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Yet another Black Screen SNES
https://forums.nesdev.com/viewtopic.php?f=12&t=17357
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Author:  pkmapd [ Sun May 13, 2018 12:43 pm ]
Post subject:  Yet another Black Screen SNES

Hello everybody.

As the tittle suggests, I am dealing with a PAL CPU-02 SNES that suffers of Black Screen. I have tested all the obvious things: voltage regulator and fuse are OK, PCB seems OK (I did not find any damage on it and cleaned with alcohol), capacitors look OK (have not tested them), etc.

As the power/electrical part seemd to be working, I decided to test the electronics themselfs. I got access to an oscilloscope and started trying things. I had the schematics, so I could get some information about what output to spect from the oscilloscope. I could only test some basic things (controller pins in CPU at HIGH, same for all RESET and the button working well and a few more things).

The problem is that I did not find any information around the internet about how to test each chip independently with the oscilloscope. I read the this may be a failing CPU, but how to get sure about it? Next time I have access to an oscilloscope, what pins should I test and what should I expect to get from there to ensure that the chip (CPU or PPU) is working?

By the way, taking some measurements, I noticed that, for two different games (SSF II Turbo and Bomberman), for the CPU: JPIO pins are HIGH, D0-D7 there is data through them, same for RAMSEL and CPUSEL, pin 18 LOW, from 19-28 HIGH, 29, 30 and 31 LOW, from 32 to 36 HIGH, 37, 38 and 39 LOW, regular pulses on 40, 41 LOW, 42 LOW, 43 HIGH, 44 LOW, 45 HIGH, 46 noisy and I got tired of taking measurements. I checked that there wasdata through the bus that conected both PPU. I suspect that this "random" measurements are not useful at all, since the system had been turned on for a while when I started taking them (maybe it is more useful to check during boot/reset sequence?). I would not trust a lot these measurements, so I plan to repeat them.

I barely know some things about electronics (I'm taking my engineering degree right now). I have experience repairing electronics (old PCs and handheld consoles), but never at a "logic" level I have not found a forum with more SNES experts than this one, so that is why I seek for help out here.

Thank you in advantage and sorry for my bad english.

Author:  Fisher [ Mon May 14, 2018 4:04 am ]
Post subject:  Re: Yet another Black Screen SNES

Other than the CPU, as you already tought of, one of the PPU ICs or even the video memories can be the problem.
The best advice I can give is to try to get an SNES test cart.
This cart sometimes can display things on screen even when some games can't, wich is a nice way to narrow down problems.
Should be a nice start, and it's not too difficult to make a repro of it.
Good luck!

Author:  pkmapd [ Mon May 14, 2018 9:50 am ]
Post subject:  Re: Yet another Black Screen SNES

Thank you for the advie, I will buy some eproms and try. By the way, is there any test cartridge better than others? Any recommendation?

And what about testing the CPU by using the oscilloscope? I will have access to on enext week and I would like to know what to test exactly.

Thanks.

Author:  koitsu [ Mon May 14, 2018 10:45 am ]
Post subject:  Re: Yet another Black Screen SNES

By "SNES test cart" he means exactly that: the official one that Nintendo made for their SNES hardware test kiosks. There are dumps of this cart (i.e. ROMs) floating around, so you can burn that + make your own cart.

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