What is this cap doing?

http://console5.com/techwiki/images/1/1 ... et-CIC.png

The image is rather large, so I cropped out the power section:



C67 (1000uF), it's located just to the left of the power switch.

Console5 lists C67 as present on the SFC but only present on the earlier North American SNES. Apparently on a later model NA SNES, there's a spot on the motherboard for C67 but the cap is missing. It's in the PAL SNES, but it's 2200uF there. It's becomes C58 on the 1chip and SNES mini, and C58 is present in the SFC but missing in the NA SNES.

What effects does C67 have on the 5v rail? What difference would it make leaving it out? Or rather, on a SNES that omits it, what difference would it make by adding it?

It might be worth mentioning that the power supply output for a NTSC-U SNES is 10v .83A, but 9v 1.3A for the PAL SNES. I don't know what the power supply specs are for a SFC.

Thanks for any input!

Normally a 7805 is "supposed" to have a relatively large capacitor on its input and output, although you can sometimes get away without one or the other depending on trace lengths.

One guess: maybe C67 is that capacitor, even though it's before the power switch. (Since C32, C33, and C63 are too small for this purpose). Then again, the wires to the switch are relatively long, so maybe not.
edit: C63's 33µF, that's passable... although it's still dwarfed by 1mF. But given the schematic, it seems to be filtering for the +11.5V supply instead of the 7805.

Another guess: maybe someone thought of D1 and C67 as forming a simple power filter.

My guess would be to minimize the effect of the transient current spike into the 7805 at the instant you flip the power switch.

Decoupling of the input voltage. They may have revised the power supply included with the system to have better internal decoupling, as so to save money on the console.

When I look at datasheets for the 7805 it shows a .33uF cap on the input and .1uF cap on the output. All I see in reference to these caps is "Input supply capacitor recommended for filtering noise on the input," and "Output supply decoupling capacitor for stabilizing the output." I've heard of putting large caps on the input/output of the 7805 before, but since I haven't seen it in a datasheet, where is that info coming from?

As far as I know, the power supply for the North American SNES never changed. At least, the model number on the brick never changed until the SNES mini redesign. Even if the power supply was redesigned, C67 becomes C58 on the 1chip/mini, and C58 is present in the SFC Jr but still missing in the NA SNES mini. As far as I can tell (from the Console 5 website) this cap is 2200uF on the PAL SNES and is present in all models.

edit: The reason I ask is because a friend was looking to recap his SNES and noticed that C67 has a spot on the motherboard but was left out. He decided to add C67 when he recapped. He claims that his vertical bar is now the faintest he has ever seen it, although admits that he can't be sure if he's just seeing what he wants to see. Quick testing looks like adding C67 has no noticeable effect on audio or video.

Maybe he is "just seeing what he wants to see" however replacing the largest cap in an NES, for example, directly affects the video. I actually wouldn't be that surprised if what he said was true. Like I said above: decoupling. That 50 or 60 Hz oscillation from your wall is a nasty thing that will leach into all nearby circuitry.