Fried the PPU?

82atari5200 · Post by **82atari5200** » Mon May 02, 2011 7:18 pm

Was doing a av mod on a famicom and was trying to clean up the picture by putting a capacitor between the 5 volt and neg pins of the PPU and accidentally switched the polarity without it discharging and I think I fried the chip. Goonies displays normally but locks up as soon as you start. SMB won't start at all and SMB3 starts but the sprites can't be seen. I was thinking about ripping one out a spare nes I have. But wondering If I might have fried something else.

socram8888 · Post by **socram8888** » Mon May 02, 2011 11:36 pm

Desolder the PPU, put there a socket and try with another PPU.

qbradq · Post by **qbradq** » Tue May 03, 2011 12:48 pm

Those are very strange symptoms to be having. Have you tried removing the cap? It seems odd that only specific functionality would be impacted.

I have heard that SMB tries to read data out of CHR-ROM for the title screen.

82atari5200 · Post by **82atari5200** » Tue May 03, 2011 12:52 pm

Yes took the cap off. Removed old ppu and am going to try to replace it tonight.

Post by **tepples** » Tue May 03, 2011 1:07 pm

qbradq wrote:I have heard that SMB tries to read data out of CHR-ROM for the title screen.

It also uses sprite 0 to detect the bottom of the status bar in order to change the scroll position.

Drag · Post by **Drag** » Tue May 03, 2011 4:27 pm

It sounds like you may have damaged the sprite hardware. Goonies II and SMB both use Sprite 0 (afaik) to do screen splits for their status bars. Given how that method of screen splitting usually requires the program to wait in a loop until the detection happens, it'd make sense that both games would hang.

SMB3's lack of sprites just backs this up.

Makes me wonder what, exactly, got damaged though, since there are a few reasons why sprites would fail.

82atari5200 · Post by **82atari5200** » Tue May 03, 2011 7:42 pm

Seems I did it again. Had it working. This time tried using a 100 uF cap. Screen was looking great and the lead slipped over to the next pin and now all I get is a yellow screen. I have never encountered an i.c. that is so fragile. Only have 1 spare PPU left before I have to run up to Play N Trade.

Drag · Post by **Drag** » Tue May 03, 2011 8:31 pm

Be careful with those! D: They don't make them anymore. :S

Drag · Post by **Drag** » Tue May 03, 2011 10:49 pm

82atari5200 wrote:Was doing a av mod on a famicom and was trying to clean up the picture by putting a capacitor between the 5 volt and neg pins of the PPU and accidentally switched the polarity without it discharging and I think I fried the chip.

Quick question, maybe it's because I'm not as experienced with electronics as I'd like to be, but what is supposed to be accomplished when you put a capacitor between VCC and GND? That sounds like a good way to cause a short circuit and nothing else. o_O

Second, when you say you switched the polarity, do you mean you put a directional capacitor on backwards?

82atari5200 · Post by **82atari5200** » Wed May 04, 2011 3:27 am

By putting the capacitor there it filters off the noise and creates a clearer picture. but if you really look at the screen close it kind of blurs the screen a little so you don't see white bars. and yes I didn't let the cap discharge for I put it on backwards.

3gengames · Post by **3gengames** » Wed May 04, 2011 4:18 am

Shouldn't the capacitor go one the VCC pin?...Shouldn't there already be on there?

TmEE · Post by **TmEE** » Wed May 04, 2011 7:56 am

I once had a short between 2 pins on the PPU and I got very peculiar results, i.e some certain colors were wrong etc.

Drag · Post by **Drag** » Wed May 04, 2011 11:19 am

82atari5200 wrote:By putting the capacitor there it filters off the noise and creates a clearer picture. but if you really look at the screen close it kind of blurs the screen a little so you don't see white bars. and yes I didn't let the cap discharge for I put it on backwards.

Wait, wouldn't the proper way to do that be to separate pin 21 (video out) from the board, stick the capacitor onto pin 21, and then connect the capacitor to the board?

Like I said, connecting VCC to GND just makes a short.

qbradq · Post by **qbradq** » Wed May 04, 2011 11:26 am

Drag, using a small capacitor from Vcc to Ground is a commonly used method of "debouncing" the supply voltage for an IC. The few electronics books I have read suggest doing this.

Keep in mind that a capacitor, especially one of low capacitance, does not create a shunt. I do not exactly understand how it "debounces" the supply voltage, but I know it does not short the circuit.

Drag · Post by **Drag** » Wed May 04, 2011 12:28 pm

qbradq wrote:Drag, using a small capacitor from Vcc to Ground is a commonly used method of "debouncing" the supply voltage for an IC. The few electronics books I have read suggest doing this.

Keep in mind that a capacitor, especially one of low capacitance, does not create a shunt. I do not exactly understand how it "debounces" the supply voltage, but I know it does not short the circuit.

"Bouncing" refers to mechanical things, like switches and buttons. As in, when you press a button, it's not a clean transition from open to closed, but the button physically "bounces" against the circuit before it rests. Basically, you see something like:
1-------0101010----------1010101------
instead of:
1---------0------------1--------
for when a button is pressed (1->0) and released (0->1).

The PPU doesn't deal with mechanical things like that when rendering video, and unless I'm misunderstanding the OP, he wants to filter the output of the video, which I think is better accomplished by fiddling with the video-out pin, instead of vcc/gnd. So am I right to be confused here? I see how it would be helpful to add a "debouncing" capacitor between vcc and gnd, but I'm having trouble seeing how it would be conductive to video output.

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Fried the PPU?

Fried the PPU?

Re: Fried the PPU?