that no U.S. Phillips screwdriver seems to fit properly (a #1 is too small and a #2 is too large, though either will work if you're careful).

Once you have removed enough of those infernal screws to access the parts-side of the motherboard, (by removing the top of the case, then the RF shield, then the cartridge carrier, then the remaining two screws by the RF modulator holding the motherboard to the bottom half of the case, then the bottom RF shield), find the lockout chip ...

... and clip the 4th pin with flush cutters (the bottom picture shows one that's already been disabled). While you have it apart, either replace the 72-pin connector with a new one (it just pulls off the motherboard) or pull the original one off and clean all the pins (both the set of pins that connect to the motherboard and the set of pins that interface with the cartridge) with a toothbrush and some rubbing alcohol (or contact cleaner if you have some). Also clean the motherboard pins that interface with the 72-pin connector and your game cartridges' card edge pins in the same way.

If you replace the 72-pin connector, the replacements are usually tight, so first try playing games with the cartridge just pushed in but not pushed down.

Either way, this should result in a 100% or near-100% reliable front-loader NES. It will have better picture quality than your top-loader, and it is far more nostalgic. I use mine on a 32" standard resolution, spherical tube CRT TV with composite ("A/V") cables, and the picture is beautiful (and it works the first time, every time).

 

[keithsarcade]

What are your thoughts on "grounding" that pin or solder pad? Ive heard a few people say you should ground it to the shield, but most of the time I see people just clip that leg off. I just pry mine back and call it done.

 

[MaximRecoil]

What are your thoughts on "grounding" that pin or solder pad? Ive heard a few people say you should ground it to the shield, but most of the time I see people just clip that leg off. I just pry mine back and call it done.

I just clip the 4th leg and call it good. I've never had any problems, and I've been using NESs with disabled lockout chips for about 6 years now. I've also seen that some people run a jumper wire from the clipped leg to ground, but I don't see the point, since it works perfectly without doing that, and it is much less of a hassle to not do that.

 

[RetroHacker]

I just clip the 4th leg and call it good. I've never had any problems, and I've been using NESs with disabled lockout chips for about 6 years now. I've also seen that some people run a jumper wire from the clipped leg to ground, but I don't see the point, since it works perfectly without doing that, and it is much less of a hassle to not do that.

+1. Usually you never want to leave a TTL input floating, but in this case, the chip has enough internal pulldown that you don't have to ground the pin. It works fine (or rather, it doesn't work...) with the pin clipped.

I, too, have never had a problem just clipping the pin.

-Ian

 

[PrairieDillo]

damn I didn't know aobut this before and when I opened up my NES, Cleaned every edge connector, installed a new cart connector and changed the LED to blue... I didn't do this mod.

Now I have to go back in and dissable this chip!!!

and yes isn't A/V actually called Composite to be correct?

 

[RetroHacker]

and yes isn't A/V actually called Composite to be correct?

If you want to get technical, yes - composite referrs to the type of video signal... but doesn't say anything about the audio output. So, I suppose you could call it NTSC composite video with a line level audio signal on the RCA socket right next to it.

Or A/V ports.

-Ian

 

[MaximRecoil]

My toploader doesn't have the lines on the screen and the different horror stories that people talk about. Seems like I lucked out with that part, the only thing is the color doesn't seems as good.

This is what the lines look like on a toploader:

This was caused by a design flaw that Nintendo acknowledged at the time. If you complained about the lines to them, you could send it in for repair and they would send you back a toploader with a revised motherboard, which in some cases had the same A/V multiport that the A/V Famicom, SNES, N64, and Gamecube have; like so:

I suppose the lines could be less or more noticeable depending on your overall setup. Maybe Nintendo revised the motherboard for the normal retail units at some point in the production so that some line-free versions made it to stores.

I would like to have one of those A/V multiport versions for the novelty factor, though you can already get as good of picture quality as is possible from a standard-PPU NES by using a dime-a-dozen front loader, which has zero lines and great color, even when using RF (but it is better to use its A/V jacks).