Super Famicom, Sprites behind Background.

[Overlord_Manny]

I was having issues with my SFC not loading any game rom on boot. If I cycled the power a few times it would start and was perfectly playable. My wife and I played for a couple hours like this. I had no sound or graphic issues, but the reset button would not reset the game, it would only blank the screen when pressed and the game would continue where it left off. I ensured that the edge connector and socket were well cleaned. I de-soldered the reset button and checked to make sure I didn't have a stuck switch, it was fine but I gave it a good cleaning anyway. I know that the cic can throw the game into a reset loop like the NES. I decided to recap the board and change the 7508, seeing as this is the original caps and that C62 is part of the reset circuit.

After recapping the SFC the game roms load great the first time but the sprites would be behind the first BG layer. I tried several games, Super Mario World, Super Ghouls and Ghosts, and Castlevania IV and all had the same issue. I pulled the caps back off and checked them all with a meter and they are all reading correct capacitance. I re-soldered them back in place and still had the same issue.

I began checking traces and found noting noticeably broken, though a couple pins on PPU1 were really close together. They didn't tone as shorted but I went ahead and de-soldered PPU1 with my hot air station, straightened the pins and replaced the chip.

I started up the system and all looked great, but after a minute or two Mario would slowly, piece by piece de-materialize behind the bushes on the SMW intro. And this is where I am now. I'm not sure what to do. If I turn the system off for a few minutes I can restart it and it looks great, but the sprites will slowly move back behind the BG layer.

I have the schematic for my revision but I don't know enough about the inner workings of the PPU's to say it's for sure one of them. But I don't know what else it could be. Could a DAC cause this? Seems like if it were a data or address issue that it would be constant or at least affect all layers some how. It seems like something's heating up and shorting out. I have a logic probe and an o-scope with an logic analyzer, but if it's the PPU I may as well toss the board and get a new one.

Here's a link to a video for example. Everything looks great at first... actually maybe that jumping piranha plant is odd being in the foreground. I'm not sure if that's right on the Japanese rom. Also the game froze on me for the recording, but that's a first.
https://youtu.be/gTGVnRYGyqA

Anyone have any ideas?

[Josh128]

Bumping for a buddy-- this is a real strange problem, I witnessed it firsthand, prior to him straightening and reflowing some pins on the PPU. It was way worse than what is shown in his video then, in SMW, mario would run behind the bushes (the closest "background") but still in front of more distant backgrounds. We popped in Super Ghouls N Ghosts and Arthur, in the same fashion, would run behind trees and shrubs. Castlevania 4, same business, looked very unique, but made the game very hard to play! I wish we had gotten video of it then.

It was a most strange problem, and it appeared to go away after he worked on the PPU, but as you can see in the video it gradually comes back as the SFC heats up (we assume). Anybody know if something like this could be caused by anything other than the PPU?

[Arthrimus]

Could be a failing PPU2. I've never seen this particular behavior, but I've had PPU2s be responsible for sprites failing to render properly or at all. Then there's always the possibility of a failing CPU as well, those can cause an enormous range of strange symptoms. Do you have a way to run the Burn In Test cartridge?

[Overlord_Manny]

Sorry for the long response time, I've had a few other projects I've been working on. I'm guessing one of the PPU's as well. I may try to get some old systems for parts (if I can get some with working PPU's) and see if changing the PPU's will help. I don't have a test cart. I figure I'd have other issues as well if it were the CPU, but maybe not.